This type is dominated by the low temperatures which fall below zero for at least 4 to 5 months, causing severe limitations to the development of life and enforcing strong adaptation of living organisms to low temperatures. There are two bioclimates within this macrobioclimate. I.a. Tundral. The highest latitudes, oscillating between 55° and 70°, are covered by tundral vegetation in which the trees do not develop due to the short duration of the period of plant activity in the short summers and due to the existence of permanent ice on the ground (permafrost). Permanent ice occupies large areas, especially in Antarctica and Greenland. Continental and oceanic versions can be distinguished. I.b. Boreal. This type occupies the latitudes above a border that oscillates between 40° and 66° depending on the marine currents, where winters are long and cold, and summers are short and rainy. Depending on the amplitude of the seasonal variation of temperatures, oceanic and continental versions can be distinguished. In general, the territory is dominated by cold-resistant coniferous forests, which are accompanied by some deciduous trees, such as birch, willow and aspen. In both cases, there are high elevation versions in the mountains of the temperate and tropical zones.

This is the most climatically diverse and the one that hosts the greatest diversity of terrestrial ecosystems. Its lower latitude limit oscillates between 25° and 30°, bordering the warm and arid bioclimates. Its entire area is subject to a thermal seasonality, with a clearly differentiated winter and summer, the more pronounced the greater the latitude and the distance from the influence of the seas. In addition to this, the seasonality of rainfall, basically determined by the polar front, plays an important role in differentiating the bioclimates: II.a. Temperate ombroestival (summer-rainy), occurs in latitudes immediately higher than the tropics and in contact with them. These areas are influenced by the polar front dragged by the westerlies, trade winds and even the monsoonal regime, ensuring summer rainfall in sufficient quantity. There are very oceanic areas, in the regions close to the coast, and others more continental, in which winter rainfall is lower than in summer due to the influence of high polar pressures and, in East Asia, due to the effect of the winter monsoon. It spans Europe, eastern North America, and eastern Asia in the Northern Hemisphere (hereafter NH), while in the south it is found in southern Chile, southeastern sides of Australia and Africa, and New Zealand. The dominant vegetation is temperate deciduous forests and non-sclerophyllous evergreen temperate forests. In the tropical zone, mountains above 1000 to 1500 m asl have a cooler climate and higher precipitations, and thus their climatic conditions approach rainy temperate. II.b. Temperate aridiestival (summer-dry or Mediterranean s.l.), located in the latitudinally lower fringe of this zone (subtropical), between 25° and 45°, on the western sides of the continents and in contact with the extratropical deserts. In the summer there is an intense drought of at least two consecutive months caused by the strengthening and expansion of the subtropical highs, while the winters are under the influence of the polar front that moves towards lower latitudes. The countries bordering the Mediterranean Sea and Middle East, the central-southern area of California, central Chile and Argentinian Patagonia, the Cape region and the south and southwest of Australia correspond to this variant, where the typical plant response is the evergreen sclerophyll-microphyll, both wooded and shrubby. There is a more continental variant of this type extended across central western North America and West-Central Asia. II.c. Temperate steppic, spread over the interior regions of the great continental masses, subject to strong thermal seasonality and low rainfall throughout the year. The powerful winter anticyclones that form on these continents limit the rains, which are somewhat more abundant in summer when there are sporadic incursions of maritime air masses. This climatic variant extends mainly through the interior regions, strongly continentalized, of the great land masses of the Northern Hemisphere (NH), such as that spanning from eastern Central Europe to western China and Mongolia, as well as the interior regions of North America (Great Plains). Its characteristic vegetation is the steppe, formations dominated by grasses, either in mosaic with scattered forest patches (forest-steppe) or without them (grass-steppe).

This is characterized by climatic aridity and appears in regions with very low rainfall (Ombrothermic Index Io (Rivas-Martínez et al. 2011) lower than 1, exceptionally 2; see below). The response of the vegetation is a desert due to dryness, with a notable presence of succulents in certain regions, with a scattered distribution, low cover, and little biomass. They can be found in a wide latitudinal range between 10° and 45°. Depending on its temperature and rainfall regime, we can distinguish three main bioclimates: III.a. Continental cold desertic, in mid to high latitudes of the central areas of the large NH continents, North America and Eurasia, where thermal seasonality is very marked in the innermost regions and in higher latitudes. III.b. Temperate desertic, in mid-latitudes above the tropics in which there is a seasonal variation in temperatures, or in the highlands of some tropical desertic areas where temperatures are lower. Sporadic rains occur in the winter solstice in latitudes above tropics and are caused by sporadic entries from the polar front. These extratropical deserts extend through the interior regions of the great continents (Eurasia, Africa, North America, Australia), due to the decrease in rainfall caused by the distance from the seas and by rain shadow phenomena generated by mountain systems. III.c. Warm desertic, in low latitudes around the tropics and where temperatures are higher and do not show a notable annual oscillation. In this case, the scarce rainfall will fall in the months close to the summer solstice, as it is in the tropical zone, and will be caused by the sporadic approach of the Intertropical Convergence Zone (hereafter ITCZ), which will contribute to them. The deserts of the southern Sahara, Arabia and South America can be included in this variant.

This roughly encompasses the regions between the Tropics of Cancer and Capricorn (intertropical), with some subtropical extensions in certain lowlands influenced by warm ocean currents. Within the intertropical zone, all its points receive solar rays with an inclination of 90° twice a year. Therefore, it is the area that receives the most solar energy and is the warmest on the planet, not being subjected to thermal seasonality, although it is to the rainfall seasonality in many of its parts. As for its bioclimates, we distinguish two. IV.a. Tropical pluviseasonal, in which there is a clear seasonality in the rains, which fall mainly in the summer months, leaving a dry season for the remainder of the year, in which the temperatures often reach very high values. It occupies the bands between 5° to 10° and 20° to 25°N and S, to which ITCZ moves on the spring equinox to cause seasonal rains. From the autumn equinox, the movement is towards the opposite hemisphere and the strip comes under the dominion of the high subtropical pressures, giving rise to the dry season. The monsoon regime is also the cause of this climatic variant in the regions where it operates and the trade winds spread this climatic type along the eastern sides of the continents. Vast tropical territories of the Americas, Africa, Asia and Australia are under this variant, where deciduous or evergreen-sclerophyll formations predominate, adapted to survive a hot and arid season. IV.b. Tropical pluvial, in which the rains are abundant for the whole year (aseasonal). It occupies the latitudes closest to the equator, between 10°N and S and is under the permanent influence of the ITCZ, registering high amounts of precipitation and notable thermal uniformity throughout the year, with little or moderate seasonal variation. Its characteristic biome is the tropical rainforest and the typical regions are the Amazon, the Congo basin and the Indo-Pacific archipelagos.

As long ago established by Huguet del Villar (1929) and further developed in Rivas-Martínez et al. (2011), there is a relevant influence of the continentality of an area in the altitudinal span of the zonation of the thermotypes in the mountains that are within it.

Continentality and the elevation of the upper limits of the thermotypes. It is noteworthy that the upper limit of the cryoro is higher at latitudes of 20–30° in the NH and 10–20° in the SH, than in the interval between 20°N and 10°S (Figure 5). In both tropical intervals, upper limits reach altitudes higher than 5100 m, while in the equator interval the limit is somewhat below 5000 m, a difference of about 200 m in elevation (Pfadenhauer and Klötzli 2014, Figure 7). This effect can be explained by the lack of thermal seasonality near the equator. As soon as the latitude increases in the tropics, the seasonality also increases, and this makes for a hotter summer and the biota can live at higher elevations. Interestingly, there is an asymmetry between both hemispheres so that the southern peak is closer to the equator than the northern one. This asymmetry between both hemispheres was noticed by Troll (1948) and can be explained by the same phenomenon: cold stressing conditions are achieved at lower altitudes in climates without thermal seasons. This is due to the lack of a warm season that can compensate for a cold winter, this being more accentuated in the SH where the conditions are more oceanic. In areas with similar latitude, the more continental is the regional climate, the higher are the thermotypes zonations. As an example, in the Colorado Rocky Mountains (USA), in latitudes about 37°N, the cryoro is found above the 3500 m asl, whereas in similar latitudes in Europe, as in Sierra Nevada (Spain) it starts regularly above 2800 m asl (Fernández Calzado and Molero Mesa 2011).

The change of the thermal definition of the thermotypes. The thermotypes are basically defined by the Tp (positive temperature, Table 5). Along the latitudinal gradient, as a general pattern, the Tp range of the thermotypes changes, being lower towards the poles, and highest at latitudes near the equator (Figure 6). This effect is more pronounced in the warm thermotypes than in the colder ones. The reason for this changes is that plants need a minimum period of high temperatures to complete their vegetative and reproductive cycle, an essential condition for their intergenerational survival. In a seasonal climate, the long and warm summer is sufficient to allow these cycles to complete, no matter how cold the winter is. In a non-seasonal climate, where winter is similar to summer, to reach the survival conditions of the plants it is necessary that the temperature values are high enough all along the year, winter and summer, as the difference between both seasons is small or null. This results in higher annual total values. For this reason, the limit values of Tp between thermotypes become lower the closer we get to the poles. Areas within the same latitudinal zone, thermal seasonality (i. e. continentality) strongly influences the altitudinal belts zonation. If seasonlity is low (very oceanic areas), the cold thermotypes are very wide and their limits with those immediately below are in lower elevations, while if the thermal seasonality is accentuated (continental areas), the warmer thermotypes expand to higher altitudes.

In short, the proposed system provides a typology in which the macrobioclimates reflect the general conditions of the great climatic zones of the earth, the bioclimates the seasonal regimes, the thermotypes the temperature regime (which will depend on the latitude and the elevation of the land above sea level), and the ombrotypes the abundance of rainfall and water availability, which are the climatic patterns determined by the General Circulation Model and the elevation of the terrain.

The climatic parameters and indices used are indicated in Table 5.

The climate of this domain is governed by the clearly negative radiative energy balance, with more radiation issued than received. This entails the permanence of the polar high pressures, which diffuse extremely cold air masses causing low temperatures and long frost periods in the higher latitudes of the Planet, mostly above 60°N and S. The long cold periods alternate with brief and cool summer seasons which can be longer and warmer in the case of the boreal forest. As a general rule, the values of Tp units rarely exceed 1000. In the case of the tundra, summers are practically non-existent since the temperature does not exceed 10°C on average in the warmest month, being also short but mild in the case of the taiga. The differences between winter and summer are extreme, with very long daylight in the central days of summer, which lasts up to 24 hours on the corresponding days in the latitudes above those of the Arctic and Antarctic Polar Circles, while in winter the situation is the inverse. There is a great difference between the seasons regarding the solar radiation received.

The vascular vegetation of this domain presents the entire syndrome of adaptations to low temperatures. Its origin is quite recent (Brochmann and Brysting 2008) since it is estimated that it was forged throughout the Pleistocene (Tallis 1991) in favor of the general cooling of the Planet and the elevation of the current mountain ranges. This vegetation probably differentiated in the adjacent mountainous areas and expanded through the polar regions in the warm stages, in a repeated cycle in line with the glacial-interglacial cycles. For this reason, the vegetation of the tundras and boreal forests have clear homologies with those of the alpine (cryoro) and subalpine (oro) levels of the mountains, particularly of the NH areas.

This is the only ecozone of the domain and it can be split into the tundra and the boreal forest biomes. They both present clear relationships with the high elevations of the temperate and tropical mountains, which are also under the dominance of the low temperatures and are considered a part of this ecozone and its corresponding biomes.

1. Biome of the tundra.

The term derives from the Russian тундра, which is applied to a plain area without trees, because it is a biome devoid of trees and constituted by a continuous, dense and evergreen vegetation layer formed by a combination of bushes, herbaceous vascular plants (often grasses and sedges), bryophytes and lichens; the low temperatures prevent the development of trees. It is the biome with the greatest adaptation to cold climate, both in latitude and altitude. Temperatures remain above freezing for only 2 to 6 months, during which the probability of frost does not disappear. The average temperature of the warmest month does not generally exceed 10°C. Ocean influences determine the span of temperature seasonality, being much higher in continental regions. In addition to the Arctic and Antarctic tundra, the cryoro (alpine) belts of the world’s mountains above the forest level are considered to have tundral vegetation due to similarly low temperatures and the morphologic analogies with them. The upper limit of the polar tundra is marked at 380 units of positive temperature (Tp) and 450 in the case of tropical and Mediterranean alpine tundras.

1.a. Polar (Arctic and Antarctic) tundra.

This extends through the territories closest to the poles above a border that oscillates between 65° and 70° (exceptionally it can go down to 56°) of latitude, depending on the continentality and the marine currents, following more or less the isotherm of 10°C of the month of July (January in the SH). It spans vast peri-arctic territories in the northernmost fringes of North America (Alaska, Canada, and Greenland) and Eurasia. In the SH there are few emerged lands in those latitudes but it is recognized in the extreme north of the Antarctic peninsula and in the small islands that surround that continent.

Due to the low temperatures and its recent release from the ice cover since the last glacial maximum, the soils of the tundra are poorly developed. In polar tundras the characteristic soil is permafrost, which freezes completely in the long winter and thaws only on its surface (mollisol) in the short summer, always leaving a permanently frozen layer of soil below 25 to 30 cm, which can reach a remarkable depth (pergelisol). Permafrost is one of the elements that limit the growth of trees by preventing the development of their root system. In the domain of the tundra there are the so-called polygonal soils. These are formations of geometric cellular figures of different sizes, with circular to hexagonal contours, formed by the accumulation of larger particles displaced by the freeze-thaw cycle.

In Arctic tundras rainfall is low, of the order of 200 mm per year or even less, and is mostly in the form of snow, so that a layer of snow forms lasting all winter. With the spring thaw this ice melts and in the short summer there is abundant availability of liquid water. The wind is responsible for an uneven distribution of the snow on the surface, so that it accumulates in the hollows and is swept from the ridges. This is of great importance in the distribution of the mosaic of tundral communities, as the most protected areas will be in the troughs and the most exposed at elevation.

Because of the extreme specialization required to survive in such an extreme habitat, tundras are generally species poor. The Arctic tundras support approximately 900 species while the cryoro (alpine) tundras are more diverse as they have numerous endemics typical of the mountains. Woody species of the genera Arctostaphylos, Betula (B. nana), Cassiope, Dryas, Empetrum, Ledum, Salix, Silene acaulis, Vaccinium etc., herbs of the genera Carex, Deschampsia, Saxifraga (S. oppositifolia) or Silene are frequent in the Arctic tundras. Non vascular plants are abundant such as the bryophytes Hylocomnium splendens and Polytrichum juniperinum or fruit lichens such as Cetraria nivalis, Cladonia gacilis or Cladina mitis (Klein 1970; Bliss et al. 1973).

In Antarctica, tundral vegetation has a modest representation in the Antarctic Peninsula area with only two vascular species: Colobanthus quitensis and Deschampsia antarctica (Smith and Poncet 1987), and a much larger number of lichens and briophytes.

1.b. Tundras of the temperate cryoro (alpine) belt.

In the tundras of the cryoro (alpine) belt of the mesic (temperate) zone, the thermic and luminic seasonality is lower than in the polar regions, reaching to nill in the tropical latitudes. In contrast, there is a strong daily temparature oscillation (diurnality) and a strong incidence of wind, with its intense, abrasive and drying effect upon the areas not sheltered by the snow cover. As a result, the cryoro tundras have a less severe winter and in general receive higher rainfall due to the orographic precipitations effect. Permafrost is thinner in the cryoro tundras, dissapearing in the mid latitudes where permanently iced soils are lacking. In the cryoro belt tundras, periglacial phenomena associated with movements caused by the daily cycle of melting-freezing of soil water, such as cryoturbation and solifluction, are frequent, as well as the fragmentation of rocks due to freezing of water: gelifraction (Körner 1995).

In the alpine (cryoro) tundras, rainfall can be very variable oscillating between low and very high values, depending on the climatic regime of the mountain system in question and the accumulation of snow as redistributed according to relief by the strong winds.

As the cryoro tundras are at higher elevations as latitude decreases, uv radiation increases, favouring the differentiation of new species and endemics. Thus, cryoro tundras have a particular flora resulting from the evolution of the low elevation local floras drawn by the adaptation to high mountain environments created during mountain uplift. This is particularly relevant in the mid and lower latitudes, such as the Mediterranean climatic area. In the higher latitude mountains, there is a strong participation of polar tundra flora which merge with the local mountain flora, due to migrations that took place in the Pleistocene ice ages, when the alpine tundras contacted with the polar tundra in the mid elevation mountains of the high latitudes, allowing floristic exchange between both elements (Comes and Kadereit 2003). In these cryoro tundras of the temperate latitudes, the herbaceous element increases to become dominant with the addition of representatives of Festuca, Carex, Oxytropis and others.

In the SH there are fewer mountains bearing cryoro tundras in temperate latitudes. We can mention the Southern Andes, above the tree line, where there is vegetation of dwarf scrubs and herbs, with species such as Gaultheria mucronata, Gunnera magellanica, Hamadryas magellanica, Marsippospermum grandiflorum, Oreopolus glacilis, etc. (Galán de Mera 2005). In areas of the highest mountains of SE Australia there is also alpine vegetation above the tree limit, which develops cushion-shaped plant communities with species such as Baeckea gunniana, Ewartia meredithae, Podocarpus lawrencei or Pterygopappus lawrencii and some others (Beadle 1981). New Zealand has an important representation of subalpine and alpine vegetation as the tree limit lies at quite low elevations, between 900 and 1600 m asl. Above that, a subalpine scrub vegetation develops in which Coprosma cheesemanii, Dracophyllum uniflorum, Gaultheria crassa, Podocarpus nivalis, etc. thrive. Above the shrub belt, alpine grasslands occur in which Chionochloa species dominate, with C. crassiuscula, C. oreophylla, Poa colensoi, etc. The grassland is dotted by cushion plants such as Haastia pulvinaris or Raoulia eximia, and others such as Aciphylla simplex or Celmisia viscosa (Wardle 1991).

1.c. Oro-Cryoro Tundras of the tropical mountains.

The tropical mountains show a high uniqueness because their floristic lineage is different from that of the cryoro tundras of the extratropical mountains (Rundel et al. 1994) which are more closely related to the polar tundras, particularly in the NH. Above the forest limit, in the tropical mountains there is a scrub floor or level formed by Ericaceae (Erica, Rhododendron, Vaccinium), Protea, Kniphofia, Gaultheria, Ribes or Hypericum. In South America, this level is occupied by a short woodland-shrubland, basically formed by species of Gynoxys (Asteraceae), Polylepis (Rosaceae), Clusia, Gaultheria, Escallonia and Podocarpus. It forms the upper part of the tropical mountain forest, constrained by the genuine tropical oro-cryoro tundra-like formations of the Andes: the puna or the páramo. In the northern Andes (Venezuela, Colombia, Ecuador), where rainfalls occur throughout the year, it develops the páramo, while in the central Andes (Perú, Bolivia, Northern Chile and Northern Argentina), where the rainfall annual regime has a dry season, there is the puna. In the Andean páramo, also recognized in the high mountains of Central America, there are plant species of the genera Coespeletia, Espeletia, Espeletiopsis, Hypericum or Puya (Rangel et al. 1997). In Africa the homologous ecosystem carries Dendrosenecio, Helichrysum (Asteraceae) and Lobelia (Campanulaceae), while in Indonesia Anaphalis appears and in New Guinea Leucopogon suaveolens. All of them have a particular biotype formed by a stem that emerges up to half a meter, at the apex of which the leaves crowd, forming a rosette that protects the growth bud from the sudden and extreme cold that occurs in the tropical mountains at these altitudes. Separation from the floor can be a strategy to mitigate the low temperatures by covering the buds with the leaves and to separate them from a permanently soaked soil. In the puna, grass pastures (pajonales) predominate, with genera such as Agrostis, Deyeuxia (D. vicunarum), Festuca (F. dolichophylla, F. ortophylla), Poa or Stipa (S. ichu). These are Holoarctic lineages (genera) that testify to their ancestry as migrants from North America who traversed the dorsal mountains on the South American continent and reached the southern latitudes far south of the equator (Navarro-Sánchez and Molina-Abril 2020). These grasslands occupy large areas subjected to cryoturbation by the daily freeze-thaw cycle and alternate with abundant peaty fens (bofedales). In New Guinea the cryoro tundras are represented by grasslands with Deschampsia klossii, Festuca papuana, Gleichenia vulcanica, Papuapteris linearis and Poa callosa (Paijmans 1976; Gressitt 1982; Mangen 1993).

2. Biome of the boreal and austral forest

This biome is often known by the name of taiga, from Russian тайгá, being a biome dominated by conifer forests, mostly perennial but which also include some deciduous broadleaved trees such as birches, aspen and willows. It is subjected to a shorter freezing period than the tundra, having a frost-free season of a certain duration which allows the soils to thaw at least partially and permit trees to establish. Temperatures reach very low values during winter, often lower than in the tundra, due to the high continentality of some of the areas occupied by this biome (Walter 1979). The Boreal forest biome is mostly spread in the NH, having a modest representation in the SH (southern Argentina; Tierra del Fuego), because there are very small areas in the latitude range of this biome. Tropical mountains also lack this biome as they have a much reduced temperature seasonality, and do not have cold winters. In the mountains of the temperate areas of the northern hemisphere, this biome can be recognized in the oro (subalpine) belt, between the cryoro tundra and the supra (montane) in which the biomes 3, 4 or 5 occur.

The typical zonal soil of the taiga is the podzol, which is created by the acid organic matter (mor) produced by the conifers and ericoids which grow under these climatic conditions and on acidic siliceous bedrock. On other substrates, such as volcanic rocks, andosolic podzols will be found.

2.a. Lowland boreal taiga.

The boreal taiga forest is predominantly a conifer forest, with some broadleaved trees, ericoid shrubs and an herb layer with sedges and grasses as well as bryophytes and lichens. Mycorrization is widespread due to the soil poverty. The boreal taiga develops in areas of higher thermic conditions and in lower latitudes than the Arctic tundra. It expands uninterruptedly across the NH lands both in Eurasia (Fennoscandia and northern Russia including large areas in Siberia) and North America, mostly in Alaska and Canada, in latudes oscillating between 42° and 72°N. The climate is cold with a high temperature seasonality having long and cold winters in which temperatures reach extreme negative values, particularly in some areas as Eastern Siberia (Yakutia); there are at least 120 days where temperatures are < 10°C. On the other hand, the short summers can have 30 to 120 days in which temperatures are above 10°C, and with some days of remarkable length (Grishin 1995). The plant growth period oscillates here between 130 to 150 days. Precipitations are usually higher than in the Arctic tundra, oscillating between 500 mm and over 1000 mm in some coastal areas, occurring mostly in summer. This rainfall concentration in the warm season, added to the snowmelt of the springtime, determines that there is no drought in the summer season (Pfadenhauer and Klötzli 2014).

In north Europe the most frequent boreal forests conifers are Larix decidua, Picea abies and Pinus sylvestris, being accompanied by the broadleaved Betula pubescens and Populus tremula. In east Siberia Abies sibirica, Larix gmelinii, L. sibirica, Picea obovata, Pinus pumila or P. sibirica are found, often with the broadleaved trees Betula ermanii, B. dahurica and B. platyphylla. South of the taiga, in some areas of Europe and East Asia conifers mix with deciduous temperate tree species resulting in a transtional forest zone called hemiboreal (Hytteborn et al. 2005).

The boreal forest of northwestern America is formed by conifers such as Larix laricina, Picea glauca, P. mariana and Pinus banksiana, while in the northeast Abies balsamea, Betula occidentalis, B. papyrifera, Populus balsamifera and P. tremuliodes form the broadleaved element.

The understorey flora is formed by species of the genera Arctostaphylos, Empetrum, Ledum, Rhododendron, Vaccinium, Pyrola, Linnaea, Lycopodium, Trientalis and several orchids such as Corallorhiza trifida. Bryophytes and lichens are abundant (Weber and van Cleve 2005).

2.b. Forests and shrublands of the temperate zone oro (subalpine) belt.

In the oro (subalpine) belt of the mountains of the temperate geographic zone, below the cryoro (alpine) level, this biome can be recognized as a mountain variant or subbiome dominated by conifers (Tallis 1991) and in the aridiestival mountains of Mediterranean character by a scrub of cushion shaped shrubs. They experience a lower seasonality as they are at lower latitudes, but at higher elevations, low temperatures are comparable to those of the Boreal taiga. Concerning the relationship of this subbiome with the lowland taiga, there is a parallel situation to that of the polar and cryoro tundras.

For the conifer forests of the European mountains, we can mention Larix europea, Picea abies, Pinus cembra, P. mugo and P. uncinata for the temperate pluviestival regions, while in Mediterranean latitudes Pinus nigra subsp. salzmanii and P. sylvestris (several varieties), as well as Cupressus sempervirens, Juniperus hemisphaerica and J. sabina, are typical (Timbal et al. 2005). In the north American mountains, the Rocky Mountains support Abies lasiocarpa, Larix occidentalis, Picea engelmanii, Pinus albicaulis, P. contorta, P. ponderosa, Pseudotsuga menziesii, etc., while in the Appalachians Tsuga canadensis and Pinus strobus are frequent (Powers et al. 2005). In East Asia the subalpine forests have Abies mariesii, A. veitchii, Picea jezoensis, Pinus koraiensis, Tsuga diversifolia, etc. (Nakamura and Krestov 2005).

In the mountains of the temperate aridiestival areas (Mediterranean), oro (subalpine) levels are often occupied by a thorny cushion shaped shrubland in which several spiny legume species of Astragalus, Echinospartum, Erinacea and Genista dominate. In some territories, this shrubland is dotted with scattered populations of pines, cypresses or junipers.

In southern Tierra del Fuego, a small area is subjected to a climate corresponding to that of the Boreal forest biome – in a very oceanic variant – where the vegetation is dominated by broadleaved Nothofagus species (Magellanic forest) combined with dwarf conifers of the genera Araucaria, Austrocedrus, Fitzroya, Pilgerodendron or Podocarpus, in a mosaic with extensive moss-mire areas. This unit has been named the Magellanic Antiboreal Evergreen Forest (Luebert and Pliscoff 2022). This modest area, in spite of being a lowland territory, fits climatically better in the 2.b subbiome than in 2.a due to its markedly oceanic character.

The temperate zone of the earth extends between the high latitude zone dominated by cold polar air masses and the intertropical zone where solar radiation is maximum. In it, the thermal seasons are clearly distinguished between a hot summer and a colder winter, which coincide with the contribution of major-minor radiation throughout the year. This seasonality diminishes in the vicinity of the tropics (subtropical areas), particularly in terms of the rigors of winter. Within this temperate zone, the climatic regime is governed by the influences of the Polar Front, the Subtropical Highs and the Trade Winds depending on the regions. All these regimes are subject to seasonal latitudinal displacements, so that in the winters of the NH there is a displacement of the whole set towards the south, and in summers it is the other way around. This temperate zone is subject to an extreme diversity of climatic situations, particularly in the NH, where there is a large extension of landmass compared to the SH. The northern land masses (Eurasia, North America, North Africa) cover a substantial part of the total area and are traversed by many mountain ranges permitting the effect of continentality to play a significant role in the central regions of these continents. We include in this domain the areas that register an Io index above 1 and a positive temperature Tp ranging between 1000 and 2000 units approximately, where the vegetation can develop up to a complete coverage of the ground surface, leaving the areas below that level in the desertic area of the arid climates. In the tropical latitudes, similarly to the cryocratic domain, the mountains replicate the mesocratic conditions above 1500 m of altitude due to the decrease in temperatures (meso-supra levels). For that reason, the ecosystems that inhabit areas above that altitude can be included in this domain since the decrease in temperatures is accompanied by the existence of many extratropical lineages, both of Holarctic and Gondwanan origin. The vegetation consists of a variety of forests, groves, bushes or grasslands, depending on regional conditions.

The summer season is critical in this domain because is the period in which plants can grow and reproduce. There is a phenological adjustment to seasonality of the entire ecosystem. Thus, the availability of moisture permits intense development and growth providing favorable conditions for high biomass production. In this category could also be included temperate rain forests as have been mentioned in the literature particularly for the Pacific coast of the Americas (Alaback 1991; Pew and Larsen 2001; Grubb et al. 2013). To separate this ecozone from the temperate aridiestival, the summer should have less than two months in which p < 2t, i.e, only one month with precipitations lower than double the temperatures.

3. Biome of the temperate deciduous forest.

Rainy summers in combination with a clear thermic seasonality, with well differentiated summer and winter seasons, allow the dominance of deciduous woody plants which shed their leaves in the severely cold season (criodeciduous).

3.a. Temperate deciduous forests.

This represents the only subbiome of this biome. The climatic conditions for this biome-subbiome are well represented in the main three areas of the NH continents: East Asia, East North America and Western Eurasia. In the literature, this biome-subbiome has been also called the nemoral forest (Walter 1977, 1979, 1985; Breckle 2002). In the SH there are much smaller areas occupied by this biome-subbiome as the appropriate climatic conditions are less frequent (Loidi and Marcenò 2022).

The eastern North American region is favored by the rainfall linked to the maritime Atlantic air masses brought mostly by the northern Trade Winds. The vast area covered by this biome in North America spans between the Canadian areas of the St. Lawrence river and the Great lakes to the southern Appalachians, and from the east coast to the middle Mississippi basin, between 32° and 45°N latitude and 70° to 98°W longitude (Greller 1988). Some of the most representative species are Acer sachharum, Castanea dentata, Carya ssp., Fagus grandifolia, Liriodendron tulipifera, Liquidambar styraciflua, Querus alba, Q. rubra and many others.

In temperate East Asia, this biome is widespread form northeastern China to the southern sectors of the Russian Far East, including most of both the Korean peninsula and the Japanese archipelago (Nakamura et al. 2007) between the latitudes 30° to 50°N and longitudes 123° and 103°E. The maritime air masses that favor this biome in that region come from the Pacific being drawn by the summer monsoon dominant in East Asia. The richness in species number of trees in this region is much higher than in the other two, and this is explained by the particular conditions that existed here in the Pleistocenic glaciations which permitted the survival of many taxa (Davis 1983). Some of the most relevant species are Acer mono, Davidia involucrata, Diospyros kaki, Fagus crenata, F. japonica, Pawlownia tomentosa, Quercus crispula, Q. mongolica, Q. serrata, etc.

In Western Eurasia this biome covers a large area encompassing most of Europe, a strip in central-western Siberia, the Caucasus piedmonts and the eastern and southern coastal fringe around the Black Sea (Euxinian) and south Caspian Sea (Hyrcanian), a vast area limited by the boreal forest in the north and the Mediterranean and steppic regions in the south (Jahn 1991; Loidi et al. 2021). The summer rains are provided by the westerlies coming from the Atlantic, which drag a succession of lows with fronts, which form part of the Polar Front system. Particularly in Euxinian and Hyrcanian areas, local rains are due to the northern and northwestern winds which are reloaded with moisture when they cross adjacent water bodies. The most relevant species are Acer campestre, Fagus orientalis, F. sylvatica, Fraxinus excelsior, Quercus petraea, Q. pubescens, Q. robur, Tilia cordata, T. platyphyllos, Ulmus glabra, U. minor, etc. Within this biome, as result of human induced transformations, there are abundant several types of evergreen grasslands (hay and grazing meadows), spiny shrublands and the heathlands in the western part.

In the SH there are only a few scarce relictual representations of this biome. This is likely due to the lower continentality of the SH emerged lands in comparison to those of the north (Box and Fujiwara 2015). The only representation of this biome in the SH is a narrow strip in the southern Andes at altitudes between 900 and 1600 m asl in which several species of deciduous Nothofagus constitute forests: N. antarctica, N. alessandri, N. alpina, N. glauca, N. macrocarpa, N. obliqua and N. pumilio (Luebert and Pliscoff 2022), and a small area at the highest elevations of western Tasmania in Australia where it occurs with the deciduous Nothofagus gunni. Its small extent prevent it from being represented in the accompanying map.

4. Biome of the temperate pluviestival evergreen forest, shrublands and grasslands.

This biome encompass highly diverse forms of dominant vegetation formed by broadleaved evergreen species as well as conifers and grasslands. It is often formed by perennial evergreen forests, with leaves of varied texture and size, often coriaceous with shiny surfaces, called lauroid (laurisilvae), but also conifers of various taxa. This biome is a complex and diverse one as it is represented in a variety of territories across the temperate and tropical latitudes of the world in both hemispheres.

In the temperate zone, it occupies the space between the tropics and the nemoral deciduous forest areas, under high to mid oceanic conditions entailing moderate winters and high summer precipitations. In the tropics it is also represented at the mid-elevations (meso-supra) of the mountain systems where temperatures are colder than in the lowlands. Due to orographic precipitations and frequent fogs, tropical mountains support this biome under several variants (Afromontane, American Monteverde, etc.). In spite of such a diversity and broad geographic distribution, there are sufficient features to recognize a certain unity in physiognomy and origin between them. There are three subbiomes to be distinguished within it. In some coastal areas of the western side of the continents, particularly in Chile and in northwestern North America (British Coumbia and Alaska), local conditions related to the persistence of westerlies allow the subsistence of this biome in lowlands or low elevation ranges in latitudes up to 55 and 60° respectively. In Western Europe, it may be expected to find such a biome in the comparable latitudes of some coastal areas, but it is not present. This absence may be due to the extinctions that happened during the Pleistocenic ice ages.

4.a. Lauroid subtropical evergreen forest of the lowlands.

This subbiome can be found in a zonal position in the lowlands of the coastal or ocean influenced areas of the extratropical continental land masses. In the eastern side of the continents, the summer low pressure in mid latitudes circulates oceanic air masses towards the continent in a monsoonal regime causing abundant summer rains coincident with high temperatures. Winters are relatively dry and cold as the situation is inverse and land air dominates causing relatively low temperatures and scarce precipitation. Nevertheless, tempertures remain above 5°C in the coldest month. These formations have received several names such as subtropischer Lobeerwald in German, temperate broad-leaved evergreen forests in English (Pfadenhauer and Klötzli 2014) or Lucidophyll Forest (Kira 1977).

In the Northern Hemisphere:

Eastern Asia is the most extensive area occupied by this subbiome, between 25° and 35°N latitude. Occupies the major part of Eastern continental China and the island of Formosa (Taiwan), penetrating to the west over the ranges connecting with the Tibetan Plateau and the arid regions of Central Asia, being limited by the 99° and 123°E longitude. Northwards, it reaches the southern areas of the Korean peninsula and the Japanese archipelago, in the coastal districts of the islands of Kyushu, Shikoku and southern Honsu (Yong-Chang and Liang-Jun 2016). In the south, it is constrained by the tropical rain forest (biome 9) in a transitional fringe north of Guangzhou (Canton) and connects to the west with the long narrow zone which occupies the southern humid mid-elevation slopes of the Himalayas (Sekar and Srivastava 2010).

This region is subjected to a monsoonal regime with warm humid air masses comning in summer from the east (Pacific, Seas of China) and to winter invasions of cold air from the center of the Asian Continent originated by the strong Siberian high, which are responsible for brief cold episodes that can cause snow events.

The forests in this region have been dramatically reduced by human pressure, but do survive in some areas and support a high number of tree species of families such as Fagaceae (Castanopsis, Cyclobalanpsis, Lithocarpus, Quercus), Myrsinaceae (Ardisia), Lauraceae (Cinnamomum, Machilus, Persea), Magnoliaceae (Magnolia, Michelia), Symplocaceae (Symplocos), Theaceae (Camellia) etc. In spite of anthropogenic reduction, these forests ere substantially more diverse in their tree flora compared with the eastern North American and Western Eurasian homologous portion (Latham and Ricklefs 1993). This richness is a result of the more mesic conditions that were dominant compared with Europe or North America during Pleistocenic ice ages. Gymnosperms are also numerous, with representatives of Cupressaceae (Fokienia hodginsii, Cryptomeria japonica, Cunninghamia lanceolata, Metasequoia glyptostroboides), Pinaceae (Pseudolarix amabilis), Podocarpaceae (Podocarpus macrophyllus) or Taxaceae (Cephalotaxus harringtonia, C. fortunei, Pseudotaxus chienii). Some of these are old taxa, particularly Ginkgo biloba, and additionally bamboo-type grasses of the genus Phyllostachys are also frequent.

Southeastern North America is another of the regions where this subbiome is zonal, occupying a vast area in the USA between SW Texas and North Carolina, encompassing southern Louisiana, Mississippi, Alabama, Georgia, northern Florida and South Carolina. This area is under the influence of the Gulf Stream which flows across the Gulf of Mexico and the Atlantic coast and is associated with summer precipitations brought by the eastern trade winds. The forests of this region are substantially poorer than the homologous forests of East Asia, likely due to the higher intensity of the Pleistocenic extinctions that took place in North America. This relative poverty can be also related to the lower precipitations occurring in this region (Kira 1991). A number of evergreen species, such as Magnolia grandiflora, Myrica cerifera, Persea borbonia, Quercus hemisphaerica, Q. virginiana or the palm Sabal palmetto, combine with deciduous species of wider distribution such as Fagus grandifolia, Liquidambar styraciflua or Liriodendron tulipifera (Greller 1988). An important part of this territory is currently covered by secondary pine forests (Pinus elliotii, P. palustris, P. taeda), which occupy the sandy soils of the coastal plain (Christensen 1988).

In the Southern Hemisphere:

The South African southeast, also called “Southern African Afrotemperate Forest” (Mucina et al. 2021), is under the influence of the humid trade winds from the Indian Ocean and the warm currents of Agulhas and Mozambique, having thus a temperate humid oceanic climate. It spans from Western Cape along the southern extreme of the continent across Eastern Cape and KwaZulu-Natal, being the only zonal representation of this biome in the African continent. It occupies the southern and western slopes of the Great Escarpement (including Drakensberg). Some of the species are Afrocarpus falcatus, Cola greenwayi, Cunonia capensis, Curtisia dentata, Faurea macnaughtonii, Ilex mitis, Leucosidea sericea, Nuxia floribunda, Ocotea bullata, Olea capensis subsp. macrocarpa, Olinia ventosa, Platylophus trifoliatus, Podocarpus latifolius, Pterocelastrus tricuspidatus, Rapanea melanophloeos and Widdringtonia schwarzii. Among the tree-ferns we can mention Alsophila (Cyathea) capensis and A. dregei.

The region of Southeastern Brazil and the Pampas expands between 23° and 39°S latitude, on the Atlantic side of the South American continent encompassing the adjacent countries of the Río de la Plata estuary including southeastern Brazil (Paraná, Santa Catarina, Rio Grande do Sul), Uruguay and Northeastern Argentina. It is the SH counterpart of SE USA. The southern part of this territory is occupied by the Pampas, from Rio Grande do Sul, Uruguay to NE Argentina. It is a vast area which is currently almost entirely occupied by grasslands and crop fields; the arboreal vegetation is thus restricted to marginal positions. The anthropic origin of these grasslands has been previously discussed, as this area experiences completely different climatic conditions to those of the steppes. The overwhelming dominance of the grasslands indicate that the environmental conditions are optimal for them (Cabrera 1971; Oyarzábal et al. 2018); however, it is evident that the human influence with the pressure of grazers has favored them by reducing the cover of woody vegetation. In any case, it is likely that the influence of large grazing mammals present before humans appeared in this region led to the development of these Pampean grasslands.

The remnants of forest in these areas are formed by some species of regional distribution such as Acacia bonariensis, Cassia corymbosa, Baccharis articulata, Colletia paradoxa, Ocotea acutifolia, Sambucus australis, Schinus longifolius, Scutia buxifolia and particularly the ombú or Phytolacca dioica and the palm Butia yatay. Other species of broader distribution are Acacia caven, Aloysia gratissima, Baccharis tandilensis, Celtis ehrenbergiana, Jodina rhombifolia, Prosopis alba, Zanthoxylum fagara, Margyricarpus pinnatus etc.

The grasslands are dominated by grasses of the genera Andropogon, Aristida, Briza, Bromus, Eragrostis, Melica, Panicum, Paspalum, Piptochaetium, Poa and Stipa, which share their dominance with other herbaceous and woody plants of other families such as Adesmia, Alicropsis, Aster, Baccharis, Berroa, Chaptalia, Heimia, Margyricarpus, Oxalis, Vicia, etc.

This region is continued with the mid elevation areas of Paraná and Santa Catarina in south Brazil, where Araucaria angustifolia and Podocarpus lambertii occur with Dicksonia sellowiana in the understorey, being included in this subbiome due to the subtropical climatic conditions dominant in these territories (Rizzini 1997).

In Southeastern Australia, this subbiome is represented in the coastal strip of land east of the Great Dividing Range, which crosses the eastern side of the continent and separates the arid regions of the interior from the moist coastal areas that are affected by the humid air masses from the Pacific Ocean. The southern stretch of this strip, which extends from the Rockhampton district to the southeastern tip of the continent and the island of Tasmania, is covered by this unit. The climate is humid all the year round and winters are mild. A high representation of species of the genera Acacia and Eucalyptus is present, some of them of great size such as Eucalyptus globulus, E. obliqua and E. regnans, reaching 100 m in stature. Other taxa are Atherosperma moschatum, Doryphora sassafras, Ceratopetalum apetalum, and conifers such as Athrotaxis laxifolia, Phyllocladus aspleniifolius and the endemic Wollemia nobilis, as well as several species of Callitris, Macrozamia and Podocarpus. Some Antarctic remnants are present such as Nothofagus cunninghamii, or the tree ferns Cyathea australis and Balantium (Dicksonia) antarcticum (Beadle 1981).

The New Zealand archipelago is located in a position where abundant rainfalls occur all year round, particularly on the western side of the islands. This entails a high oceanity which permits the existence of evergreen temperate forests in the whole archipelago lowlands and midlands. The lower levels are occupied by a conifer/broad-leaved forest: among the conifers we can mention Agathis australis, Dacrycarpus dacrydioides, Dacrydium cupressinum, Libocedrus bidwillii, Phyllocladus trichomanoides, Podocarpus totara, Prumnopytis ferruginea and others, while among the broadleaved there are Laurelia nova-zelandiae, Metrosideros umbellata, Weinmannia racemosa, W. silvicola and the palm Rhopalostylis sapida. Several species of tree-ferns of the genera Cyathea and Dicksonia occur in the understory. The evergreen southern beeches (Nothofagus) occupy higher elevations than the conifer/broad-leaved forest with species such as Nothofagus fusca, N. menziesii, N. truncata or N. solandri (Wardle 1991).

Southern Chile. In the temperate southern part of Chile, around the 38°S, the climate shifts from aridiestival to pluviestival under the influence of the westerlies of the South Pacific. This region extends to Tierra del Fuego at 54°S, because conditions remain extremely oceanic all along the strip. Only in the southwesternmost strip of the area will occur boreal conditions (called antiboreal by local authors such as Luebert and Pliscoff 2022). The forests in this area (Valdivian and Magellanic) are evergreen with many lauroid species such as Drimys winteri, Eucryphia cordifolia, Gevuina avellana, Laurelia sempervirens, Luma apiculata, Nothofagus betulodes, N. dombeyi, Persea lingue and bamboos such as Chusquea quila (Grau 1992). In the cooler areas of the montane levels, conifers such as Araucaria araucana, Fitzroya cupressoides and Pilgerodendron uviferum become dominant. As mentioned in biome 3, at mid elevations there is a level of deciduous southern beech forests occupying a narrow strip in the western slopes of the southern Andes (Luebert and Pliscoff 2022).

4.b. Conifer coastal forests.

This subbiome is also zonal and differs from the previous one in that it presents a high dominance of conifers although there are always a significant presence of evergreen hardwoods. It can be recognized in northwestern North America, where it is known as the Pacific Northwest Forest (Franklin 1988), and occupies a 60–120 km width strip along the coast from central-northern California to southern Alaska, up to 60°N latitude. Tree species, either conifers or broadleaved, are adapted to a mild, moist, maritime climate with high oceanic influence in the form of frequent fogs. In the southern part of its area (mostly in California and Oregon), a clear summer drought is observed, as a result of the influence of the North Pacific High, but this drought is offset by the frequent summer fogs coming from the Pacific Ocean. Conifers, many of them of large size, combine with evergreen and deciduous hardwoods. Some important tree and shrub species are Abies procera, Arbutus menziesii, Chamaecyparis lawsoniana, Chrysolepis chrysophylla, Libocedrus decurrens, Notholithocarpus densiflorus, Pinus lambertiana, P. radiata, Pseudotsuga menziesii var. menziesii, Quercus chrysolepis, Q. garryana, Q. sadleriana, Rhododendron macrophyllum, Sequoia sempervirens, Umbellularia californica etc. Other species reach higher latitudes along the Pacific coast such as Abies amabilis, Taxus brevifolia, Thuja plicata or Tsuga heterophylla (Powers and Fiske 2005).

4.c. Tropical and subtropical montane cloud lauroid and conifer evergreen forest and shrublands.

This subbiome can be recognized in the mid elevations of the tropical and subtropical mountains above 1000–1500 m (meso, and supra, humid and hyperhumid). They are evergreen forests formed by broadleaved trees and conifers which receive high amounts of moisture in the form of rain and fog. Conifers and broadleaved trees are both almost always present but they alternate in dominance, mostly depending on the historic evolution of the area. In any case, the broadleaved trees often have coriaceous and shiny leaves (lauroid) while the conifers often bear larger and softer leaves than in other biomes.

The Macaronesian archipelagos: Azores, Madeira and Canaries. Fragments of this subbiome (monteverde) are found in the north Atlantic volcanic archipelagos (Macaronesia), west to the Eurasian continent, being dispersed across most of its islands. Due to the influence of the Azores High, this area is overall subjected to a climatic dynamism in which summer is the driest season. This can be locally offset by orogaphic precipitations and, above all, through the cryptoprecipitations that take place due to the condensation of the fogs accumulating in the mid elevations of the windward slopes facing the trade winds (sea of clouds). Rain increases northwards and westwards so that the Azores are the most humid and the Canaries the driest, being the monteverde generally in the former and restricted to the north facing slopes in the latter. This monteverde or laurisilva (Fernández-Palacios et al. 2017) has a certain relic character as it is considered the remnant of the Tertiary lauroid forests which covered the adjacent continental regions Eurasia and North Africa.

Trees are all perennial, with Apollonias barbujana, Clethra arborea, Heberdenia bahamensis, Ilex canariensis, I. perado, Myrica faya, Laurus azorica, L. novocanariensis, Ocotea foetens, Persea indica, Picconia excelsa, Prunus lusitanica and Visnea mocanera being the most relevant among others. Some of the noteworthy ferns are Culcita macrocarpa, Davallia canariensis and Woodwardia radicans.

The southern slopes of the Himalayas are covered by this subbiome at mid-elevations between 1000 m and 2500 m, above the tropical foothills that precedes the Indo-Gangetic plain covered with biomes 8 and 9, and below the subalpine conifer forest of 2.b thriving in the upper altitudes. This territory forms a long and narrow zone that spans along the southern slopes of the Himalayas, receiving the copious rains of the summer monsoon. This strip is thought to be the original area of the tea plant (Camellia sinensis) and is characterized by rhododendron forests, of which there are several tree species. Among them, Rhododendron arboreum, the national flower of Nepal, stands out, but also can be mentioned R. barbatum, R. campanulatum or R. lepidotum (Sekar and Srivastava 2010). These forests are rich in bamboo and Araceae and cover a wide stretch of the southern slope of the mountain range, from Kashmir to the mountains of the Chinese provinces of Yunnan and Sichuan.

Afromontane forests occur at elevations above 1000 m asl, where rainfall usually exceeds 1200 mm and fogs are frequent throughout the year. Their distribution is discontinuous being found in the long mountain chain crossing eastern Africa (Aerts et al. 2006), with the northernmost extreme in the highlands of Yemen. It occupies large areas in the many ranges and high plateaus of Ethiopia (from which the coffee tree, Coffea arabica originates), crosses Kenia, Ruanda, Burundi, Congo and Tanzania and reaches Malawi and Zambia, to finally connect with the South African southwest (Drakensberg). In the Ecuatorial latitudes the fragments of this subbiome occupy the belt between 2000 m and 4000 m asl, benefited by the fogs that accumulate in these altitudes favored by the trade winds from the Indian Ocean. There are other African mountain areas in which this subbiome is found, such as the highlands of Angola, the mountains of Cameroon, Nigeria and even the island of Bioko in the Gulf of Guinea. Some species of this Afromontane forests are Balthasaria schliebenii, Barbeya oleoides, Chrysophyllum gorungosanum, Entandrophragma excelsum, Ficalhoa laurifolia, Hagenia abyssinica, Hypericum revolutum, Juniperus procera, Kiggelaria africana, Myrianthus holstii, Ochna holstii, Ocotea usambarensis, Olea europea subsp. cuspidata (= O. africana), Pouteria adolfi-friedericii, Prunus africana, Tabernaemontana stapfiana and Xymalos monospora.

In the Indo-Malesian region, In the mountains of Borneo, Java, Sumatra and other islands, the flora is derived from both Asian and Australian lineages of families, such as Araucariaceae, Clethraceae, Ericaceae, Fagaceae, Lauraceae, Myrtaceae, Podocarpaceae, Symplocaceae and Theaceae. They are very rich in epiphytes and forbs in the understory, with genera such as Begonia, Cyrtandra and Musa. Some species of this SW Asian region are Agathis dammara, Dacrycarpus imbricatus, Castanopsis buruana, Lithocarpus celebicus, Magnolia carsonii, Myrsine affinis, Pinus kesya, Prunus mirabilis etc. (Edwards et al. 1990; Kiatayama 1992; Buot 2002).

In the mountains of New Guinea, there are up to 14 species of Nothofagus which comprise forests from the mid elevations up to 3000 m (Robbins 1961; Paijmans 1976; Mangen 1993; Read and Hope 1996). Fagaceae and gymnosperms are frequent in these forests, with species such as Agathis spatulata, Araucaria hunsteinii, Callitris neocaledonica, Castanopsis acuminata, Dacrydium novo-guineeense, Dacrycarpus compactus, Libocedrus papuana, Lithocarpus megacarpus, Nothofagus carrii, N. flaviramea, Papuacedrus papuana, Phyllocladus hypophyllus, Podocarpus brasii, Prunus pullei etc.

The island of New Caledonia is a fragment of Gondwana which has migrated northwards, entered into the intertropical zone and currently is immersed in a tropical climate. However, the indigenous flora is largely derived from the temperate Gondwanan and retains up to five species of Nothofagus and numerous gymnosperms (Read and Hope 1996). The mountain range that runs through the island along its length, crowned by elevations of modest altitude (1500 m), receives abundant rainfall from the east and in its upper section there is a montane forest with numerous gymnosperms such as Araucaria humboldtensis, A. montana, A. laubenfesii, A. schmidii, Agathis montana, Podocarpus longifoliatus, Dacrydium lycopodioides, Libocedrus chevalieri, Retrophyllum comptonii, Acmopyle pancheri as well as other broadleaved such as Metrosideros elegans, Nothofagus aequilateralis, N. balansae and several species of Quintinia, Weinmaniia etc.

In the Indian subcontinent, these forests are recognized in the mountains of the south of the country, in the chain known as the Western Gaths, between the states of Kerala and Tamil Nadu (Greller et al. 2016). Some species are Cullenia exarillata, Dyospiros barberi, Garcinia rubro-echinata, G. travancorica, Guta travancorica, Knema attenuata, Mesua ferrea, Palaquium ellipticum, as well as the podocarp Nageia wallichiana. In the island of Ceylon (Sri Lanka), there is a significant representation of these mountain lauroid forests above 1000 m asl (Greller et al. 2016), which are under the influence of constant fogs in the massifs of the south-central part of the island. The famous Ceylon tea is grown in its territory. The arboreal rhododendron stands out, having an endemic subspecies for the island, Rhododendron arboreum subsp. zeilanicum. Additionally, Palaquium rubiginosum, Stemonoporus cordifolius and S. rigidus can be mentioned. There are also several arboreal ferns such as Cyathea hookeri, C. sledei and C. sinuate, while in the understory several Strobilanthes species are found.

In Meso and South America there are important areas with montane forests where temperatures are lower than in the lowlands, precipitations higher and fogs frequent. In Meso- and Central America there are the Mexican Sierra Madre Oriental and Occidental, as well as the Mountains of Central America. In the Mexican sierras, above 1500 m asl of elevation, precipitations increase and are concentrated in the central months of the year (summer). There thrive evergreen lauroid forests, in particular the locally called Mesophylous Montane Forest (Miranda and Sharp 1950), with trees such as Clethra mexicana, Fagus grandifolia subsp. mexicana, Meliosma alba, Quercus ocoteifolia, Q. xalapensis and many others. Under basically similar climatic conditions extend a major part of the encinares, or evergreen oak forests, with a high number of species of Quercus such as Q. durifolia, Q. microphylla, Q. obtusata, Q. rugosa, or Q. sideroxyla. As they occupy similar elevations and receive higher rainfall, they are assimilated to this subbiome too. The pinares, conifer forests with a high number of pine species such as Pinus montezumae or P. teocote, are subjected to a similar pattern of annual rainfall and can also be included in this subbiome (González Medrano 2004; Rzedowsky 2006). Southwards rises the Talamanca range (Costa Rica-Panamá) where Quercus copeyensis, Q. costaricensis and Ilex pallida appear, as well as other lauroid species such Drimys granadensis and Weinmannia pinnata, the tree fern Cyathea fulva and bamboos of the genus Chusquea, forming the luroid montane forest locally called Monteverde.

The Guiana Shield reaches elevations of ca. 3000 m asl in the Guiana Highlands, forming a mountainous area known as the Tepuys, which is covered by a vegetation very rich in endemics. Some frequent species are Catostemma durifolius, Pouteria rigida, Protium neglectum, etc. There are also several species of Bonnetia, such as B. roraimae, B. steyermarkii or B. tepuiensis, living above 2000 m asl (Delgado et al. 2009; Lozada et al. 2014).

In the northern Andes, in Ecuador, Colombia and Venezuela, the montane forests have species such as Aegiphila bogotensis, Cecropia telenitida, Quercus humboldtii, Saurauia ursina, Vallea stipularis, Weinmannia tomentosa, and several species of the genus Cinchona, such as C. calisaya, C. macrocarpa, C. officinalis, C. pubescens etc. (Rangel et al. 1997). The central Andean eastern slopes receive the wet air masses from the Atlantic drawn by the trade winds. At mid elevations, the cloud montane forest called Yunga or Andean Brow Forest occupies a narrow zone between 600 m and 2500 m asl between northern Peru and northern Argentina, crossing Bolivia. It occurs under extremely rainy and foggy conditions and is formed by a dense canopy up to 30 m high, which creates a humid and dark understory where a mass of shrubs, epiphytes, herbs and lianas live. Some of the trees are Alnus acuminata, Escallonia myrtilloides, E. paniculata, E. pendula, E. resinosa, Podocarpus parlatorei, Polylepis pepei or Sambucus peruviana. In the southernmost stretches appear Blepharocalyx salicifolius, Cedrela angustifolia, Cynnamomum porphyrium, Duranta serratifolia, Fuchsia boliviana, lex argentina, Juglans australis, Ocotea porphyria, Polylepis australis and Schinus gracilipes, and the bamboo Chusquea lorentziana, among others (Cabrera 1971; Oyarzábal et al. 2018).

Unlike the previous ecozone, if the summer is dry (at least 2 months) the ecosystem is subjected to double stress, that of cold in winter and drought in summer, severely limiting biomass production and inducing adaptations to drought in the traits of plants. This summer aridity is due to the fact that in that season these areas are under the influence of the subtropical highs, since they are in the vicinity of the domain of arid climates and desert biomes, in the zonal scheme, or are in rain shadow during the summer season.

5. Biome of the temperate aridiestival evergreen (Mediterranean) forests and shrublands.

This biome lives under Mediterranean climatic conditions (s.l.), and the vegetation is formed by trees, shrubs and scrub with reduced and hard leaves (Tomaselli 1995). This sclerophylly, combined with microphylly in the leaves and other photosynthetic organs, is an adaptation to survive under the arid conditions of the summer months. There are also an abundance of ephemeral therophytes which present a life cycle adapted to this type of climate.

5.a. Oceanic sclerophyllous-microphyllous evergreen forests and shrublands (Mediterranean s.str.).

The territories which are under oceanic conditions, not distant from the seashores, are those which are considered as genuine Mediterranean. There are five areas in the world which can be included in this subbiome: the Mediterranean coastal countries of Europe, Near East and North Africa, the central and southern Californian area, the central Chilean region, the fynbos in the Cape province of South Africa, and the southwestern Australian territories.

The Mediterranean Basin territories extend along the shores of the Mediteranean Sea encompassing relevant areas in southern Europe, from Portugal to Greece, northern Africa (Magrebian countries and Cyrenaica), the Middle East from Anatolia to Siria and Palestine to the Gaza Strip and all the islands within that sea. The territory is covered by sclerophyllous-microphyllous vegetation (Quézel and Barbero 1982) dominated by forests (Quercus ilex, Q. suber, Q. calliprinos, Pinus halepensis, P. pinea), shrublands (maquis, high matorral; Querus coccifera, Pistacia lentiscus, Rhamnus alaternus, Juniprus phoenicea) and scrub (garrigue, low matorral, phrygana; Rosmarinus officinalis, Thymus vulgaris, Sarcopoterium spinosum) having a particular flora with a high endemic component (Quézel 1995).

Central and southwestern California is another region where this subbiome can be recognized, having a clear summer drought that is responsible for sclerophyll vegetations types. These forests are usually formed of oak and pine species such as Quercus agrifolia, Q. chrysolepis, Q. douglasii, Q. dumosa, Q. wislizenii, Pinus sabiniana and others, and the shrublands, locally called chaparral, have a high number of species of Arctostaphylos (manzanita) and Ceanothus, as well as Adenostoma fasciculatum, Rhamnus californica and others. In this region, fire plays a relevant role in the ecosystem and the chaparral is closely associated with it. The abundance of serpentines shapes a substantial number of vegetation types. In the coastal strip, the sagebush formation with several Salvia species is also charactaristic (Barbour et al. 2007).

In South America there is a region under similar Mediterranean climatic conditions in Central Chile, between 31 and 38°S. In this area thrive evergreen sclerophyllous forests with Cryptocarya alba, Jubaea chilensis, Lithraea caustica, Peumus boldus and Quillaja saponaria, which are replaced by a shrubland (matorral) when disturbance occurs, with Adesmia microphylla, Bahia ambrosioides, Fuchsia lycioides and other species (Grau 1992).

The small Cape region in South Africa is covered with the shrubby evergreen vegetation called fynbos, which is comprised of sclerophyllous small-leaved scrubs of an extremely high variety of species, with high endemicity (Mucina and Rutherford 2006). This subbiome is adapted to extremely nutrient-poor soils and to frequent fires. With very few exceptions (Protea nitida), all the species of the fynbos are scrubby, many of them belonging to the Erica genus, as well as species of Proteaceae, Asteraceae, Rhamnaceae, Thymleaceae and Restionaceae.

Southwestern Australia is another of these “Mediterranean” regions having precipitations concentrated in the winter season and a low continentality (Beard et al 2000). The region stretches along the southern fringe of the continent, occupying the western side of its two main salients, where the main cities of Perth and Adelaide are located. Southwestern Australian flora is extremely rich, with a high number of endemics (Beard 1975; Beard et al. 2000) and forms several types of forests in which Eucalyptus species are dominant: Eucalyptus (Corymbia) calophylla (marri), E. diversicolor (jarrah), E. marginata (karri), and many others. Other outstanding plants are Anigozanthus manglesii (kangaroo paw), Banksia attenuata or Xanthorrhoea preissii (black boy). Additional to forests, there are shrub formations such as that of the kwongan (Mucina et al. 2014), rich in endemic species as Banksia formosa, Hakea victoria etc., and mallee, formed by shrubby eucalypts with many stems arising from a lignotuber (Beadle 1981).

5.b. Continental scrub and woodlands.

Related to the previous subbiome, when continentality increases due to altitude or distance from the oceans and winter temperature descends frequently below zero, sclerophyllous-microphyllous broadleaved woody plants decrease and there is an increase in conifers and deciduous tree species. Conifers belonging to Juniperus or Pinus genera form a vegetation of open woodland with a scrub covering the floor. There are several regions with this subbiome, all of them in the NH: western United States, Central Asian highlands and the Zagros range, as well as the highlands of the southern Atlas ranges in North Africa. In North America it is represented by the Pinyon-Juniper Woodland (West 1988) with relevant species such as Juniperus monosperma, J. osteosperma, Pinus edulis and P. monophylla. In the central Asiatic areas of this subbiome there are several Juniperus species comprising this unit (J. excelsa, J. polycarpos, J. seravschanica, etc.), which often combine with some broadleaved trees such as Pistacia atlantica or Prunus scoparia and with Quercus brantii particularly in the Kurdo-Zagrosian area (Zohary 1973). In central Spain there is a small area that can be attributed to this subbiome with Juniperus thurifera as one of the main species (Peinado et al. 2017). In the highlands surrounding the mid elevations of the Atlas ranges, in Morocco and Algeria, continentality increases and woodlands of Juniperus africana and Cupressus atlantica thrive (Taleb and Fennane 2019; Rivas-Martínez et al. 2020).

5c. Patagonian shrublands.

Due to the coincidence of particular geographic and orographic conditions, this subbiome is only found in the Argentinian Patagonia, on the eastern side of the southern Andean Range. It is a very unique unit from both the climatic and floristic-vegetation point of view. It is a region with pronounced summer aridity due to the rain shadow of the Andes combining with the southern westerlies coming off the Pacific. However, temperatures and precipitations are quite low, but the ombrothermic index (Io) is not so low to be considered as a true desert. This results in a cool and dry climate where summer is dry matching the aridiestival conditions in the broad sense. Due to the maritime influence, the annual temperature range is low. The vegetation is a grassy shrubland formed by species with clear drought adaptations such as spiny aphyllous or cushion dwarf shrubs. Some species are: Chuquiraga rosulata, C. avellanedae, Ephedra ochreata, Grindelia anethifolia, G. chiloensis, Malguraea tridens, Larrea ameghinoi, L. nitida, Mulinum spinosum, Nardophyllum bryoides, Nassauvia glomerulosa, Prosopis denudans, Prosopidastrum globosum, Retanilla patagonica, etc. Grasses are common, with species such as Festuca argentina, F. pallescens, Pappostipa humilis, Poa alopecurus etc. There are some species in common with the Central Chilean region, such as Lycium chilense or Mulinum spinosum, showing the climatic proximity of western Patagonia with central Chile (Cabrera 1971; León et al. 1998; Oyarzábal et al. 2018).

Low levels of precipitation throughout the year, accompanied with a high annual temperature variation range (continentality or BIO7 > 300), produce a type of vegetation often dominated by turf-grasses in which other growth forms are less abundant. The extreme cold of the winter combined with a not so low summer rainfall are the conditions of the steppes persisting in the central areas of the big continents of the NH. In North America, these occupy a vast extension in the center of the continent, where the Rocky Mountains buffer the influence of the Pacific Ocean and are separated from the east coast and the influence of the Atlantic Ocean by the wide rainy regions of the east and the Appalachians. Though some authors recognise similar biotic units in the SH like the Patagonian steppe and grasslands in South America (Luebert and Pliscoff 2022), in our scheme they are not presented at all due to the lower levels of continentality which, we believe, are crucial for their development.

6. Biome of the steppe.

The steppes form a zonal biome whose dominant vegetation is made up of grasses and grass-like plants that constitute the dominant element mixed with a large number of herbs and a mixture of chamaephytes. The main climatic characteristic of the steppe is continentality; that is, the high annual temperature range, which implies usually very cold winters. The precipitations are generally moderate or low, taking place mainly in summer; in winter they are scarce and are produced mainly in the form of snow. Usually, the summer coincides with the peak of rains, leading to no or moderate drought in the growing season. Thus, winters are dry and windy, while summers are relatively rainy except in the driest versions in which a certain summer aridity is manifested. Such a climate, a combination of a cold or very cold dry winter and a growing season with sufficient rainfall or moderately dry, cannot support the development of taller and more closed woody vegetation than that of a grassland or dwarf-shrubland (Wesche et al. 2016). These conditions are combined with the frequency of natural fires and the existence of large browsers and wild herbivores, adding difficulties to developing larger woody vegetation (Sims 1988). The soils typically belong to the chernozem type with their thick, blackish surface layers rich in organic matter.

6.a. Forest-steppe.

In Eurasia, this subbiome covers a large extent between eastern Central Europe and East Asia, occupying an intermediate spatial position between the closed forests (taiga or Temperate nemoral forests) and the treeless steppes, under transitional climatic conditions. It is formed by a typical pattern of mosaic of forest and grassland vegetation (Lavrenko 1970; Erdos et al. 2018). It is under the influence of the huge Mongolian winter anticyclone where high pressures determine low temperatures and precipitations over a vast area across the Central Eurasian area.

The most common trees in this subbiome are those present in the nearby forest areas, such as Betula pendula, B. pubescens, Pinus sylvestris, Populus tremula, Querus robur, Ulmus pumila, etc. In the eastern territories there are also Betula platyphylla, Larix sibirica and others. The grasslands are dominated by grasses such as Festuca valesiaca, Koeleria macrantha, Stipa capillata, S. pennata, S. tirsa and herbs such as Fragaria viridis and Filifolium sibiricum.

In North America, this subbiome can be roughly recognized in the tallgrass prairie that extends in a north-south strip in the eastern side of the biome as a transition to the temperate deciduous forest. Accordingly, this subbiome is moister than the following one, likely enabling it to bear a tree component. It has a scattered-tree overstory of Quercus marilandica and Q. stellata wth an understorey of Andropogon gerardii, Panicum virgatum, Schizachyrium scoparium, Sorghastrum nutans, etc. (Bailey 1998).

6.b. Grass-steppe.

When climatic conditions are more extreme, i.e. lower precipitations and colder and longer winters, the trees become scarce, or even disappear, and we have the true grass-steppe which form a grassy treeless landscape. It occupies the temperate areas subjected to higher continentality, both in North America and in Eurasia.

In central Eurasia this subbiome covers a large extent between the northern Black Sea and East Asia, in a fringe south to the forest-steppe. The southern fringe of this area usually has a drier climate and a “desert-steppe” is recognized locally. Species are numerous, many of them grasses but also other herbs. We can mention some as Agropyron cristatum, Allium strictum, Bothriochloa ischaemum, Cleistogenes squarrosa, Krascheninnikovia ceratoides, Salvia nutans, Sisymbrium polymorphum, Stipa glareosa, S. lessingiana, S. zalesskii, Veronica spicata, Scorzonera austriaca, etc. (Hurka et al 2019; Liu et al. 2022).

The North American grass-steppe zone comprises the so-called “short-grass prairie” and the “mixed-grass prairie” (Sims 1988). It occupies the central-western belt of the prairies in central North America, receiving less precipitations than the forest-steppe. Common grasses in this region include Andropogon gerardii, Bouteloua gracilis, Buchloe dactyloides, Eragrostis trichodes, Panicum virgatum, Schizachyrium scoparium, Sorghastrum nutans, Sporobolus compositus and others. Although perennial grasses are dominant, forbs supply the majority of the plant diversity: Amorpha canescens, Ambrosia psilostachya, Aster ericoides, Chenopodium belandieri, Echinacea atrorubens, Helianthus hirsutus, Lesquerella ovalifolia, Liatris punctate, etc.

The most decisive climatic characteristic of this domain is the extreme aridity, which is mostly due to the predominance of permanent subtropical highs (Logan 1968; Mares 1999). There are areas in which its influence is constant throughout the year and other areas in which there are some short periods in which some irregular rainfall occurs, both in the post-solstice period in the areas surrounding the temperate zone, as in the post-equinox in those closer to the tropical domain. In any case, deserts and semi-deserts occur when the Io index is generally lower than 1, with arid and hyper-arid ombrotypes, and vegetation is reduced to only partially cover the ground and very rarely to develop large trees or even shrubs. This formation can receive the names of desert and semi-desert, depending on the coverage and the meaning that these words have in the different languages.

This is the only ecozone recognized in this domain.

7. Biome of the deserts and semi-deserts of arid regions

This is the only biome recognized in this ecozone.

7.a. Cold deserts and semi-deserts.

In central Asia, there is a vast arid territory from the eastern shores of the Caspian Sea to southern Mongolia and northeastern China, encompassing southern Kazakstan, vast portions of Uzbekistan and Turkmenistan, Dzungaria as well as the Taklamakan and the Gobi Deserts. Aridity, measured by Io, keeps usually below 1 but due to altitude in some areas exceptionally increases near to 2. This huge continuous arid territory is created by the rain decrease due to distance from the oceans and several mountain ranges rising around. It still receives scarce winter and spring precipitations of Atlantic origin in its western part, whereas its eastern part is under the influence of the monsoonal regime, receiving some rains from the Pacific in summer (Walter 1977). These arid areas include many patches of saline soils due to topography and climate, which are populated by a halophytic usually succulent vegetation. Some of the relevant species in these regions are Artemisia lerchiana, A. terrae-albae, Potaninia mongolica or Stipa tianshanica as well as some Anabasis, Zygophyllum and Peganum (Pfadenhauer and Klötzli 2014). In North America some areas in the Great Basin Desert in the US can be included in this subbiome, with species such as Artemisia tridentata.

7.b. Temperate deserts and semi-deserts.

These occur in arid areas in which thermic seasons are clearly distinguished with a winter in contrast to a warm and hot summer.

The deserts and semi-deserts of western North America occupy the depression between the Rocky Mountains and the Sierra Nevada-Cascades range, which create a strong rain shadow between them. The Mohave Desert, dominated by Larrea tridentata (creosote bush) is included in this category, and in Mexico the Chihuahua desert, confined to the depression between Sierra Madre Occidental and Oriental, penetrating much to the south. In spite of its low latitude, this desert is relatively cool and rainy (Io between 1 and 2) due to its high altitude, with elevations mostly between 1100 m and 1500 m. The Neotropical flora is dominant with a high proportion of succulents such as Agave (A. lechuguilla), Yucca (Y. torreyi) and cacti species (MacMahon 1988; Rzedowski 2006).

In Australia the temperate arid regions extend across the southern half of the central areas of the continent, bearing a semi-desert with a number of shrubby species such as Acacia aneura (mulga). There are also several species of spiny grasses of the genera Triodia (T. basedowii and others) and Plectrachne schinzii called spinifex grasses (Walter 1977) as well as Crotalaria cunninghamii, Zygochloa paradoxa and Atriplex vesicaria (Beadle 1981).

In the Old World, the temperate deserts have a large representation which includes the northern half of the Desert of the Sahara, the northern part of the Arabian Peninsula, Mesopotamia with Iraq, Jordan and Siria, as well as central Iran and Afghanistan, and western Pakistan. They are under extremely arid conditions with a marked thermic seasonality and rains falling in winter. Some relevant species are Aristida acutiflora, Calligonum polygonoides, C. comosum, Zilla spinosa and Calotropis procera, whereas in the western areas close to the Atlantic coasts, Euphorbia stem-succulents appear such as Euphorbia echinus (Quézel 1965).

In southwestern Africa there is an important region of aridity formed by the coastal Namib desert, the Karoo, the Succulent Karoo, Nama-Karoo and the Kalahari towards the inland (Mucina and Rutherford 2006). In general, the aridity is due to the dry air masses provided by the South Atlantic Subtropical High, which is a result of the descending arm of the Hadley cell in that area, in combination with the rain shadow created by the South African Great Escarpment range which dries up the SE humid trade winds from the Indian Ocean. The coastal Namib desert strip is influenced by the Benguela current in which cold waters favour the formation of frequent fogs. These areas are mostly included in the temperate deserts as they are influenced by the cool waters of the Benguela current in the coastal areas and the highlands in inland, which attenuate the high temperatures. The vegetation varies from the coastal desert where Aloe dichotoma is found, to the Succulent Karoo with great diversity and abundance of succulent Aizoaceae and Crassulaceae as well as Asteraceae and geophytes. Acacia mellifera subsp. detinens and Lycium cinereum inhabit more in inland districts.

7.c. Warm deserts and semi-deserts.

The arid regions within the tropical climatic latitudes, i.e. with a reduced thermic seasons, post equinox precipitations and expanding in lowlands, are included in this subbiome. In Tropical America there are two areas in which these deserts can be recognized, the Sonoran desert in North America and the Pacific Coastal Desert in South America. The Sonoran Desert occupies an area south of the Mojave Desert, from southern California and Arizona extending to the Mexican territories of coastal Sonora and most of Baja California. The vegetation is dominated by a high number of cacti as well as some Fouqueria (ocotillo) and Parkinsonia species (palo verde). The Pacific Coastal Desert occupies a narrow strip between the Andes range and the Pacific shores, from northern Peru to mid Chile spanning between 6° and 27°S latitude. This arid strip is a result of the rain shadow created by the high Andes against the southeastern trade winds, which is enhanced by the cold waters of the Humboldt stream. The vegetation is very sparse and there are communities of Tillandsia which survive by taking the humidity from the frequent coastal fogs (garúas), and some cacti (Neoraimondia, Armatocereus, etc.) in inland sectors (Galán de Mera 2004; Galán de Mera et al. 2009).

The southern Sahara desert is continued in the Arabian Peninsula and southern Iran and Pakistan until the Indian Sind-Thar. This vast strip is under tropical climatic influence and the rainfalls occur in the summer months. Some species of this territory are Acacia senegal, Aristida pungens, Cornulaca monacantha, Indigofera oblongifolia or Neurada procumbens (Quézel 1965) as well as Euphorbia caducifolia and Prosopis cineraria in the eastern stretches (Enright et al. 2005).

The northern section of the western South African deserts, in the coastal regions of northern Namibia up to southern Angola, can be included in the category of the warm deserts. It is well known that Welwistchia mirabilis is a remarkable element in the flora of this area.

The tropical Australian desert occupies the northern fringe of the arid region of the continent where the rainfalls are received predominantly in summer (Beadle 1981). Some species are several mallee form eucalypts including Eucalyptus brevifolia, E. microtheca or E. odontocarpa as well as Acacia shirleyi, A. pachycarpa, shrubs like Hakea subarea and spinifex grasses such as Triodia pungens or T. wiseana among others (Beard 1990).

This domain is extended through the lowland areas spanning approximately between both tropics, receiving the maximal amount of solar radiation and having a positive radiative balance. Thus, Pt is usually over 2000 units. Thermic seasons are not distinguishable in this tropical zone because the solar radiation falls on the earth almost vertically all year round, and there are at least two days each year in which the sun rays are perpendicular to the ground. Seasonality, when it exists, is due to yearly rainfall distribution. Precipitations can be continuous all year in the areas influenced by the ITCZ, or seasonal in the areas more separated from the lowest latitudes and subject to a monsoonal regime. In the last case, the rain season occurs in the “summer” (post-summer equinox) months. Vegetation is adaptad to these thermic and pluviometric circumstances and phenology does not follow the mandatory concentration of flower and fruit production of the temperate world but only the rain seasonal regime.

Mountains rising within this tropical zone reproduce tropical versions of the biomes of the mesocratic and cryocratic domains in their vegetation belts, which are formed partially by local lineages combined with others migrated from higher latitudes in different climatic episodes. In our classification, these mountain formations will be explained within the biomes of the mesocratic and cryocratic domains as the lower temperatures of the higher elevations determine substantial convergences with them in terms of climate, growth forms and often biogeographical relationships. Thus, we will consider warm (tropical) biomes stricto sensu to those of the lowlands up to 1000–1500 m asl (infra and thermo).

As indicated above, in the intertropical latitudes the major part of the rainfalls occur in the post-summer equinox (summer) months and if this entails a wet and a dry season, so we will find an important set of adaptations to the drought. In many areas trees and shrubs become deciduous or semideciduous to prevent water loss. Woody plants often become spiny to prevent herbivory and the woody formations adopt a low density leaving room for many other smaller plants, like grasses and shrubs. Sclerophylly and evegreeness appears when soils are poor in nutrients.

8. Biome of the tropical pluviseasonal forests and woodlands.

In general, the vegetation is adapted to pluvial seasonality and in the dry season there is a complete or partial shedding of the leaves. In some areas where soils are very poor, woody plants become sclerophyllous and evergreen, as in the Brasilian cerrado (Rizzini 1997) or the north of Australia (Beadle 1981). Within this biome are included the savannas (from çabana, a term taken from the taino natives of the island of Hispaniola and incorporated to the Spanish language; Oviedo 1535) meaning that the vegetation is formed by sparse trees including a substantial proportion of grasses.

8.a. Tropical xeric woodlands and shrublands

This subbiome represents the transition from the warm desert (7.c) to the pluviseasonal areas with a longer rainy season and abundant precipitations, which in many cases follows a flat gradient which leaves substantial areas of dubious qualification. The seasonality of the precipitation becomes strongly unbalanced in favor of the dry season, which covers over 8 months of the year and with mostly arid and sub-arid ombrotypes. This results in a shrubby and spiny vegetation, often with many succulents.

In Africa, it is greatly extended in the Sahel, between Senegal and Sudan, in the western African Horn (Somalia and Masai areas) and in the transitional areas to the deserts of the southern part of the continent.

In the vast territories of the Sahel the vegetation comprises thorny shrubs or trees, mainly acacias, such as Acacia nilotica, A. tortilis, Senegalia laeta and S. senegal. Other tree species are Balanites aegyptiaca, Boscia senegalensis, Comminphora africana or Faidherbia albida. The grass layer is formed by annuals such as Aristida stipoides, Cenchrus biflorus, Panicum turgidum and Schoenefeldia gracilis, which dry up in the dry season when the trees and shrubs shed their leaves (Quézel 1965).

In the southern African territories covered by this subbiome, Acacia erioloba, Colophospermum mopane and Terminalia sericea with grasses of Aristida, Eragrostis or Panicum, are frequent (Mucina and Rutherford 2006).

In tropical America, it is dominant in the southern Chaco and Monte in Argentina and nerby countries, the Guajira in Venezuela and the Caatinga in Brazil. The Guajira-Maracaibo in northern Venezuela and Colombia occupies an area around the gulf of Maracaibo, with some relevant species including Castela erecta, Cercidium praecox, Cesalpinia coriaria, Gyrocarpus americanus, Haematoxylon brasiletto, Myrospermum frutescenes, Prosopis juliflora and the cacti Acanthocereus columbianus, Cephalocereus russelianus and Ritterocereus griseus (Mares 1999; Josse 2014; Morrone 2017; Navarro-Sánchez and Molina-Abril 2021).

Northeastern Brazil is occupied by the caatinga, a dry deciduous spiny vegetation rich in legumes with species such as Auxemma oncocalyx, Caryocar cuneatum, Caesalpinia bracteosa, Maytenus rigida, Mimosa caesalpiniifolia, Piptadenia viridifolia, Ziziphus joazeiro and others (Rizzini 1997).

In India, this subbiome ranges over a large area around the west of the Deccan plateau, including east of Rajasthan, south-west Punjab, Haryana, Gujarat and parts of Uttar Pradesh, Madhya Pradesh, Maharastra, Telangana and Andra Pradesh. The species of Indian distribution such as Acacia planiforns or Azadirachta indica, mix with others of broader range within this subbiome including Acacia nilotica, Butea monosperma or Prosopis cineraria (Greller et al. 2016).

In Australia, the distribution of the subbiome occupies a strip in the central northern regions, between the pluviseasonal forest and the warm desert (Beard 1990). It occupies vast portions in north Western Australia, southern sections of the Northen Territory and southwest of Quuensland. Some of the common woody species are Acacia aneura, A. estrophiolata and A. kempeana.

8.b. Tropical pluviseasonal forests

This subbiome corresponds to the mesic version of the biome and is formed basically by tree-dominated vegetation, often with a grassy (savanna) or scrubby understorey. Climatic conditions entail a dry season between 1–2 to 6–7 months, where double temperatures are higher than precipitation.

In tropical Asia and Malesia, the vast areas occupying most of the Indian subcontinent, as well as territories in Indochina and Malesia, are subjected to a tropical climate with a dry season, and a deciduous forest develops which corresponds to this biome. In it species such as Anisoptera thurifera, Anogeissus latifolia, Areca catechu, Azadirachta indica, Bombax ceiba, Butea monosperma, Dalbergia sissoo, Diospyros melanoxylon, Ficus religiosa, Garuga floribunda, Haldina cordifolia, Intsia bijuga, Madhuca longifolia, Phyllanthus emblica, Protium macgregorii, Pterocarpus indicus, Schleichera oleosa, Senegalia catechu, Shorea robusta, Tectona grandis and others can be found (Paijmans 1976).

In tropical America, this biome is widely represented all over the continent, from Mexico to the south American Paraná-Paraguay river basin, encompassing a range of more humid to more xeric variants. There are a couple of well defined regions which are occupied by this biome in their particular versions: In Mesoamerica (southern Mexico and Central America, as well as in the Caribbean islands), there are important areas occupied by this biome, particularly on the Pacific side, with deciduous trees such as Swietenia humilis, Bombacopsis quinatum, Cedrela fissilis, C. mexicana, C. odorata, C. salvadorensis, Ceiba aesculifolia, Dalbergia retusa; many legumes including Enterolobium cyclocarpum, Hymenaea courbaril, Cassia grandis, Platymiscium dimorphandrum, Samanea saman; and the palms Acrocomia aculeata, A. crispa, etc. and succulents (Cactaceae and Agavaceae) (Miranda and Hernández 1963; Vargas Ulate 1997; Rzedowski 2006).

The area of the Orinoco watershed shared by Colombia and Venezuela, known as the Llanos, is a flat region which is influenced by extensive floods (Pantanal). In the non flooded areas a woody vegetation of evergreen semideciduous trees is established, with tropical American species such as Bowdchia virgilioides, Brysonima crassifolia or Curatella americana, which combine with other endemics such as Borreria aristeguietana, Duguetia riberensis or Randia venezuelensis (Galán de Mera et al. 2006).

The Brazilian cerrado occurs widely across central Brazil (Rizzini 1997) and western Bolivia, where it forms a mosaic with the Bosque Chiquitano (Chiquitania; Navarro-Sánchez 2011). It encompasses a more or less open woodland with sclerophyllous evergreen or semideciduous shrubs and trees such as Aspidosperma tomentosum, Cavanillesia umbellata, Licania sclerophylla, Palicourea rigida, Qualea parviflora, Schinopsis brasiliensis, Terminalia actinophylla, Vochysia elliptica, V. thyrsoidea, and many others, accompanied by dwarf palms of the genus Allagoptera and herbs such as several species of Paepalanthus with tropical grasses such as Axonopus aureus, Trachypogon spicatus which form a dense and continuous herb layer. The cerrado grows on siliceous poor substrata rich in free aluminium, being the sclerophyll-evergreeness of the woody plants an adaptation to these harsh edaphic conditions. Where substrates change to base-rich, the forests become deciduous.

The Chaco is a vast region extending along the plain east of the central Andes through southwestern Brazil, most of Bolivia and Paraguay and northern Argentina. It is occupied by a deciduous forest in which occur species such as Aspidosperma quebracho-blanco, Ceiba speciosa, Piptadeniopsis lomentifera, Prosopis ruscifolia, Schinopsis balansae, S. cornuta, Sideroxylon obtusifolium, Ziziphus mistol and others, with some grasses such as Cenchrus pilcomayensis and Elionorus muticus (Oyarzábal et al. 2018; Navarro-Sánchez et al. 2006).

In tropical Africa this subbiome occupies its largest extent, surrounding the tropical rain forest biome area and constrained by the 8.a subbiome (White 1983). It encompasses the vast territories of central, east and south Africa (Sudano-Zambesian territories) and most of Madagascar, which are covered by several forms of this subbiome, with trees of genera such as Adansonia, Brachystegia (miombo), Combretum, etc, and several Bombacaceae. Some frequent woody species are Boscia angustifolia, Cussonia arborea, Daniellia oliveri, Erythrina abyssinica, Pterocarpus angolensis, Vachellia nilotica or the palm Borassus aethiopum, while grasses of the genera Hyparrenia, Heteropogon and Anropogon are frequent in the herb layer. In Madagascar, this subbiome occupies the entire central and western side of the island and there are many endemics in it, such as Adansonia rubostipa, A. za, Givotia madagascariensis or Stereospermum euphorioides.

In Australia this subbiome occupies a vast area encompassing the northernmost regions of the Continent such as the Kimberly Plateau, the area of Darwin and the Carpentaria Plains (south of the Gulf of the same name), most of the Cape York peninsula and other areas in the northeast. It occupies the landscapes between the shores of the Timor and Arafura seas and the xeric inland regions of the continent. The vegetation is dominated by woodlands formed by evergreen and hard-leaved eucalypts. This sclerophyllous evergreen life form is consistent with the poverty of the soils widespread on the Australian continent. Some important trees are Acacia shyrleyi, Corymbia confertiflora, C. grandifolia, C. polycarpa, Eucalyptus brevifolia, E. dichromophloia, E. grandifolia, E. miniata, E. phoenicea, E. pruinosa, E. tectifica, E. tetrodonta, etc. Some other woody plants are Acacia humifusa, A. mountfordiae, Hakea arborescens, Grevillea pteridifola or Lysiphyllum cunninghamii (Beadle 1981).

In the tropical areas where rainfall persists all year round (Rainfall seasonality BIO15 < 60) in a sufficient intensity and frequency so that no or a very brief (1 month) dry season can be recognized, the tropical pluvial ecozone is established. Temperatures are high and present low daily and yearly oscillations, and precipitations rarely fall below 2000 mm per year and occur usually at zenital showers in the afternoon. The soils undergo an intense leaching leading to the formation of laterites.

9.Biome of the tropical rain forests.

This biome occurs where the tropical warm and moist forest thrives, usually called tropical rainforest or pluviisilva. It has a high number of vascular plant species, including trees, which bear particular adaptations to this favorable environment: thin barks, leaves with entire margins, gutation organs and other well documented features of this tropical rainforest (Schroeder 1998; Primack and Corlett 2011; Pfadenhauer and Klötzli 2014).

9.a Tropical rain forests

This is the only subbiome recognized withis this biome.

In Tropical America, there are several areas occupied by this biome-subbiome:

In Central- and Mesoamerica, this biome-subbiome develops mostly in the Caribbean side, occupying the entire isthmus of Panamá and peripheral areas in the Antilles, including the southern half of the Florida peninsula.

The most important area is the vast Amazonian plain (the hylea amazonica of Humboldt and Bonpland 1805) which spans from the high Orinoco and the Guianas to the central regions of Brazil, southern Colombia, western Ecuador and Perú and northwestern Bolivia, i.e. all the area between the Atlantic coast and the Andean piedmonts, constrained by the drier regions to the north and the south. The vegetation is dominated by dense forests very rich in species of plants, with many epiphytes and lianas, hosting most of the representatives of the genus Hevea (Rizzini 1997). Noteworthy is the high number of ant-pollinated species (myrmecophyllous) and the large extent of flooded areas by the regular oscillations in the level of water characteristic of the Amazonian drainage network. Among the overwhelming richness of the Amazonian tree flora, we can mention Bertholletia excelsa, Calophyllum brasiliesnse, Caraipa grandifolia, Caryocar villosum, Clarisia racemosa, Clathrotropis macrocarpa, Clusia spathulifolia, Euterpe precatoria, Hevea brasiliensis, Hyeronima oblonga, Pterocarpus amazonum, Taralea oppositifolia and many others (Josse 2014).

Another characteristic region is the Darién-Chocó in the Pacific coastal strip from southern Panamá to Colombia and Ecuador, which receives abundant precipitations from the east. The tropical rain forest of this area has many endemics and there are species such as Billia columbiana, Exarata chocoensis, Guettarda crispiflora subsp. sabiceoides, Humiriastrum procerum, Virola dixonii and others (Rangel 1997).

The low altitudes of the Atlantic coastal fringe of Brazil, spanning between Bahia and Santa Catarina are covered by this biome-subbiome. It is represented by the Mata Atlântica or Floresta Atlântica, a highly diverse tropical forest maintained by the moist trade winds of the south Atlantic, which provide abundant rainfall all year. Due to the higher latitudes and elevations in the mountains of the coastal range, the thermic regime has lower temperatures than in the Amazonian river basin. The natural forest is extremely rich in species, particularly epiphytes with many bromeliads, making each tree resemble a true garden in miniature (Rizzini 1997). Among the trees we can mention Alseis involute, Astrocaryum aculeatum, Caryocar edule, Cordia taguahyensis, Ficus gomelleira, Lecitis pisonis, Licania salzmanii, Tabebuia obtusifolia, Virola oleifera, etc., many of them growing also in the Amazonian river basin.

The Guineo-Congolian region is the most important of tropical Africa, encompassing the coastal strip of the gulf of Guinea from Sierra Leone to Nigeria and entering deep into the African continent in the Congo watershed until the chain of mountains and lakes crossing east Africa, including large parts of Cameroon, Gabon and Ecuatorial Guinea (White 1983). Some near-endemic species are Afzelia bipindensis, Pouteria altissima, Diospyros gabonensis, Garcinia punctata, Sterculia tragacantha, Syzygium owariense, Trichilia prieuriana or Xylopia aethiopica (Sayer et al. 1992).

The Madagascar eastern coast is homologous to the Mata Atlântica region of Brazil. The flora is mainly African with a high number of endemics, among them Ravenala madagascariensis. (Moat and Smith 2007).

The Indo-Malesian region is a large territory encompassing south China, the Philippines, the islands of Malesia (Java, Sumatra, Borneo, Celebes, Moluccas, etc.), the peninsula of Malacca, parts of Indochina and the gulf of Bengal to the piedmonts of the eastern Himalayas. For Malesia we can mention some species such as Artocarpus elasticus, Macaranga hispida, Nephelium lappaceum, Palaquium rostratum, Platea excelsa, Prunus arborea, Shorea leprosula, Trevesia sundaica etc. (Edwards et al. 1990; Kitayama 1992).

The Malabar-Ceylon region lies along the southwestern coast of the Indian subcontinent, at the piedmonts of the Gaths range and the western half of the Sri Lanka (Ceylon) island.

The Pacific Oceanic region encompass the numerous islands widespread over the Pacific Ocean. The flora and vegetation of this inmense number of islands is full of endemic species having originated from neighbouring continental Godwanic islands (basically New Zealand and New Caledonia) and from tropical East Asia across the Malesian archipelagos. There are representatives of gymnosperms of the genera Agathis, Dacrydium or Podocarpus and some species of a more general distribution are Calophyllum neoebudicum, Canarium vitiense, several species of Metrosideros such as M. collina, among many others (Mueller-Dumbois and Fosberg 1998).

The Papuan-Australian region includes the island of New Guinea and the northeastern extreme of the Australian continent. In Australia only a narrow area in Queensland can be assigned to this biome, in the eastern coastal strip of the Cape York Peninsula, bearing similarities with the near island of New Guinea. Some species are Agathis microstachya, Balanophora fungosa, Balanops australiana, Cordyline cannifolia, Macaranga inamoena, Musa banksia, Pandanus monticola, Syzygium wilsonii and others (Beadle 1981).