The Origins and Historical Assembly of the Brazilian Caatinga Seasonally Dry Tropical Forests

Moabe F Fernandes,Luciano P De Queiroz,R Toby Pennington,Domingos Cardoso

doi:10.3389/fevo.2022.723286

Abstract

The Brazilian Caatinga is considered the richest nucleus of the Seasonally Dry Tropical Forests (SDTF) in the Neotropics, also exhibiting high levels of endemism, but the timing of origin and the evolutionary causes of its plant diversification are still poorly understood. In this study, we integrate comprehensive sampled dated molecular phylogenies of multiple flowering plant groups and estimations of ancestral areas to elucidate the forces driving diversification and historical assembly in the Caatinga flowering plants. Our results show a pervasive floristic exchange between Caatinga and other neotropical regions, particularly those adjacent. While some Caatinga lineages arose in the Eocene/Oligocene, most dry-adapted endemic plant lineages found in region emerged from the middle to late Miocene until the Pleistocene, indicating that only during this period the Caatinga started to coalesce into a SDTF like we see today. Our findings are temporally congruent with global and regional aridification events and extensive denudation of thick layers of sediments in Northeast (NE) Brazil. We hypothesize that global aridification processes have played important role in the ancient plant assembly and long-term Caatinga SDTF biome stability, whereas climate-induced vegetation shifts, as well as the newly opened habitats have largely contributed as drivers of in situ diversification in the region. Patterns of phylogenetic relatedness of Caatinga endemic clades revealed that much modern species diversity has originated in situ and likely evolved via recent (Pliocene/Pleistocene) ecological specialization triggered by increased environmental heterogeneity and the exhumation of edaphically disparate substrates. The continuous assembly of dry-adapted flora of the Caatinga has been complex, adding to growing evidence that the origins and historical assembly of the distinct SDTF patches are idiosyncratic across the Neotropics, driven not just by continental-scale processes but also by unique features of regional-scale geological history.

Highlights

The Brazilian Caatinga is recognized as the largest and most species rich nucleus of the Seasonally Dry Tropical Forest (SDTF) biome in the New World (Pennington et al, 2000; Banda et al, 2016; Queiroz et al, 2017; Fernandes et al, 2020), yet it is one of the least studied neotropical biogeographic regions (Fiaschi and Pirani, 2009)
Since our aim is to assess the evolution of the dry-adapted flora, we considered Caatinga in its narrow sense, including only vegetation types connected to the SDTF biome, which encompasses distinct formations that mostly occur on areas of low altitude in the region
Few studies, most of which are lineage-specific, have investigated plant evolution in the isolated patches of the SDTF biome. Despite their important biogeographical insights, we lacked basic understanding on the timing of origin and the evolutionary causes that shaped the high levels of diversity and endemism in the key SDTF nucleus of the Caatinga

Summary

Introduction

The Brazilian Caatinga is recognized as the largest and most species rich nucleus of the Seasonally Dry Tropical Forest (SDTF) biome in the New World (Pennington et al, 2000; Banda et al, 2016; Queiroz et al, 2017; Fernandes et al, 2020), yet it is one of the least studied neotropical biogeographic regions (Fiaschi and Pirani, 2009). Caatinga plant communities have been classically divided into: (i) Crystalline Caatinga, established over fertile-soils primarily derived from Precambrian crystalline basement of the Depressão Sertaneja, ranging from woodlands (Caatinga s.s.) on driest sites to tall forests (Arboreal Caatinga) on moister areas; (ii) Sedimentary Caatinga, open scrub occupying the patchily distributed nutrient-poor sandy soils of ancient sedimentary basins and continental sand dunes and; (iii) the vegetation established over the karst deposits of the Bambuí and Jandaíra groups (Figures 1A–F; Queiroz, 2006; Moro et al, 2016; Queiroz et al, 2017). Increased aridity resulted in massive erosion of a thick sedimentary cover accumulated during wetter and warmer climatic conditions of the Early Cenozoic (Figure 2A). This process culminated in the exhumation of the Paleo-Mesozoic sediments and the formation of the large-scale, lower-level peneplain of the Depressão Sertaneja (Figure 2B). In the Pleistocene, the large karst systems belonging to Bambuí and Jandaíra groups were exhumed (Silva et al, 2017), adding to the increased environmental heterogeneity in the Caatinga (Figure 2D)

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