Research Paper |
Corresponding author: Sebastián R. Zeballos ( sebazeba@hotmail.com ) Academic editor: Jose Alejandro Velazques Montes
© 2023 Sebastián R. Zeballos, Alicia T. R. Acosta, Walter D. Agüero, Rodrigo J. Ahumada, Martín G. Almirón, Daihana S. Argibay, Daniel N. Arroyo, Lisandro J. Blanco, Fernando N. Biurrun, Juan J. Cantero, Justo Márquez, Alejandro Quiroga, Raúl E. Quiroga, Marcelo R. Cabido.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Zeballos SR, Acosta ATR, Agüero WD, Ahumada RJ, Almirón MG, Argibay DS, Arroyo DN, Blanco LJ, Biurrun FN, Cantero JJ, Márquez J, Quiroga A, Quiroga RE, Cabido MR (2023) Vegetation types of the Arid Chaco in Central-Western Argentina. Vegetation Classification and Survey 4: 167-188. https://doi.org/10.3897/VCS.100532
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Aims: We address the following questions: 1) Which are the main vegetation types that currently occur in the Arid Chaco? 2) Do those vegetation types differ in terms of floristic composition, endemism, chorotypes and life forms? and 3) Is there any spatial association between the vegetation types and the environmental heterogeneity of the Arid Chaco? Study area: The southwestern extreme of the Gran Chaco, in Central-Western Argentina. Methods: The survey was based on a dataset comprising 654 relevés collected according to the Braun-Blanquet method. Data were classified by the hierarchical ISOmetric feature mapping and Partition Around Medoids (ISOPAM), and ordinated through isometric feature mapping (ISOMAP). Bioclimatic and edaphic variables were related to the ISOMAP ordination. Results: We recorded 439 vascular plant species, 62 endemic at the national level and 22 endemic species restricted to the study and surrounding environments in Central-Western Argentina. A total of nine vegetation types, belonging to four major clusters, were identified. The most prominent chorotypes included species distributed in the Chaco region and in the Arid Chaco/Monte phytogeographic units. The predominant life forms were micro- and nano-phanerophytes, followed by hemicryptophytes, chamaephytes and mesophanerophytes. Conclusions: Major results highlighted that xerophytic shrublands are the most common vegetation types in this area as a result of the historical and present use, while old growth forests were constrained to areas with low anthropogenic disturbance in the last decades or to protected areas. Most vegetation types (with the exception of halophytic environments) are poorly differentiated from a floristic point of view; however, they clearly differ in physiognomy. The floristic composition of the vegetation types described revealed numerous species in common with other sectors of the Chaco of northern Argentina, Bolivia and Paraguay. Although the number of species restricted to the Arid Chaco was quite low, the most relevant chorotype included species with Western and Eastern Chaco distribution, conferring a clear Chaquenian identity to this area and discriminating it from other phytogeographic units.
Taxonomic reference: Catálogo de las Plantas Vasculares del Cono Sur (
Abbreviations: ISOMAP = isometric feature mapping; ISOPAM = isometric partitioning around medoids.
Arid Chaco, Aspidosperma forest, chorotype, endemism, halophytic succulent, shrubland, species richness, vegetation classification
The “Gran Chaco” or “Chaco” is a natural ecoregion in South America, comprising about 1,000,000 km2, expanding in Argentina, Bolivia, Paraguay and a narrow strip in Mato Grosso do Sul, Brazil (
Over the last century, several phytogeographical synthesis were proposed for Argentina including also the Arid Chaco vegetation (
Formerly an almost continuous forest, the Arid Chaco vegetation has been fragmented and converted mostly into closed and open shrublands with only scattered emergent trees (
The aim of this study was to classify the main vegetation types occurring in the Arid Chaco of Central-Western Argentina. To accomplish this, the following questions were addressed: 1) Which are the main vegetation types that currently occur in the Arid Chaco? 2) Do those vegetation types differ in terms of endemic species, chorotypes and life forms? and 3) Is there any spatial association between the vegetation types and the environmental heterogeneity of the Arid Chaco?
The survey was conducted in the Arid Chaco, the southwestern extreme of the Gran Chaco, extended across ca. 9.6 million ha in Central-Western Argentina (64°00’ and 67°50’W – 28°20’ and 34°00’S) (
The study area: a) The location of the Arid Chaco in Argentina and within the Gran Chaco; and b) Arid Chaco and major features of the environment of the study region (in grey are shown the elevation gradient). Cover adapted from MapBiomas Chaco Project Collection 3.0 of the annual land use and land cover maps, accessed on December 2022. Datum: WGS 84, EPGS: 4326.
Bioclimatically the study area belongs to the tropical xeric bioclimate (
Previous authors have associated the distribution of the vegetation of the Arid Chaco in Argentina with topography and geomorphology (
Similar to what has been reported for other sectors of the Chaco in northern Argentina (
A total of 654 phytosociological relevés were sampled during the last 20 years to represent the wide geographic, topographic and ecological heterogeneity comprised in the Arid Chaco. All vegetation plots can be found in Suppl. material
Bioclimatic variables were interpolated from the CHELSA (Climatologies at High Resolution for the Earth’s Land Surface Areas) database (
The data set of 654 plots and 439 species was classified into the major vegetation clusters using the ISOmetric feature mapping and Partition Around Medoids (ISOPAM) method (
Incidence-based rarefaction and extrapolation (R/E) curves using sample size-based and coverage-based methods were performed to evaluate whether plant species from the different vegetation types classified by the ISOPAM method were well represented (
Finally, one-way ANOVAs were performed in order to evaluate the differences in the mean percentage of life forms, endemic species and the mean percentage of each chorotype per vegetation type, as well as the differences in bare soil, height and cover of different layers among the vegetation types. The normality of the data and the homoscedasticity of variances were previously evaluated and when these requirements were not accomplished the data were natural log transformed. All statistical analysis and graphics were performed in R software version 4.0.2 (
We recorded a total of 439 vascular plant species, distributed in 60 families and 241 genera (Table
We should note that 62 endemic species at the national level (14.0% of the total) and 22 (4.9%) endemic species restricted to the study area and surrounding environments of the Monte and the Western Chaco were recorded. The life-form spectra showed a predominance of micro- and nanophanerophytes (16.6%), followed by hemicryptophytes (15.0%), chamaephytes (12.7%) and low trees (mesophanerophytes) (10.7%). The most relevant chorotype was the Eastern and Western Chaco (39.4% of all species), while the Arid Chaco and Monte (15.7%) and the Southern-Brazilian/Paranaense (11.1%) chorotypes were also represented. Ubiquitous species were also relevant (26.1%).
Although the sample-size based rarefaction showed that the curves of all the vegetation types had not yet reached the asymptote (Suppl. material
Summarized synoptic table obtained through the ISOPAM classification showing the vegetation types along with the percentage constancy and mean Braun-Blanquet cover-abundance values (as superscript) based on 654 relevés recorded in the Arid Chaco, Central-Western Argentina. Species are sorted by decreasing fidelity within each vegetation type. Dark, light dark, medium, light grey and low light grey indicate phi > 0.5, phi > 0.4, phi > 0.3, phi > 0.2 and phi > 0.1, respectively. Vegetation types are: 1.1, Aspidosperma quebracho-blanco forest; 1.2, Larrea divaricata shrubland; 1.3, Prosopis flexuosa woodland; 2.1, Larrea cuneifolia shrubland; 2.2, Larrea cuneifolia – Bulnesia retama shrubland; 3.1, Mimozyganthus carinatus – Bulnesia foliosa shrubland; 3.2, Tricomaria usillo shrubland; 4.1, Suaeda divaricata – Atriplex argentina shrubland; 4.2, Heterostachys ritteriana – Allenrolfea patagonica succulent shrubland. Only those species with constancy ≥ 25% in at least one vegetation type were included in the table. CT, chorotypes: Southern-Brazilian/Paranaense (1), Western and Eastern Chaco (2), Espinal/Pampean (3), Arid Chaco/Monte (4), High and Andean Mountains/Puna (5), Patagonian (6), Non-native (7), Ubiquitous (8). LF, Life forms: C, Chamaephytes (low shrubs, sub-shrubs, some perennial herbs); E, Epiphytes; F, Ferns; G, Geophytes; H, Hemicryptophytes; Kc: Columnar cacti; Ko, Opuntioid cacti; L: Climbers; P, Parasites and hemi-parasites; Ph, Phanerophytes (Phsa, sub-shrubs; Phs, shrubs; Phss, succulent shrubs; Pht, trees); T: Therophytes. Symbols: †, endemic species at the national level, and *, endemic species at more local level.
Plant communities | 1.1 | 1.2 | 1.3 | 2.1 | 2.2 | 3.1 | 3.2 | 4.1 | 4.2 | ||
---|---|---|---|---|---|---|---|---|---|---|---|
Numbers of relevés | 80 | 171 | 61 | 52 | 78 | 56 | 68 | 58 | 30 | ||
Species | CT | LF | |||||||||
Justicia squarrosa | 2 | Fsa | 652 | 131 | 8+ | 2+ | 2+ | 1+ | |||
Senegalia gilliesii | 4 | Fs | 832 | 231 | 11+ | 152 | 41 | 142 | 31 | ||
Sarcomphalus mistol | 2 | Ft | 581 | 15+ | 8+ | 8+ | 2+ | 1+ | |||
Tillandsia duratii | 2 | E | 73+ | 27+ | 10+ | 35+ | 6+ | 131 | 13+ | 7+ | |
Tillandsia xiphioides | 2 | E | 60+ | 19+ | 5+ | 21+ | 1+ | 16+ | 6+ | 5+ | |
Tillandsia aizoides | 2 | E | 36+ | 12+ | 2+ | 6+ | 1+ | 4+ | 1+ | ||
Tillandsia pedicellata | 2 | E | 46+ | 17+ | 2+ | 17+ | 111 | 3+ | |||
Tillandsia rectangula | 2 | E | 26+ | 7+ | 3+ | 2+ | 1+ | ||||
Cardiospermum halicacabum* | 1 | T | 25+ | 5+ | 2+ | 10+ | 4+ | 3+ | |||
Lycium elongatum* | 4 | Fs | 54+ | 28+ | 16+ | 19+ | 131 | 16+ | 22+ | 5+ | |
Ayenia cordobensis* | 2 | Fsa | 30+ | 16+ | 11+ | 6+ | 4+ | 1+ | |||
Castela coccinea | 2 | Fs | 36+ | 15+ | 8+ | 10+ | 3+ | 5+ | 12+ | 5+ | 3+ |
Cleistocactus baumannii | 2 | Ko | 31+ | 12+ | 2+ | 12+ | 9+ | 91 | 3+ | ||
Setaria lachnea | 2 | H | 441 | 22+ | 211 | 17+ | 91 | 181 | 3+ | 3+ | |
Cereus forbesii | 2 | Kc | 30+ | 8+ | 3+ | 21+ | 7+ | 3+ | 2+ | ||
Tragia hieronymii | 2 | Fsa | 41+ | 21+ | 31+ | 12+ | 5+ | 14+ | 4+ | ||
Justicia xylosteoides | 2 | Fs | 26+ | 9+ | 5+ | 15+ | 1+ | 71 | 31 | ||
Tillandsia myosura | 8 | E | 34+ | 19+ | 8+ | 21+ | 4+ | 91 | 6+ | 2+ | |
Deinacanthon urbanianum | 2 | H | 511 | 311 | 111 | 291 | 211 | 111 | 6+ | 411 | 7+ |
Ephedra triandra | 2 | Fsa | 26+ | 17+ | 26+ | 13+ | 61 | 13+ | 11 | 2+ | |
Digitaria californica | 8 | H | 51+ | 44+ | 301 | 38+ | 24+ | 451 | 32+ | 9+ | |
Neobouteloua lophostachya* | 4 | H | 43+ | 721 | 302 | 541 | 381 | 48+ | 181 | 91 | |
Parkinsonia praecox | 8 | Ft | 741 | 821 | 611 | 771 | 21+ | 541 | 76+ | 401 | 3+ |
Sporobolus pyramidatus | 8 | H | 60+ | 701 | 341 | 541 | 35+ | 46+ | 16+ | 591 | 501 |
Mimozyganthus carinatus | 2 | Fs | 802 | 802 | 341 | 711 | 491 | 843 | 822 | 522 | 71 |
Sida argentina | 2 | H | 15+ | 26+ | 20+ | 12+ | 8+ | 27+ | 7+ | ||
Aloysia gratissima | 8 | Fs | 21+ | 16+ | 381 | 10+ | 4+ | 4+ | 31 | 2+ | |
Leptochloa crinita | 8 | H | 651 | 612 | 772 | 651 | 28+ | 381 | 12+ | 591 | 371 |
Celtis pallida | 2 | Fs | 681 | 27+ | 432 | 82 | 21 | 3+ | 2+ | ||
Selaginella sellowii | 8 | Pt | 512 | 272 | 82 | 152 | 14 | 41 | |||
Monteverdia spinosa | 2 | Fs | 60+ | 34+ | 311 | 6+ | 1+ | 9+ | 3+ | 2+ | |
Amphilophium carolinae | 2 | L | 54+ | 28+ | 26+ | 10+ | 1+ | 7+ | |||
Pseudabutilon pedunculatum | 2 | Fs | 64+ | 43+ | 25+ | 23+ | 12+ | 21+ | 13+ | ||
Talinum polygaloides | 2 | G | 50+ | 33+ | 11+ | 27+ | 5+ | 9+ | 1+ | 9+ | |
Atamisquea emarginata | 8 | Fs | 811 | 57+ | 821 | 60+ | 15+ | 391 | 34+ | 41+ | 3+ |
Leptochloa pluriflora | 8 | H | 351 | 321 | 71 | 81 | 31 | 131 | 7+ | 171 | 3+ |
Pappophorum caespitosum | 4 | H | 391 | 351 | 161 | 15+ | 9+ | 11+ | 13+ | 5+ | 32 |
Condalia microphylla† | 4 | Fs | 65+ | 40+ | 611 | 12+ | 4+ | 16+ | 9+ | ||
Prosopis flexuosa | 4 | Ft | 70+ | 801 | 932 | 751 | 711 | 591 | 241 | 411 | |
Lycium tenuispinosum† | 4 | Fs | 8+ | 161 | 311 | 10+ | 13+ | 7+ | 71 | 331 | 131 |
Aspidosperma quebracho-blanco | 2 | Ft | 942 | 801 | 642 | 902 | 511 | 711 | 902 | 361 | |
Larrea cuneifolia* | 4 | Fs | 5+ | 191 | 71 | 1003 | 993 | 301 | 132 | 211 | |
Setaria pampeana | 8 | H | 491 | 511 | 30+ | 63+ | 33+ | 36+ | 28+ | 45+ | 20+ |
Aristida adscensionis | 8 | T | 10+ | 181 | 161 | 15 | 601 | 382 | 151 | 10+ | 3+ |
Bulnesia retama | 4 | Ft | 62 | 191 | 131 | 251 | 551 | 301 | 15+ | 12+ | |
Cottea pappophoroides | 4 | H | 4+ | 91 | 23+ | 501 | 30+ | 29+ | 14+ | ||
Bougainvillea spinosa | 4 | Fs | 1+ | 6+ | 31 | 15+ | 351 | 131 | 9+ | 9+ | |
Bouteloua barbata | 4 | T | 1+ | 5+ | 101 | 271 | 51 | 1+ | 9+ | 3+ | |
Bouteloua aristidoides | 4 | T | 13+ | 92 | 52 | 331 | 602 | 502 | 711 | 91 | |
Bulnesia foliosa | 2 | Fs | 3+ | 61 | 5+ | 6+ | 101 | 302 | 161 | 5+ | |
Zinnia peruviana | 8 | C | 5+ | 121 | 111 | 21+ | 191 | 301 | 13+ | 3+ | |
Aloysia ovatifolia† | 2 | Fsa | 1+ | 41 | 52 | 81 | 141 | 291 | 32+ | ||
Pappophorum krapovickasii | 2 | H | 4+ | 4+ | 6+ | 121 | 71 | 691 | 3+ | ||
Chloris castilloniana | 2 | H | 1+ | 2+ | 8+ | 5+ | 471 | 31 | |||
Aristida mendocina | 4 | H | 431 | 401 | 261 | 231 | 371 | 521 | 972 | 5+ | |
Gomphrena martiana | 2 | T | 31 | 21 | 61 | 121 | 2+ | 401 | 2+ | ||
Croton bonplandianus | 1 | Fsa | 1+ | 5+ | 5+ | 41 | 6+ | 11+ | 43+ | 2+ | |
Prosopis pugionata | 2 | Ft | 30+ | 32+ | 15+ | 211 | 331 | 141 | 821 | 211 | 7+ |
Vachellia aroma | 2 | Ft | 301 | 161 | 15+ | 6+ | 8+ | 23+ | 631 | 2+ | |
Tricomaria usillo* | 4 | Fs | 18+ | 311 | 201 | 331 | 501 | 431 | 882 | 331 | |
Gaya parviflora | 2 | Fsa | 4+ | 2+ | 41 | 6+ | 14+ | 26+ | 2+ | ||
Euploca mendocina* | 4 | C | 1+ | 11 | 2+ | 2+ | 10+ | 131 | 25+ | 3+ | |
Senna aphylla | 4 | Fs | 28+ | 601 | 481 | 671 | 65+ | 611 | 851 | 601 | |
Gomphrena tomentosa | 2 | C | 4+ | 161 | 13+ | 25+ | 191 | 301 | 411 | 101 | |
Setaria hunzikeri | 2 | H | 161 | 151 | 131 | 291 | 21+ | 29+ | 40+ | ||
Pseudabutilon virgatum | 8 | Fs | 8+ | 12+ | 13+ | 27+ | 15+ | 25+ | 32+ | ||
Justicia gilliesii | 2 | Fsa | 61 | 161 | 251 | 12+ | 101 | 20+ | 291 | 2+ | |
Cordobia argentea | 2 | L | 841 | 52+ | 311 | 331 | 211 | 861 | 882 | 2+ | |
Larrea divaricata | 4 | Fs | 952 | 953 | 822 | 401 | 191 | 822 | 942 | 142 | |
Gouinia paraguayensis | 2 | H | 73+ | 681 | 611 | 541 | 351 | 731 | 791 | 2+ | |
Ximenia americana | 2 | Fs | 29+ | 39+ | 11+ | 62+ | 49+ | 64+ | 62+ | 21+ | 7+ |
Pappophorum phillippianum | 8 | H | 9+ | 181 | 11+ | 48+ | 47+ | 341 | 10+ | 261 | 17+ |
Opuntia sulphurea | 4 | Ko | 36+ | 29+ | 101 | 42+ | 10+ | 9+ | 4+ | 411 | 10+ |
Stetsonia coryne | 2 | Kc | 231 | 131 | 21 | 311 | 11 | 51 | 6+ | 312 | 171 |
Suaeda divaricata | 4 | Fss | 21 | 32 | 6+ | 191 | 2+ | 12 | 722 | 10+ | |
Atriplex lampa† | 4 | Fss | 1+ | 2+ | 2+ | 5+ | 11 | 281 | 101 | ||
Maytenus vitis-idaea | 2 | Fss | 4+ | 111 | 81 | 13+ | 61 | 9+ | 1+ | 431 | 202 |
Lycium infaustum† | 2 | Fss | 1+ | 1+ | 5+ | 6+ | 81 | 21 | 311 | 231 | |
Strombocarpa strombulifera | 4 | Fs | 1+ | 2+ | 191 | 131 | |||||
Neltuma sericantha | 4 | Fs | 3+ | 26+ | 18+ | 29+ | 10+ | 71 | 10+ | 451 | 31 |
Geoffroea decorticans | 2 | Ft | 44+ | 531 | 641 | 461 | 361 | 34+ | 131 | 792 | 231 |
Echinopsis leucantha† | 4 | Ko | 20+ | 18+ | 2+ | 19+ | 12+ | 7+ | 6+ | 41+ | 27+ |
Atriplex argentina† | 8 | Fss | 2+ | 52 | 41 | 121 | 4+ | 792 | 631 | ||
Plectrocarpa tetracantha* | 4 | Fs | 1+ | 51 | 31 | 101 | 131 | 5+ | 3+ | 711 | 631 |
Grahamia bracteata* | 2 | Fss | 3+ | 81 | 17+ | 17+ | 41 | 6+ | 661 | 601 | |
Ehretia cortesia | 4 | Fs | 1+ | 8+ | 3+ | 4+ | 4+ | 2+ | 1+ | 471 | 371 |
Cyclolepis genistoides | 4 | Fs | 1+ | 2+ | 2+ | 2+ | 5+ | 2+ | 451 | 531 | |
Lippia salsa | 4 | Fs | 4+ | 2+ | 2+ | 4+ | 2+ | 1+ | 31+ | 371 | |
Strombocarpa reptans | 2 | Fs | 1+ | 4+ | 4+ | 2+ | 1+ | 381 | 701 | ||
Alternanthera nodifera | 2 | Fsa | 2+ | 6+ | 281 | 43+ | |||||
Heterostachys ritteriana | 8 | Fss | 2+ | 5+ | 832 | ||||||
Allenrolfea patagonica† | 2 | Fss | 51 | 733 | |||||||
Distichlis acerosa* | 2 | H | 2+ | 3+ | 431 |
The ISOPAM analysis divided the data set into four major clusters (Table
Cluster 1: This cluster comprised three vegetation types, two of them including forests, woodlands and shrublands with tree cover (mainly in Type 1.1 Aspidosperma quebracho-blanco forest and in Type 1.3 Prosopis flexuosa woodland) ranging from 30–40% to only scattered emerging individuals. The remaining type includes shrublands mainly dominated by Larrea divaricata (Type 1.2 Larrea divaricata shrubland). The floristic differentiation among these vegetation types was poor and mostly based in changes in the frequency and cover of a common set of species.
Diagnostic species: Justicia squarrosa, Senegalia gilliesii, Sarcomphalus mistol, Tillandsia duratii, Tillandsia xiphioides, Tillandsia aizoides, Tillandsia pedicellata, Tillandsia rectangula, Cardiospermum halicacabum, Lycium elongatum, Aspidosperma quebracho-blanco (for the complete list of diagnostic species, see Suppl. material
This vegetation type was mainly found on flat plains and lower bajadas in the eastern Arid Chaco with higher rainfall, but also in western humid sites located in the proximity of mountain ranges. The plains are homogeneous and have a gently to very gently undulating relief, with slopes of less than 1%. The sediments are very permeable so there is no defined drainage network. The direction of runoff is controlled by the central sectors or bottoms of the bolson. In the higher terrain the soils are loam, well drained and poorly developed (Torripsaments) while in the lower sectors silty-loam soils predominate (typical Camborthides) and the water retention capacity is greater.
This vegetation type included forests with a predominantly open tree canopy dominated by the evergreen tree Aspidosperma quebracho-blanco, and an open to close shrub understory, but also shrublands with scattered emergent trees. Other common deciduous trees were Sarcomphalus mistol, Castela coccinea, Parkinsonia praecox and Prosopis flexuosa. In the understory the most common shrubs were Larrea divaricata, Justicia squarrosa, Senegalia gilliesii, Mimozyganthus carinatus, Celtis pallida, Monteverdia spinosa, Condalia microphylla and Atamisquea emarginata. These shrubs may alternatively dominate in sites where the tree canopy is more open. A relevant feature in these forests was the presence of several epiphytic species of Tillandsia, as well as the bromeliad Deinacanthon urbanianum, as diagnostic of this vegetation type.
Diagnostic species: Larrea divaricata, Neobouteloua lophostachya, Parkinsonia praecox, Sporobolus pyramidatus, Mimozyganthus carinatus, Sida argentina (for the complete list of diagnostic species see Suppl. material
This vegetation type is a rather open to closed shrubland, nowadays highly widespread in the Arid Chaco. In some patches the cover of thorny shrubs may be over 75% turning the vegetation into a closed, inaccessible thicket. This vegetation type occurs in different geomorphological units in the alluvial and eolian plains, but in general on rather elevated sites with silty loam to sandy loam soils, well to excessively drained. This vegetation type frequently constitutes the replacement community of the Aspidosperma quebracho-blanco forest after anthropic disturbances and is therefore often found in similar habitats.
The dominant species is Larrea divaricata, an evergreen shrub widely distributed in north-western Argentina, with emergent individuals of the tree species Aspidosperma quebracho-blanco and Prosopis flexuosa. Alternatively, other deciduous thorny shrubs such as Mimozyganthus carinatus, Parkinsonia praecox and Condalia microphylla, and the evergreen shrub Atamisquea emarginata, may codominate in some relevés. Some C4 grass species such as Neobouteloua lophostachya, Sporobolus pyramidatus, Leptochloa pluriflora, Gouinia paraguayensis and Pappophorum caespitosum, are diagnostic for this vegetation type.
Some of the most representative vegetation types in the Arid Chaco in Central-Western Argentina belonging to: a) Type 1.1 Aspidosperma quebracho-blanco forest in fluvio-eolian plains; b) Type 1.2 Larrea divaricata shrubland (with emergent low trees) in sandy plains; c) Type 2.1 Larrea cuneifolia – Bulnesia retama shrubland in western sites; d) Type 3.1 Mimozyganthus carinatus – Bulnesia foliosa shrubland on stabilized dunes; e) Type 4.1 Suaeda divaricata – Atriplex argentina shrubland in muddy saline areas; and f) Type 4.2 Heterostachys ritteriana – Allenrolfea patagonica succulent shrubland in saline depressions. Photo credits: A. Cuchietti and G. Conti (a); S.R. Zeballos (b); M.R. Zak (e); W.D. Agüero (f).
Diagnostic species: Aloysia gratissima, Leptochloa crinita, Celtis pallida, Prosopis flexuosa (for the complete list of diagnostic species see Suppl. material
The relevés of this vegetation type also occurred in mesic habitats in the eolian and fluvio-eolian plains and fluvial terraces, with a higher frequency in the wettest parts of the study area. Soils are well to moderately well drained and soil texture is predominantly silty to sandy loam. The algarrobal may also be found in a wide range of habitats across the entire Arid Chaco, such as lower sites with silty loam texture and the drainage are, at least temporarily, poor. The dominant physiognomy was that of low woodlands and tall shrublands.
The dominant species in the canopy was Prosopis flexuosa, a deciduous tree diagnostic to this cluster, but also Aspidosperma quebracho-blanco may codominate in some mixed woodland patches. The floristic composition of the shrub layer is similar to that of vegetation types 1.1 and 1.2, with Aloysia gratissima, Celtis pallida, Atamisquea emarginata and Condalia microphylla as diagnostic in this vegetation type. In lower sites, Celtis pallida, Lycium chilense and Lycium tenuispinosum may dominate in the understory.
Cluster 2: This cluster included shrublands dominated by Larrea cuneifolia and was further divided in two vegetation types (i.e., 2.1 and 2.2): the first one was characterized by widely distributed shrublands while the second one is confined to the drier parts of the study area and included characteristic species of the Monte deserts and semi-deserts.
Diagnostic species: Larrea cuneifolia, Setaria pampeana, Ximenia americana, Pappophorum phillippianum, Opuntia sulphurea, Stetsonia coryne (for the complete list of diagnostic species see Suppl. material
These shrublands were distributed across most of the Arid Chaco, especially in low areas in the alluvial and aeolian plains with moderately well to somewhat imperfectly drained soils consisting mainly of fine sediments and sands. The edaphic conditions are controlled by the micro- and meso-relief: in the higher sectors the soils have a higher percentage of sands and are less saline (typical Torripsaments), while in the intermediate and lower levels the soils are finer and somewhat saline (natric Camborthides).
The predominant physiognomy was that of a low and open shrubland. In sites where soils are well drained, Aspidosperma quebracho-blanco was frequent, also forming small forest patches. The dominant shrub was Larrea cuneifolia, an evergreen shrub, accompanied by other diagnostic species such as the deciduous shrub Ximenia americana, the columnar cacti Stetsonia coryne and the opuntioid cacti Opuntia sulphurea. Also, the aphyllous shrub Senna aphylla, though not diagnostic for this vegetation type, reached high cover values. In low-lying areas of the plains, where water retention capacity increases promoting the accumulation of salts by evaporation, Stetsonia coryne was dominant forming characteristic communities locally named “cardonales”. The presence of this candelabra-like cacti is also conspicuous in some Aspidosperma quebracho-blanco forests (“quebrachal with cardón”), also included in this vegetation type.
Diagnostic species: Larrea cuneifolia, Bulnesia retama, Aristida adscensionis, Cottea pappophoroides, Bougainvillea spinosa, Bouteloua aristidoides (for the complete list of diagnostic species see Suppl. material
This vegetation type was commonly found in the western driest parts of the Arid Chaco, on loessoid plains with well drained, loam to sandy loam soils. Some relevés were also recorded in the eastern part of the study area close to saline depressions and muddy saline areas, on imperfectly drained soils.
The characteristic physiognomy was that of low shrublands dominated by Larrea cuneifolia; however, many relevés exhibited also scattered emergent individuals of Aspidosperma quebracho-blanco. The main floristic difference with vegetation type 2.1 is the presence of several shrubs and small trees typical of the Monte Phytogeographical Province such as Bulnesia retama, Bougainvillea spinosa and Zuccagnia punctata, being the first two diagnostics for this vegetation type. Also, the presence of C4 grasses widely distributed across northwestern arid Argentina, such as Bouteloua barbata, Munroa mendocina and Bouteloua aristidoides, among others, characterized this vegetation type.
Cluster 3: This cluster comprised the typical vegetation of highly disturbed sites, split into two vegetation types (3.1 and 3.2) and embracing open shrublands with high values of bare soil (“peladares”) and a heterogeneous mosaic of xerophytic/thorny scrublands (“fachinales”), with the absence of trees or only very scattered individuals. We should note that the floristic differentiation between these two vegetation types was very poor (Table
Diagnostic species: Bulnesia foliosa, Zinnia peruviana, Aloysia ovatifolia (for the complete list of diagnostic species see Suppl. material
This vegetation type is widespread across the entire study area in flat to moderately sloped plains of different origin on well to excessively drained sandy loam soils formed from deposits of alluvial and eolian materials. On the flatter sites, this type may occupy areas where Aspidosperma quebracho-blanco forests probably thrived in the past. Patches of this vegetation type were also observed on stabilized dunes and distal parts of the bajadas, on excessively drained, sandy soils.
The prevailing physiognomy was that of a closed and medium height shrubland or scrubland, usually with scattered individuals of tree species such as Aspidosperma quebracho-blanco, Prosopis flexuosa, and Parkinsonia praecox, among others. Though not diagnostic for this vegetation type, the evergreen shrub Larrea divaricata and the deciduous shrub Mimozyganthus carinatus, were usually the dominants along with many other shrubs; however, in the central and northern parts of the study region, Bulnesia foliosa predominated. As in most of the clusters, C4 grasses such as Digitaria californica, Neobouteloua lophostachya, Sporobolus pyramidatus, Aristida adscensionis, Aristida mendocina and Gouinia paraguayensis were frequent in the relevés of this vegetation type.
Diagnostic species: Aristida mendocina, Pappophorum krapovickasii, Gomphrena martiana, Chloris castilloniana, Croton bonplandianus, Prosopis pugionata, Tricomaria usillo, Vachellia aroma, Senna aphylla, Gomphrena tomentosa (for the complete list of diagnostic species see Suppl. material
This vegetation type was mainly found in the central and western part of the Arid Chaco, in dune fields or sand dunes, semi-stabilized or stabilized, alternating in the lowlands with forests and shrublands described in Cluster 1. The dunes are generally low, with maximum heights between four and six meters, and the materials that gave rise to the dunes have been deposited by the wind. The soils have a sandy loam to sandy texture and are very poorly developed. They are excessively drained and are very susceptible to erosion when vegetation cover is lost. In the lowlands the soils have a loamy texture and are somewhat more developed (Camborthides ustólicos); this development indicates a relatively long time of stabilization of the system.
The prevailing physiognomy was that of open shrublands, though shrub cover reached up to 80% in some relevés. The number of tree species was low and sometimes only individuals of Aspidosperma quebracho-blanco were present. The dominant shrubs were the diagnostic shrubs Tricomaria usillo (with ephemeral leaves) and Senna aphylla (a leafless species) and the no diagnostic shrubs Larrea divaricata and Mimozyganthus carinatus, while the woody climber Cordobia argentea was also relevant. The C4 grasses Pappophorum krapovickasii, Chloris castilloniana, Aristida mendocina and Gouinia paraguayensis exhibited high constancy values in this vegetation type.
Cluster 4: This cluster included halophytic and sub-halophytic vegetation and was further split into two groups: vegetation type 4.1 represented mostly the vegetation locally known as “barreales” (muddy saline areas), while vegetation type 4.2 comprised succulent shrublands in the outer perimeter of salt flats and playas. Both vegetation types were characterized by halophylous and sub-halophylous species widely distributed across the whole Chaco and Monte Phytogeographical Provinces.
Diagnostic species: Suaeda divaricata, Atriplex lampa, Maytenus vitis-idaea, Lycium infaustum, Prosopis sericantha, Geoffroea decorticans, (for the complete list of diagnostic species see Suppl. material
This vegetation type was found across the entire Arid Chaco, but confined to the margins of low-lying areas, mostly muddy saline areas (barreales), and also in the outer perimeter of playas and salinas. The mudflats receive runoff water, loaded with fine sediments that accumulate in the lower areas. Therefore, the soil is of alluvial origin, formed by the deposition mainly of silts and clays, and the drainage is almost impeded.
From the central, more saline sector of barreales and playas devoid of vegetation, up to the contact with the fluvio/aeolian plains, a vegetation zonation with communities gradually replacing each other, developing a series of rings around the salty playas and barreales, was found. This sequence comprised the following communities: a) open succulent shrubland with Heterostachys ritteriana and Allenrolfea patagonica (see vegetation type 4.2); b) “zampales” (dominated by Atriplex lampa), “cardonales” (dominated by the candelabra-like columnar cacti Stetsonia coryne), “cachiyuyales” (dominated by other species of Atriplex), and, mostly, shrublands dominated by Suaeda divaricata and some succulent species; c) mixed shrublands (“chañarales”, “latales”) with sub-halophylous species (they tolerate moderate salt concentrations) and other taxa shared with clusters 1 and 2, but frequently dominated by shrubs such as Geoffroea decorticans and Mimozyganthus carinatus; and d) Aspidosperma quebracho-blanco forests and Larrea divaricata shrublands but still with the presence of a few halophyllous species. This vegetation type included mostly communities b) and c). Some relevés were also found in the base of sand dunes nearby saline depressions. In addition to the diagnostic species, the more relevant taxa were: Stromobocarpa strombulifera, Strombocarpa reptans, Plectocarpa tetracantha, Grahamia bracteata, Cortesia cuneifolia, Cyclolepis genistoides, Lippia salsa, and Alternanthera nodifera. All of these species are shared with vegetation type 4.2.
Diagnostic species: Heterostachys ritteriana, Allenrolfea patagonica, Distichlis acerosa, (for the complete list of diagnostic species see Suppl. material
This vegetation type was found in the perimeter of salt lakes and salt flats, on soils frequently flooded or wet during the rainy season (summer), but dry, hard and with white salt efflorescence in winter and spring. The salinas have all the characteristics of ephemeral salt lakes, originated by the conjunction of structuring (closed basins) and climatic (evapotranspiration higher than the water supply) factors. The water supply is provided by ephemeral streams that flow only occasionally during the rainy season. The degree of salinity is maximum among the nine vegetation types, limiting the establishment and development of plant species. However, a distinction is made between (a) the saline depressions with clay-loam sediments, occasionally floodable, with polygonal drying cracks and a superficial saline crust, and (b) sandy hills between the lower areas (bajos). This variability in the salinity conditions of the soils causes changes in the composition and physiognomy of the vegetation.
This vegetation type included open succulent shrublands (community a) in the zonation described in vegetation type 4.1, dominated by Heterostachys ritteriana and Allenrolfea patagonica, both succulent shrubs that tolerate high salt concentrations in the soil.
The vegetation types differed significantly in their mean species richness per relevé (F(8, 644) 13.09; p < 0.0001; Figure
In all the vegetation types described, significant differences in the representation of the different chorotypes were observed (Statistics and p values for the response variables analyzed are reported in Suppl. material
The average total cover differed significantly among the vegetation types (Statistics and p values for the response variables analyzed are reported in Suppl. material
Life forms spectra varied significantly among vegetation types (Statistics and p values for the response variables analyzed are reported in Suppl. material
The ISOMAP ordination (Figure
Isometric feature mapping plot (ISOMAP), based on Bray-Curtis dissimilarity of 654 relevés × 439 plant species matrix corresponding to the Arid Chaco, Central-Western Argentina. Vegetation types codes as in Table
In this study we classified the vegetation of the Arid Chaco located in Central-Western Argentina and recognized four main clusters comprising nine vegetation types. With the exception of the halophytic vegetation, the floristic differentiation among the remaining vegetation types described was quite poor. The vegetation types differed in the relative abundance of species and in their physiognomy, ranging from forests to open shrublands. Despite the floristic composition of the different vegetation types was related to elevation, climate and soil variables, the strong impact of human activities on the original Aspidosperma quebracho-blanco forests could have become one of the main drivers determining the physiognomy and composition of the current vegetation of the Arid Chaco. Although the number of species restricted to the Arid Chaco was quite low, the most relevant chorotype included species with Western and Eastern Chaco distribution, conferring a clear Chaquenian identity to this area and discriminating it from other phytogeographic units. Therefore, the floristic composition of the vegetation types described revealed numerous species in common with other sectors of the Chaco of northern Argentina, Bolivia and Paraguay.
The most recent phytogeographical scheme at the national level recognized the Arid Chaco as a separate unit, the “xerophyte forest with Aspidosperma quebracho-blanco in transition to steppe” (
Many authors have reported that different types of shrublands with a similar composition to ours, constitute the current most widespread vegetation type in the Arid Chaco (
At the scale of the whole Chaco, our vegetation types share numerous species with communities described for the northern Paraguayan (
The vegetation types of the Arid Chaco also maintain floristic relationships with the Northwestern Chaco Sector of the Bolivian Chaco (
The vegetation types occurring in saline habitats correspond to the phytosociological class Suaedetea divaricatae and the order Stenodrepano-Prosopietalia reptantis, described for the warm saline desserts of north-western Argentina (
As stated by
As stated above, one of the relevant physiognomic features of the Arid Chaco is the dominance of shrublands and scrublands. However, some physiognomic variations could be observed among the different shrubland types, possibly related to edaphic variations and different intensities of disturbance (
In accordance with the dominant physiognomy in the Arid Chaco, shrubs were the most prominent life form in terms of cover and average number of species per relevé in all the vegetation types. Different kinds of shrubs (microphyllous, both evergreen and deciduous, aphyllous, thorny and non-thorny) have been reported for the Arid Chaco vegetation (
Previous studies revealed that not only plant diversity, but also arthropod richness, was reported highest in forests and woodlands in relation to scrublands and very open shrublands in the Arid Chaco (
It has been proposed that the low number of endemism reported for the Gran Chaco was related to the lack of geographical barriers separating the Chaco from other phytogeographical regions such as the Espinal, the Pampa and the Cerrado (
The ISOMAP ordination revealed some trends of floristic change that were weakly, though significantly, related to elevation, climate and soil. The limited explanatory power of the bioclimatic variables and elevation suggested that the entire Arid Chaco region exhibits a certain climatic (mainly thermal) uniformity.
Previous surveys in other sectors of the Gran Chaco have also revealed the influence of climate, geomorphology and soils, mostly soil drainage, on the composition, physiognomy and distribution of vegetation (
Schematic representation of the toposequence observed from the distal part of the bajadas to the saline bottom of the bolsón in the Arid Chaco of Central-Western Argentina. a) sequence from the bajada to the bottom of the bolsón showing the following vegetation types: 1) Mimozyganthus carinatus – Bulnesia foliosa shrubland; 2) Aspidosperma quebracho-blanco forest; 3) Larrea divaricata shrubland; 4) Prosopis flexuosa woodland; 5) Tricomaria usillo shrubland; 6) Stetsonia coryne shrubland; and 7) Heterostachys ritteriana – Allenrolfea patagonica shrubland. b) Detailed vegetation profile of saline environments: 1) Aspidosperma quebracho-blanco forest with Stetsonia coryne; 2) Stetsonia coryne shrubland; 3) Suaeda divaricata – Atriplex argentina shrubland; 4) Geoffroea decorticans shrubland; Heterostachys ritteriana – Allenrolfea patagonica shrubland; 5) saline playa. Art work: Fernando A. Gallará.
Studies of the vegetation in specific sites of the Arid Chaco associated the spatial distribution of vegetation to geomorphological and edaphic characteristics (
Saline soils occupy large areas in Central-Western Argentina and in some places, they are on the rise, mainly due to the use of inadequate management techniques (
Despite our considerations on the relationships between vegetation distribution and soils, a definitive explanation of the spatial patterns of vegetation will require the detailed study of the effects of disturbance (logging, livestock and fire) on vegetation cover, as well as the variability of soils at a higher spatial resolution than that used in this study.
It has been proposed that the original landscape of the Chaco region consisted of patches of old-growth forest alternating with patches of grasslands resulting from fires set by the Amerindian people for their hunting activities (
Stages in the degradation (regressive succession) of vegetation cover in the Arid Chaco. The figure shows in a simplified schematic form the regressive sequence from the Aspidosperma quebracho-blanco forest (vegetation type 1.1) to the vegetation types exhibiting the greatest degradation of vegetation (the peladares and fachinales represented by vegetation types 3.1 and 3.2). Vegetation types are those reported in Table
The, preliminary outline of major trends of change representing different degradation stages of the original forest is presented in Figure
In this study we provide baseline information concerning the floristic heterogeneity of the vegetation of the Arid Chaco in Central-Western Argentina. Despite the studied region still includes communities in a good conservation status, most of the territory exhibits secondary communities (mainly shrublands and scrublands) replacing vegetation Type 1.1 Aspidosperma quebracho-blanco forest, as a consequence of human activities. Old growth forests are restricted to areas free from disturbances for decades or to protected areas (
The database of occurrence records of the Arid Chaco vegetation in Central-Western Argentina is available in Suppl. material
M.R.C, A.T.R.A, J.M., F.N.B. and L.J.B. planned the research; M.R.C., A.T.R.A., J.M., F.N.B., J.J.C., S.R.Z., L.J.B., W.D.A., R.J.A., M.G.A., D.N.A., A.Q. and R.E.Q. conducted the field work; S.R.Z. and M.R.C. performed the statistical analyses; S.R.Z. prepared the tables, figures 2 to 4 and the Supplementary materials; D.S.A. performed Figure
The authors wish to acknowledge the financial support of the following institutions: the National University of La Rioja, the National Institute of Agricultural Technology, the National University of Catamarca, the National University of San Juan, the National Scientific & Technological Research Council (CONICET), the National University of Córdoba (SECyT – UNC, Programme PRIMAR), the National University of Río Cuarto (SECyT), the Córdoba Province Ministry of Science and Technology (MINCyT). The authors declare no conflict of interest related to this publication. We thank the Editor and reviewer who made valuable suggestions and commented on the first versions of the manuscript.
Individual relevé matrix comprising 439 species × 654 plots (Table S1.1) recorded in the Arid Chaco, Central-Western Argentina.
Appendix S1: Distribution of relevés belonging to different vegetation types.
Appendix S2: Sample-size-based and coverage-based rarefaction, extrapolation sampling curves jointly with species observed and non-parametric estimators of species richness.
Appendix S3: National and regional percentage of endemic species (mean ± sd) per vegetation type.
Appendix S4: Mean (± sd) species percentage per relevé of all chorotypes and non-natives species.
Appendix S5: Mean cover and height with their respectively standard error and maximum and minimum cover values recorded for tree, shrub and herb layers.
Appendix S6: Life forms percentage (mean ± sd) per vegetation type in the Arid Chaco.
Floristic table. Table S1.1: extended synoptic table.