Abstract
Studies based on contemporary plant occurrences and pollen fossil records have proposed that the current disjunct distribution of seasonally dry tropical forests (SDTFs) across South America is the result of fragmentation of a formerly widespread and continuously distributed dry forest during the arid climatic conditions associated with the Last Glacial Maximum (LGM), which is known as the modern-day dry forest refugia hypothesis. We studied the demographic history of Tabebuia rosealba (Bignoniaceae) to understand the disjunct geographic distribution of South American SDTFs based on statistical phylogeography and ecological niche modeling (ENM). We specifically tested the dry forest refugia hypothesis; i.e., if the multiple and isolated patches of SDTFs are current climatic relicts of a widespread and continuously distributed dry forest during the LGM. We sampled 235 individuals across 18 populations in Central Brazil and analyzed the polymorphisms at chloroplast (trnS-trnG, psbA-trnH and ycf6-trnC intergenic spacers) and nuclear (ITS nrDNA) genomes. We performed coalescence simulations of alternative hypotheses under demographic expectations from two a priori biogeographic hypotheses (1. the Pleistocene Arc hypothesis and, 2. a range shift to Amazon Basin) and other two demographic expectances predicted by ENMs (3. expansion throughout the Neotropical South America, including Amazon Basin, and 4. retraction during the LGM). Phylogenetic analyses based on median-joining network showed haplotype sharing among populations with evidence of incomplete lineage sorting. Coalescent analyses showed smaller effective population sizes for T. roseoalba during the LGM compared to the present-day. Simulations and ENM also showed that its current spatial pattern of genetic diversity is most likely due to a scenario of range retraction during the LGM instead of the fragmentation from a once extensive and largely contiguous SDTF across South America, not supporting the South American dry forest refugia hypothesis.
Highlights
Dry tropical forests (SDTF hereafter) are deciduous and semideciduous forests globally distributed on tropical regions and characterized by pronounced seasonality in rainfall [1]
Previous studies based on contemporary plant occurrences and pollen fossil records proposed that the current disjunct distribution of seasonally dry tropical forests (SDTFs) across South America is the result of recent fragmentation of a formerly more widespread and continuously distributed dry forest during the arid climatic conditions associated with the Last Glacial Maximum (LGM; 5, 6), which is known as the dry forest refugia hypothesis [7,8], supported by phylogeographic analyses [9]
We modeled four demographic scenarios (Fig 2) according to the hypothesis supported by ecological niche modeling and biogeographic hypotheses (PLAH, PPPH, “Both”, or “Range Retraction”), following the framework described in Collevatti et al [9,17]
Summary
Dry tropical forests (SDTF hereafter) are deciduous and semideciduous forests globally distributed on tropical regions and characterized by pronounced seasonality in rainfall [1]. The SDTFs occur usually scattered among other vegetation types such as wet forests, savannas and woodlands [2], and the transition forest-savanna is determined mainly by soils conditions and fire frequency [3]. SDTFs occur in regions with similar climates of savannas, but restricted to eutrophic and oligotrophic soils, with moderate pH and low levels of aluminum [2]. In South America, the SDTFs are distributed in multiple and isolated patches across a "dry diagonal" [4], including the cactus thorn scrub in more arid areas in the Northeastern Brazil (the Caatinga Biome), the savannas of Central Brazil (the Brazilian Cerrado Biome), and the Chaco woodlands across the ‘Misiones’ nucleus in Bolivia. Previous studies based on contemporary plant occurrences and pollen fossil records proposed that the current disjunct distribution of SDTF across South America is the result of recent fragmentation of a formerly more widespread and continuously distributed dry forest during the arid climatic conditions associated with the Last Glacial Maximum (LGM; 5, 6), which is known as the dry forest refugia hypothesis [7,8], supported by phylogeographic analyses [9]
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