Aridity drives plant biogeographical sub regions in the Caatinga, the largest tropical dry forest and woodland block in South America

Abstract

Our aims were to quantify and map the plant sub regions of the the Caatinga, that covers 844,453 km2 and is the largest block of seasonally dry forest in South America. We performed spatial analyses of the largest dataset of woody plant distributions in this region assembled to date (of 2,666 shrub and tree species; 260 localities), compared these distributions with the current phytogeographic regionalizations, and investigated the potential environmental drivers of the floristic patterns in these sub regions. Phytogeographical regions were identified using quantitative analyses of species turnover calculated as Simpson dissimilarity index. We applied an interpolation method to map NMDS axes of compositional variation over the entire extent of the Caatinga, and then classified the compositional dissimilarity according to the number of biogeographical sub regions identified a priori using k-means analysis. We used multinomial logistic regression models to investigate the influence of contemporary climatic productivity, topographic complexity, soil characteristics, climate stability since the last glacial maximum, and the human footprint in explaining the identified sub regions. We identified nine spatially cohesive biogeographical sub regions. Current productivity, as indicated by an aridity index, was the only explanatory variable retained in the best model, explaining nearly half of the floristic variability between sub regions. The highest rates of endemism within the Caatinga were in the Core and Periphery Chapada Diamantina sub regions. Our findings suggest that the topographic complexity, soil variation, and human footprint in the Caatinga act on woody plant distributions at local scales and not as determinants of broad floristic patterns. The lack of effect of climatic stability since the last glacial maximum probably results from the fact that a single measure of climatic stability does not adequately capture the highly dynamic climatic shifts the region suffered during the Pleistocene. There was limited overlap between our results and previous Caatinga classifications.

Aridity, soil and biome stability influence plant ecoregions in the Atlantic Forest, a biodiversity hotspot in South America

“…Based on a data-driven approach that relied on community composition patterns, we identified and mapped discrete woody plant ecoregions in the Atlantic Forest domain in South America, and uncovered their relationships with environmental and historical correlates. These results are of relevant for the global effort of mapping terrestrial ecoregions (Olson et al 2001, Saiter et al 2016, Dinerstein et al 2017, Moura et al 2017, Silva and Souza 2018a. Our results give support to the point made recently (Särkinen et al 2011, Saiter et al 2016, Silva and Souza 2018a, that vegetation maps based on physiognomic variation, as valuable as they are, should not be used as proxies to plant diversity ecoregions.…”

“…These results are of relevant for the global effort of mapping terrestrial ecoregions (Olson et al 2001, Saiter et al 2016, Dinerstein et al 2017, Moura et al 2017, Silva and Souza 2018a. These results are of relevant for the global effort of mapping terrestrial ecoregions (Olson et al 2001, Saiter et al 2016, Dinerstein et al 2017, Moura et al 2017, Silva and Souza 2018a.…”

“…In order to address the historical hypothesis we used the historical stability of the Atlantic Forest over the last 21 and 120 kyr, measured as the frequency in which each grid cell changed between forest-type and open vegetation covers, irrespective of plant community composition, estimated by Carnaval et al (2014). This index has been regarded as the most accurate estimation of aridity (Quan et al 2013), which is the long-term water deficit produced by the imbalance between rainfall and temperature-driven evapotranspiration, and has been shown to be more relevant to our understanding of plant ecology than either rainfall or temperature alone (Silva and Souza 2018a). This index has been regarded as the most accurate estimation of aridity (Quan et al 2013), which is the long-term water deficit produced by the imbalance between rainfall and temperature-driven evapotranspiration, and has been shown to be more relevant to our understanding of plant ecology than either rainfall or temperature alone (Silva and Souza 2018a).…”

Short gradient, but distinct plant strategies: The CSR scheme applied to subtropical forests

“…This might explain the more important contribution of temperature, when compared to rainfall, in our results. In contrast, rainfall and water availability have been shown to be important in driving shifts in plant ecological strategies along vegetation gradients in tropical zones, e.g., in the variation in species composition along Caatinga dry forests (Silva & Souza, 2018) and changes in species richness across Amazonian forests (Esquivel‐Muelbert et al.,). Indeed, annual precipitation showed a significant association only with component R, as was the case for climate water deficit, which showed marginal significance with this component.…”

Climate and soils at the Brazilian semiarid and the forest-Caatinga problem: new insights and implications for conservation

“…Another possibility is that the arboreal Caatinga is reminiscent of seasonal forests that covered a larger area in the Brazilian semiarid region during the Pleistocene, and wetter phases in the Holocene (Behling et al 2000, Werneck et al 2011, Arruda et al 2018, Bouimetarhan et al 2018, Medeiros et al 2018. Nowadays, they are the result of the combination of vestigial Dry Forest with species of the Caatinga, which have advanced with the dryness increase in the region (Silva and Souza 2018), which justifies its greater distribution in the border region of the Caatinga biome, in contact with Dry Forests.…”