Floristic structuring of woody plants from the Chaco in light of abiotic factors

Abstract

The phytogeographic domains are structured by different geoclimatic (abiotic) factors, such as altitude, temperature and precipitation. Thus, our goal was to investigate the possible abiotic factors that are related to the floristic richness of the tree-shrub component throughout the Chaco remnants. This study is based on data of the presence and absence of species in different remnants related to geoclimatic data. In addition, World Wildlife Fund shapes were used to differentiate dry and wet Chaco. The database aggregated 36 areas and 522 species. The different clusters obtained by floristic affinity between groups in Chaco was mainly related to temperature, altitude and seasonality of precipitation. In remnants where the seasonality of precipitation and altitude were more evident, there is greater dissimilarity in the composition of the tree-shrub vegetation. On the other hand, the temperature gradient explained the floristic homogeneity in the humid Chaco. Thus, the richness of the tree-shrub component of the Chaco is mainly attributed to three geoclimatic factors. Dry Chaco presents greater floristic dissimilarity compared to Humid Chaco. Still, the seasonality of the precipitation and the altitude play a dominant role in the structuring of trees and shrubs in the Dry Chaco and the temperature in the most homogeneous formations that integrate the humid Chaco.

Keywords:
floristic similarity; precipitation; temperature; vegetation

Resumo

Os domínios fitogeográficos são estruturados por diferentes fatores geoclimáticos (abióticos) tais como altitude, temperatura e precipitação. Assim, nosso objetivo foi investigar quais os possíveis fatores abióticos que estão relacionados à riqueza florística do componente arbóreo-arbustivo em remanescentes de Chaco, ao longo de sua extensão. O estudo é baseado em dados de presença e ausência de espécies em diferentes remanescentes relacionados com dados geoclimáticos. Somado a isso, foram utilizados shapes da World Wildlife Fund para diferir o Chaco seco do úmido. O banco de dados agregou 36 áreas e 522 espécies. A afinidade florística entre os grupos foi relacionada principalmente à temperatura, altitude e sazonalidade da precipitação. Em remanescentes onde a sazonalidade da precipitação e a altitude foram mais evidenciados, há maior dissimilaridade na composição da vegetação arbóreo-arbustiva. Por outro lado, o gradiente de temperatura explicou a homogeneidade florística no Chaco úmido. Assim, a riqueza do componente arbóreo-arbustivo do Chaco é atribuída principalmente a três fatores geoclimáticos. O Chaco seco apresenta maior dissimilaridade florística comparado ao Chaco úmido. Ainda, a sazonalidade da precipitação e a altitude têm papel preponderante na estruturação de árvores e arbustos no Chaco seco e a temperatura nas formações mais homogêneas que integram o Chaco úmido.

Palavras-chave:
similaridade florística; precipitação; temperatura; vegetação

Introduction

The distribution of species among different phytogeographic domains can be evaluated by considering abiotic (geoclimatic) filters, such as soil type, precipitation and topography. Different filters can act in species selection according to the scale ( Morello 1967; Lewis 1991; Oliveira-Filho & Fontes 2000), resulting in a heterogeneous environment. Thus, it is essential to consider scale when analyzing patterns in biological communities ( Lewis 1991; Oliveira-Filho & Fontes 2000; Vellend 2010). Among spatial scales, local scales are often considered as those up to 102 km2 in area and landscape or regional scales are those between 102 km2 and 108 km2 ( Gurevitch et al. 2009 ).

The seasonally Dry Forests of South America (Caatinga, Chaco, Cerrado and Atlantic forest) are influenced by a gradient rainfall seasonality and/or decreasing soil moisture content ( Oliveira-Filho et al. 2006 ). However, there is also substantial influence of temperature gradients, soil fertility and fire frequency, which results in distinct phytophysiognomies ( Oliveira-Filho et al. 2006 ; Neves et al. 2015 ).

Among the different domains in South America, the Chaco is distinguished by its marked climatic seasonality ( Pennington et al. 2000 ). The Chaco is a large sedimentary plain of ~1,000,000 km2, extending north and south of the Tropic of Capricorn over northern Argentina, western Paraguay, eastern Bolivia and part of southeastern Brazil ( Adamoli et al. 1990 ). The variation in annual rainfall separates the domain into two sectors: dry (500 to 700 mm.yr-1) and wet (over 1,200 mm.yr-1), which confers distinct floristic and phytophysiognomic types within the same domain ( Ramella & Spichiger 1989; Adamoli et al. 1990 ; Prado 1993; Navarro et al. 2006 ). The dry portion occurs in Argentina, Bolivia, and Paraguay and the wet one in Argentina, Brazil, and Paraguay. According to Bueno et al. (2017 ), ecotonal regions in Paraguay present distinct species richness in the Chaco due to rainfall and soil, among other factors.

Another factor that likely interferes in the vegetation composition of the Chaco is altitude. In general, the topography of the Chaco is flat terrain and in its eastern portion the altitude does not exceed 100 m ( Hueck 1972). However, in Chaco Serrano remnants the altitude varies from 1,000 to 2,600 m s.n.m. and represents 10% of Gran Chaco ( Biani et al. 2006 ; Hernández & Giménez 2016). This portion of the Chaco is poorly studied, and little is known about the precise delimitation of its territory ( Morales et al. 2019 ) and the influence of abiotic factors.

Regarding temperature, in the Chaco there is a decreasing gradient from north to south ( Hueck 1972). In Santa Cruz, Bolivia and Porto Murtinho in Brazil, the average annual temperature is 25 °C ( Hueck 1972; CEMTEC - Monitoring Center for Weather, Climate and Water Resources of the state of Mato Grosso do Sul, Brazil), while in Paraguay’s central Chaco average annual temperatures between 23 and 24 °C are observed ( Hueck 1972). In Santiago Del Estero, in northwest Argentina, it is 20.6 °C ( Hueck 1972). In Tucumán, at the western limit of Chaco and in Córdoba, at the southern limit, 19 °C and 17 °C are recorded, respectively ( Hueck 1972).

In the Neotropics the precipitation contributes positively to the diversity of species ( Leigh Jr. et al. 2004 ; Esquivel-Muelbert et al. 2017 ). In seasonally dry tropical forest (SDTF) and Chaco woodland in the South American Dry Diagonal, the temperature followed by the precipitation contributed to the characterization of vegetation ( Neves et al. 2015 ). The floristic affinity in the Argentine Chaco can be attributed to temperature variations in the north/south direction and to the east/west humidity gradient ( Lewis 1991; Leigh Jr. et al. 2004 ). In the Paraguayan Chaco, precipitation contributes to species richness according to soil drainage and possible flooding events ( Navarro et al. 2006 ). Studies of abiotic factors in the structuring of the woody vegetation of Chaco in Brazilian territory have revealed the important effect soil fertility has on vegetation ( Assunção et al. 2020 ; Baptista et al. 2020 ). The Brazilian Chaco has the smallest territorial extension, ranging from 0.8% to 8% ( Hueck 1972; Abdon et al. 2007 ) within the domain.

It is important to highlight that floristic structuring in the Argentine Chaco communities can be attributed to variations in the temperature and humidity gradient ( Lewis 1991; Leigh Jr. et al. 2004 ), while precipitation and possible flooding events are determinants in the Paraguayan Chaco ( Navarro et al. 2006 ) and the altitudinal variations affect floristic structuring in the Chaco Serrano ( Biani et al. 2006 ). However, temperature was the factor that most influenced the flora of Chaco and SDTF ( Neves et al. 2015 ). Based on this information, we assumed that Chaco is a heterogeneous domain and that abiotic factors can act on different scales in floristic structuring throughout Chaco remnants. Therefore, our goal was to investigate which and how abiotic factors are related to floristic structuring of woody vegetation (tree/shrubs) throughout the Chaco remnants. We expected that, among the variables, the higher the precipitation, the greater the dissimilarity in the vegetation within the entire South American domain since various studies ( Ramella & Spichiger 1989; Lewis 1991; Leigh Jr. et al. 2004 ; Navarro et al. 2006 ) showed that precipitation directly influenced the structuring of vegetation in different regions of the domain.

Materials and Methods

Study areas and data collection

The study considered tree and shrub species recorded in floristic and phytosociological studies carried out in Chaco ( Fig. 1). A database was obtained from studies performed in Brazilian Chaco remnants ( Noguchi et al. 2009 ; Seleme 2010; Padilha 2011; Lima 2012; Carvalho & Sartori 2014; Assunção et al. 2020 ). Additionally, 29 other floristic lists from Oliveira-Filho (2014) were added to the data bank. Scientific names were checked. From the database, only presence and absence data for tree and shrub species were used. The database aggregated 36 areas and 522 species. Geoclimatic data were obtained for each Chaco domain remnant from the WORLDCLIM ( Hijmans et al. 2005 ) with a resolution of 30 seconds.

Data analyses

The Jaccard Similarity Index ( Mueller-Dombois & Ellenberg 1974) was used to calculate the similarity between the compiled areas. The grouping method used was the Unweighted Pair-Group Method (UPGMA), and the results were converted into a dendrogram ( Sneath & Sokal 1973). Dry and humid Chaco were classified by plotting remnant coordinates in the QGIS Program using the shapes available at Olson et al. (2001 ) and the IBGE for Brazil ( IBGE 2015). An ANOSIM nonparametric analysis ( Clarke 1993) with 10,000 permutations was carried out to verify the significance of the groups formed. Environmental variables were selected by Principal Component Analysis (PCA) with only the least correlated between each other maintained (mean annual temperature, maximum temperature of the coldest month, temperature of the three coldest months of the year, annual precipitation, seasonality of precipitation and altitude). The climate gradient and the altitude of the remnants along the plots were obtained using the Canonical Correspondence Analysis (CCA). The analyses were performed using R software ( R Core Team 2014).

Figure 1 –
a. map of South America. b. emphasis on the Chaco domain represented by the dry (blue) and humid (green) sector in relation to precipitation. c. details of some remnants measured. 1–36 represent different listings of woody species of Chaco. BR = Brazil; AR = Argentina; BO = Bolivia; PY = Paraguay.

Results

The evaluated remnants of Chaco domain formed five groups (values between S = 0.12 and S = 0.24), differing from one another (ANOSIM: R = 0.81, p < 0.0001), as seen in the dendrogram ( Figs. 1; 2). The floristic affinity between groups was mainly related to temperature, altitude and seasonality of precipitation ( Fig. 3).

Figure 2 –
Similarity dendrogram (Jaccard index) with UPGMA classification applied to woody species of the Chaco domain (Brazil, Bolivia, Argentina and Paraguay). 1–36 = each number in this range represents a list of the presence and absence of tree and shrub species obtained in the Chaco domain. From 1 to 28: Oliveira-Filho (2014); 29: Noguchi et al. (2009); 30: Carvalho & Sartori (2014); 31: Lima (2012); 32: Padilha (2011); 33: Seleme 1 (2010); 34: Seleme 2 (2010); 35: Assunção (2020 - Santa Vergínia Farm); 36: Assunção (2020 - Retiro Conceição Farm) (the index yielded a 0.71 cophenetic correlation coefficient). G1 to G5 represent the groupings.

The first two axes of the CCA covered 63.3% of the total variation. The areas represented by the G1 cluster were related to the seasonality of precipitation and altitude (from 130 to 867 m); G2 annual mean temperature; G3 mean annual temperature and average rainfall; G4 average temperature of the coldest month, average annual temperature, temperature of the three months plus cold and precipitation; and G5 all the previous plus precipitation, all the details of which can be found in Figures 1, 2 and 3 and Table 1. Using the World Wildlife Fund shapes that separate the Chaco domain according to precipitation, the floristic affinity indicated that the drier portion was more heterogeneous regarding floristic affinities than the humid portion.

Discussion

Temperature, seasonality of precipitation, and altitude were critical factors for determining floristic affinity among woody plants in different Chaco remnants. However, the affinity did not follow the same pattern observed for floristic affinity in the Argentine Chaco, where temperature contributes to the affinity of remnants in the north-south direction and precipitation contributes to it in the east-west direction ( Lewis 1991).

The remnants with the highest elevation were those closest to the Andes, which are higher in altitude than the other areas ( Biani et al. 2006 ). The altitude of the remaining remnants ranged from 62 to 867 m. In this range, the remnants between 130 and 867 m contributed to explain the heterogeneity of the flora of trees and shrubs together with the seasonality of precipitation. When evaluating the geographical ecology of the Arecaceae at different scales, Eiserhardt et al. (2011 ) observed that the altitude corresponded to the composition on a local and regional scale, possibly through the effects in conjunction with the hydrology and dynamics of the forest and soil. In other words, the altitude, together with other factors, likely contribute to explain the floristic dissimilarity on a regional scale.

Studies related to taxonomic diversity in Chaco remnants under different conditions and altitudes showed variations. In the Chaco Serrano in an intermediate successional stage above 1,800 m altitude, the Shannon (H’) index was 2.65 ( Hernández & Giménez 2016). In different remnants of “bosque chaqueño serrano,” whose maximum altitude was 580 m above sea level, Varela et al. (2002 ) observed different values ​​of diversity: “sur-Oeste” (H’ = 2.97), “fondo de quebradas” (H’ = 2.74), “cumbral area” (H’ = 2.54), “piedemonte” (H’ = 2.32) and “exposición nor-este” (H’ = 2.30). In “Savana Estépica Arborizada” the diversity was H’ = 2.02 ( Lima 2012) and in “Savana Estépica Florestada” the values ​​were higher than H’ = 3 ( Padilha 2011), both with an altitude of approximately 80 m. As a result, the diversity of the woody plant composition can likely be attributed to greater altitudinal differences ( Biani et al. 2006 ), as well as to small elevations and depressions (between 10 to 20 m) ( Ramella & Spichiger 1989), which contribute to vegetational particularities in the Chaco. However, it is important to note that the data from the studies mentioned above were obtained using different methods and performed on different phytophysiognomies.

Lewis (1991) pointed out that the more extreme the environmental factors, the more homogeneous Chaco rainforests are. Our evidence indicates that temperature contributed to the homogeneity found throughout the domain, unlike what was found when analyzing Chaco species in conjunction with SDTF, in which temperature explained the dissimilarity in woody plant composition ( Neves et al. 2015 ). Therefore, the peculiarities of Chaco are observed when studied separately from the other domains present in South America.

As in other forest formations in Japan, diversity was negatively related to temperature ( Mori 2018), which differed from the results observed by Butterfield & Munson (2016), who found that community structuring was attributed to temperature variation throughout a dry region in the southwestern United States.

Interestingly, Argentina presents different phytophysiognomies throughout its territory ( Giménez et al. 2007 ) and presents Chaco flora that is most similar to the Bolivian flora (G1 of Fig. 2),

Figure 3 –
Canonical Correspondence Analysis showing the ordering of the first two axes of 36 areas with the presence and absence of Chaco tree and shrub species related to environmental variables (bio_1 = average annual temperature; bio_6 = average temperature of the coldest month; bio 11 = temperature of the coldest three months of the year; bio 12 = annual precipitation; bio 15 = precipitation seasonality; alt = altitude). From 1 to 28: Oliveira-Filho (2014); 29: Noguchi et al. (2009); 30: Carvalho & Sartori (2014); 31: Lima (2012); 32: Padilha (2011); 33: Seleme 1 (2010); 34: Seleme 2 (2010); 35: Assunção (2020 - Santa Vergínia Farm); 36: Assunção (2020 - Retiro Conceição Farm). Blue circle represents the G1, Yellow G2, Red G4 and Green G5. G1 to G5 represent the groupings obtained from the Similarity dendrogram.

Table 1 –
Relation of trees and shrubs in different remnants of Chaco domain, separated by humid and dry sectors according to precipitation in different countries and coordinates.

Remnant	Longitude	Latitude	Sector	Country
1	-57.914	-21.449	Humid	Brazil
2	-57.531	-21.42	Humid	Brazil
3	-57.884	-21.498	Humid	Brazil
4	-57.835	-21.599	Humid	Brazil
5	-57.514	-22.034	Humid	Brazil
6	-58.759	-23.626	Humid	Paraguay
7	-57.573	-25.244	Humid	Paraguay
8	-57.938	-26.509	Humid	Paraguay
9	-59.476	-27.409	Humid	Argentina
10	-58.753	-27.428	Humid	Argentina
11	-60.22	-29.488	Humid	Argentina
12	-57.892	-21.445	Humid	Brazil
13	-59.933	-18.253	Dry	Bolivia
14	-60.824	-18.513	Dry	Bolivia
15	-59.78	-21.316	Dry	Bolivia
16	-61.497	-23.084	Dry	Bolivia
17	-63.222	-18.915	Dry	Bolivia
18	-62.423	-19.068	Dry	Bolivia
19	-60.521	-19.426	Dry	Bolivia
20	-61.914	-20.116	Dry	Bolivia
21	-61.873	-24.628	Dry	Argentina
22	-63.317	-24.647	Dry	Argentina
23	-61.945	-25.966	Dry	Argentina
24	-63.723	-28.249	Dry	Argentina
25	-62.91	-19.659	Dry	Bolivia
26	-63.689	-21.209	Dry	Bolivia
27	-63.234	-20.546	Dry	Bolivia
28	-66.528	-31.288	Dry	Argentina
29	-57.885	-21.701	Humid	Brazil
30	-57.782	-21.685	Humid	Brazil
31	-57.883	-21.700	Humid	Brazil
32	-57.845	-21.701	Humid	Brazil
33	-57.897	-21.723	Humid	Brazil
34	-57.763	-21.705	Humid	Brazil
35	-57.885	-21.701	Humid	Brazil
36	-57.783	-21.685	Humid	Brazil

a fact corroborated by López (2003), who observed that the flora of the Bolivian dry valleys was similar to the Andean flora in northern Argentina. In other words, even with different phytophysiognomies, the specific regions with less precipitation were more similar compared to the others that were analyzed.

In the clusters where the seasonality (G1) and precipitation average (G3) was higher ( Figs. 2; 3), we observed greater heterogeneity in species composition. Leigh Jr. et al. (2004 ) pointed out that rainfall contributes to the higher tree species diversity in the tropics. The positive relationship between tree richness and precipitation also occurred in remnants in the Neotropics ( Esquivel-Muelbert et al. 2017 ). The woody vegetation of Paraguay ( Oakley & Prado 2011) is distributed according to the climatic seasonality marked by a well-defined dry season with varied duration.

It is interesting to note that for Leguminosae in the Chaco Morales et al. (2019 ) mentioned that the highest percentages of exclusive specific and infraspecific taxa correspond to the dry Chaco and sierra Chaco (23% and 12%, respectively, and ca. 16% in species growing simultaneously in both subregions), whereas the percentage in humid Chaco is lower (ca. 22%). The same authors mentioned that this is likely due to their more demanding and harsh environmental conditions which limit the dispersion of generalists or intrusive-invading species. Another aspect to be considered refers to the seasonality of precipitation that can be considered in different time scales (days, years, decades) or seasonal scales (seasons), with the seasonality of precipitation becoming unpredictable in extreme climates ( Gurevitch et al. 2009 ).

In addition, the formation of two groups (G2 and G3 in Fig. 2) in Paraguay is likely justified by soil fertility ( Oakley & Prado 2011). The proximity of these groups with the Brazilians groupings (G4 and G5 in Fig. 2) is due to the influence of the Missiones Nucleus in Brazil, which allows common species ( Oakley & Prado 2011). Also, the clusters formed in Brazil are probably due to their geographical proximity to Cerrado and Dry Forests.

The species shared with these areas in Mato Grosso do Sul include Astronium urundeuva (M.Allemão) Engl., Handroanthus heptaphyllus (Vell.) Mattos, Anadenanthera colubrina (Vell.) Brenan, Guazuma ulmifolia Lam., Helicteres brevispira A.St.-Hil., Psidium guajava L., Chomelia obtusa Cham. & Schltdl. and Randia armata (Sw.) DC. The Chaco portion with the highest mean precipitation also presents more exorheic rivers ( Ramella & Spichiger 1989); that is, diaspores could come from species in other domains following the flow of the river, with favorable conditions facilitating their establishment.

From our study, we attribute the floristic affinity of the different Chaco remnants in Chaco domain to variations in precipitation, altitude and temperature, which is in contrast to our expectations regarding precipitation as the predominant factor in species richness and structure. In addition, in remnants where the seasonality of precipitation and altitude are highest, there is greater dissimilarity in the richness of woody vegetation.

Temperature played a greater role in remnants with more floristic homogeneity. In general, it can be said that the heterogeneity verified in the Chaco is influenced by different factors, where the seasonality of precipitation and altitude prevail in the structuring of trees and shrubs in the dry Chaco and the temperature in more homogeneous formations that make up the humid Chaco.

Acknowledgments

The authors are grateful to CNPq casadinho Process 552352/2011-0, CAPES (2012-2015) and CNPq, for funding the doctoral research of VAA (process number 140504/2015-0), and the grant to ALBS and VFM (process numbers 309987/2012-1 and 308047/2013-3). We also acknowledge the following scientists for plant identification: Dr. Adriana Lobão (Annonaceae), Dr. Arnildo Pott, Dr. Cyl Farney de Sá (Nyctaginaceae), Dr. Marcelo da Costa Souza (Myrtaceae), Dr. Flávio Macedo Alves (Lauraceae), MSc. Thomaz Sinani, Dr. Ronaldo Marquete (Salicaceae) and Dr. Massimo Bovini (Malvaceae). We thank Mr. Sérgio de Oliveira, who owns the “Retiro Conceicão” farm; and the staff and owners of the “Santa Vergínia” farm, for allowing us to conduct our field experiments, as well as some colleagues and friends who helped us during our field expeditions.

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• The authors are grateful to CNPq casadinho Process 552352/2011-0, CAPES (2012-2015) and CNPq, for funding the doctoral research of VAA (process number 140504/2015-0), and the grant to ALBS and VFM (process numbers 309987/2012-1 and 308047/2013-3). We also acknowledge the following scientists for plant identification: Dr. Adriana Lobão (Annonaceae), Dr. Arnildo Pott, Dr. Cyl Farney de Sá (Nyctaginaceae), Dr. Marcelo da Costa Souza (Myrtaceae), Dr. Flávio Macedo Alves (Lauraceae), MSc. Thomaz Sinani, Dr. Ronaldo Marquete (Salicaceae) and Dr. Massimo Bovini (Malvaceae). We thank Mr. Sérgio de Oliveira, who owns the “Retiro Conceicão” farm; and the staff and owners of the “Santa Vergínia” farm, for allowing us to conduct our field experiments, as well as some colleagues and friends who helped us during our field expeditions.

Edited by

Area Editor:

Dr. Gustavo Shimizu

Publication Dates

• Publication in this collection
  04 Nov 2022
• Date of issue
  2022

History

• Received
  13 May 2020
• Accepted
  16 Sept 2021