Works considered in the analysis were divided into two time periods: 1950 to 1999 and 2000 to the present. Emphasis was placed on works that prioritize classification in the strict sense: Leopold (1950), Miranda and Hernández (1963), Rzedowski (1978), and some international references (UNESCO 1973). Approaches and main considerations of each one were synthesized, as well as the criteria they used, with a particular focus on temperate forests, through comparative tables and their respective analysis and discussion. Other works that have contributed to the development of this undertaking were reviewed, such as those from Gómez-Pompa (1965), Toledo and Ordóñez (1998), and INEGI (1999).

The analysis of the second period focused on the works of González-Medrano (2003) and Velázquez et al. (2016). In comparison with previous works, both contributions present important attributes, such as: compatibility with international classification systems and the use of physiognomic, climatic, phenological, and floristic criteria, which help to include and cover the high diversity of plant communities up to more detailed levels, such as associations. Other important publications, such as those from Palacio-Prieto et al. (2000), Velázquez et al. (2010), Villaseñor and Ortiz (2014), and the forest inventories carried out by the National Forestry Commission (CONAFOR) from 2004–2009 and 2017, contributed to enriching the analysis and discussion.

To identify the most common names used in literature to refer to these forests, a review of document titles in academic literature was carried out. This began with a search in the Scopus database, entering as many terms as possible that have been used to designate these communities.

As inclusion criteria, the term “forest” was used in all search fields, in combination with other terms used to designate the type of forest, in such a way that the expected results were as broad and inclusive as possible. The terms used were: “temperate forest”, “mix forest”, “mixed forest”, “pine forest”, “oak forest”, “fir forest”, “pine-oak forest”, “Pinus-Quercus forest”, “Quercus-Pinus forest”, “conifer forest”, “coniferous forest”, “Abies forest”, “Abies religiosa forest”, “Quercus forest”, “Pinus forest”, “broadleaf forest”, and “Alnus forest”. To include only works developed in Mexico, the term “mexic*” was added to find publications with any of the following terms in the title, abstract or keywords: Mexico, mexicano and mexicana. In addition, as a way of restricting the search to topics related to vegetation, the terms “vegetat*”, “plant*”, “flora”, and “floristic” were entered in the search engine.

A table was constructed with the most frequent ways of naming these forests, which were integrated into a corresponding field according to the hierarchical levels of the most recent vegetation classification system used in Mexico. For the graphic visualization of these results, a frequency bar chart was created. In addition, a co-occurrence analysis was constructed in the VosViewer software to identify the way in which these terms are grouped based on keywords of the considered documents. Only these terms that presented a co-occurrence ≥2 were considered first, , subsequently excluding terms that were not associated with the theme of vegetation classification or the region or country of study, as well as those related to fauna or human actions. The co-occurrence map was constructed with a total of 78 items.

At the end, a list of associations identified in the literature for temperate subhumid forests of Mexico is presented. The International Code of Phytosociological Nomenclature (Theurillat et al. 2020) specifies that two is the maximum number of species to indicate an association; however, in the literature it is possible to find the use of two or three species. In this work, cases were identified with up to four, but this is not common. Therefore, an arbitrary criterion was established to have a maximum of three species to form an association and compile the list.

The distribution of plant communities is largely influenced by meteorological elements such as temperature, pressure, wind, humidity, and precipitation – in other words, by climate. There are also other factors such as edaphic factors, topography, altitudinal gradients, and even evolution itself (Miranda and Hernández 1963; Rzedowski 1978; González-Medrano 2003). Five main climates have been identified around the world according to the system formulated by Wladimir Köppen, which are still used today (Peel et al. 2007); these are: tropical (A), warm (B), temperate (C), cold (D), and polar (E). However, in the Mexican Republic, according to modifications of the Köppen system proposed by García (2004), only A, B and C are present, which in turn are grouped into two large humid and arid groups, and five thermal categories.

Type C climate can be subdivided into sub-humid with summer rains (Cw), which occupies about 24% of the national territory (González-Medrano 2003), especially in mountains or plains with altitudes higher than 800 or 1000 meters. In Mexico, they are distributed particularly in the four physiographic provinces mentioned in the study area: SMOc, FVT, SMO, and SMS. These mountainous and climatic regions coincide with large forest areas of coniferous and broadleaf species (Figure 1).

As a consequence, proposals based on climate have been made to classify plant communities, such as those of Holdridge (1967), who proposed a diagram that represents the distribution of plant formations based on what he called “life zones”. For the case of Mexico, Leopold (1950) grouped vegetation into temperate and tropical, and Gómez-Pompa (1965) into a) temperate or cold, b) arid or subarid climate, and c) warm climates.

Environmental features and habitat are factors that influence the way vegetation is named and classified. Thus, when the expression of large vegetation obeys climatic functions, it is considered zonal vegetation; and azonal when it depends on the conditions of the geological substrate and soil (González-Medrano 2003). Velázquez et al. (2016), for their part, apply a vegetation classification based on the climatic system according to García (2004), as well as the proposal of Holdridge (1967), considering only three climatic groups: temperate, tropical, and cold, in which they subdivide the temperate into dry and humid.

Other associated factors that stand out in the expression of vegetation at both local and regional levels are: geological substrate, soil type, altitudinal gradient, orientation, and humidity (González-Medrano 2003; Sánchez-González and López Mata 2003). For example, in the case of soil, Rzedowski (1978) suggested that the predominance of oak forests over pine forests in the SMO is due to the tolerance of the former to soils approaching neutrality, as opposed to the soil conditions in the SMOc and FVT, where acidic soils predominate as a result of volcanic eruptions, which would favor the presence of pines.

The contributions of different international classification systems as well as those developed in Mexico, combined with the development in research on vegetation given the biodiverse nature of the country, have resulted in the Mexican Vegetation Classification System (SECLAVEMEX), a proposal by Velázquez et al. (2016). This system is hierarchically structured in eight nested levels. Levels I to III are based on physiognomic and environmental criteria, while IV to VI are botanical and floristic in nature. The last two levels, sub-association and facies, are established by edaphic and substrate characteristics in that order, considering the degree of disturbance or ecological succession of the community.

Some of the qualities of this system include recognition, versatility, and reorganization, which involve harmonizing and comparing classification proposals, particularly those of Miranda and Hernández (1963), Rzedowski (1978), Technical Consultative Commission for the Definition of Grazing Coefficients (COTECOCA 1994), and González-Medrano (2003). This is further complemented by compatibility with global systems, such as the European phytosociological approach and the North American EcoVeg approach (Faber-Langendoen et al. 2014, 2016; Velázquez et al. 2016) (Table 3). In addition, it seeks to satisfy both technical and academic needs, as well as those of practical use, such as for institutions responsible for natural resource management, including field data collectors. In addition, it foresees adaptations that may arise due to the dynamism of vegetation, such as changes from natural to cultural. Finally, it considers the presence of exceptions as a product of the evolutionary history of vegetation and the biogeographic history in Mexico.

In the case of temperate forests, this system would only include up to level VI Associations. As mentioned, levels VII and VIII depend on edaphic and substrate conditions, attributes that are not considered at this level. Table 3 shows the levels in which temperate forests are integrated in this classification system, and Table 4 shows the criteria for the vegetation types of these forests.

The eight levels that make up this system are designed with the purpose of grouping or naming all plant communities, although not all levels use all the criteria to define a particular type of community.

Level I Biome. The highest level encompasses all lower levels; it is defined by physiognomic criteria (forest, shrubland, herbaceous), which is a function of the dominant life form. Level II Major formation. Defined by climatic criteria, based on García (2004) and Holdridge (1967). Level III Formation. This is a function of phenological criteria, i.e. the fall or permanence of foliage (deciduous, sub-deciduous, evergreen, sub evergreen), a condition that depends on the amount or percentage of the fall or permanence of foliage.

Level IV Subformation. Depends on phenological and floristic criteria, such as leaf characteristics, presence of thorns or leaf succulence. In the particular case of humid and sub-humid temperate forests, these are characterized by the absence of dominant species with thorns or succulence, so only the characteristics of leaves, their shape and anatomy are considered. Level V Series of Associations. Part of the floristic criterion, in which two hierarchical criteria are used: family level, or dominant genera. Level VI Association. Finally, for the association level, the floristic criterion is taken into consideration, although unlike level V, it uses one to three or more dominant species as a reference.

The document search process yielded a total of 360 documents, of which 92 were considered for analysis. From a review of these, the use of 18 terms to refer to this type of plant communities was identified (Table 5). The most commonly used term was “temperate forest”, which appears in 34 titles of these documents, followed by the terms “oak forest” and “pine-oak forest”, with 14 and 10 appearances respectively. These three together account for 63% of terms used.

The term temperate forest has been used in a conventional way to refer to these formations, from the work of Leopold (1950) to Velázquez et al. (2016), as analyzed in both periods in previous sections. It is frequently used in classification systems as a reference to plant communities established in this type of climatic conditions – both in those proposed worldwide and in those of Mexico, Velázquez et al. (2016). According to the SECLAVEMEX system, this term is included within the major formation level (Level II), given that in this hierarchical system the two highest levels are integrated with floristic and climatic criteria. The other two terms, pine forest and oak-pine forest, would be included within the series level of associations of the aforementioned system.

This suggests that the vegetation classification proposals that have had an important influence on the use of one or another denomination come from the works of Rzedowski (1978), Miranda and Hernández (1963), or INEGI (1999), and to be more specific, names of well-marked plant associations such as Abies and fir forest are often used.

Coniferous, mixed coniferous, coniferous and broadleaved, and coniferous and Quercus forests are located within the Subformation level, which total 13 documents. The term conifers is based strictly on physiognomic criteria, given that the external physical appearance of the taxa grouped within is cone-shaped, which is represented in 11 titles of the documents found. It appears formally in Rzedowski’s classification system (1978) and was taken up by Palacio-Prieto et al. (2000) to designate a type of vegetation within forest formation, which is also used in CONAFOR’s forest inventories and in INEGI’s land use and vegetation inventories.

At the association series level, there are 35 documents that use the terms oak forest, pine-oak forest, pine forest and other variants, such as Quercus-Pinus and mixed. In this context, Rzedowski (1978) used the term Quercus to refer to oak forests, which is the technical name that groups all oak species and although it is uncommon to use it in the titles of academic documents, the term persists, suggesting the author’s influence. On the other hand, Miranda and Hernández (1963), used the term encinares, which was no longer used in later classification systems and was not even found in the titles of these revised documents. However, for practical uses, like in forest inventories and wherever the term oak forests is used, it can specifically refer to oak forests or to pine-oak forests.

According to the results on the level of associations of different terms, the presence of these in document titles refer to Abies religiosa or oyamel forests, which are used indistinctly and with the same number of appearances (4). The other two identified are for Pinus ponderosa and Pinus pinceana forests (1 each; Miranda and Hernández 1963).

The terms with a high co-occurrence of keywords are: “Quercus”, “temperate forest”, “coniferous forest”, “Abies religiosa”, with 22, 15, 11 and 10 respectively, and present the highest number of interactions with other terms (Figure 2). This would suggest that “Quercus” is an alternative term widely used to indicate that the titles of academic documents would refer to a type of vegetation community composed of oak forest or mixed pine-oak forest (Figure 3). Similarly, the other keywords with a high number of co-occurrences such as “coniferous forest” or “temperate forest” indicate that they are either a community with a predominance of some of the coniferous species, or any forest related to temperate climate in general.

Figure 2. 

Map of co-occurrence of terms for sub-humid temperate climate plant communities.

Figure 3. 

Frequency of the most common terms of temperate forests.

As previously mentioned, in sub-humid temperate forests, predominantly conifer and broadleaf communities are established, either independently or forming associations between one another, along with mixed conifer and broadleaf forests. The conifer group in Mexico is represented by four families (Pinaceae, Cupressaceae, Podocarpaceae and Taxaceae), 94 species, and 10 genera (Gernandt and Pérez-de la Rosa 2014). The genera Pinus, Abies, Pseudotsuga, and Cupressus mainly form the Series of Associations: Pinus-Pinus, Pinus-Abies or Cupressus-Pseudotsuga, among others. These, in turn, are integrated in Associations either with the dominance of one, two or more species (Braun-Blanquet 1974) – for example, Pinus hartwegii or Abies religiosa, Pinus pseudostrobus, or associations between species of the same genus, two or more genera as shown in Table 6, which expands on what is presented in Suppl. material 1.

The Subformation of mixed coniferous and broadleaf forests is a combination of coniferous and broadleaf species, in their various proportions. The most common are Series of Associations such as Pinus-Quercus and Quercus-Pinus, and to a lesser extent Abies-Quercus, Quercus-Cupressus, and Pseudotsuga-Quercus-Cupressus, Table 6 and Suppl. material 2.

In the broadleaf Subformation, species belonging to the Quercus genus dominate, although they are also associated with other genera such as Alnus and even Arctostaphylos. The Series of Associations that these form are: Quercus-Quercus, Quercus-Alnus, and less frequently Quercus-Arctostaphylos. Similar to what happens with Pinus associations, where a single species usually dominates, in the case of broadleaved trees, these are frequently Quercus rugosa and Quercus deserticola, among others. In other cases, they are usually formed by two or more Quercus species such as: Quercus rugosa-Quercus laurina, Table 6 and Suppl. material 3.