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

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.

Revisiting phylogeny, systematics, and biogeography of a Pleistocene radiation

“…This sedimentary process was already complete in the early Pleistocene and might be meaningful for the current range of Pilosocereus . The global and regional aridification processes during the Miocene–Pliocene that were important for the plant assembly and diversification of both the Cerrado (Simon et al, 2009; Werneck et al, 2011) and the Caatinga (Fernandes et al, 2022) seem to have had no immediate role in the diversification of Pilosocereus . In another way, the Pleistocene exhumation of the large karst limestone landforms belonging to the Bambuí and Jandaíra groups (Silva and Souza, 2018) in eastern Brazil possibly had profound influences on the diversification of karst‐dwelling Pilosocereus species as P. flexibilispinus , P. floccosus subsp.…”

“…Our results support that the clade Pilosocereus s.s. diversified in situ within the modern Caatinga landscapes. Phylogenetic analysis of multiple taxa of flowering plants suggests that the present-day Caatinga communities were already well established by the mid to late Miocene (Fernandes et al, 2022). The main geomorphological events shaping the modern heterogeneous landscapes of the Caatinga and Cerrado and exposing habitat characteristics of several cactus species had already occurred when Pilosocereus began to diversify.…”

Mapping the root systems of individual trees in a natural community using genotyping‐by‐sequencing

“…(2011), yielding a World Reference Base (IUSS, 2015) soil classification of ‘Haplic Lixisol (Loamic, Hypereutric, Ochric and Magnesic)’. The Brazilian Caatinga is recognised as the largest and most species‐rich forests of the Seasonally Dry Tropical Forest biome in the New World (Pennington et al ., 2000; Fernandes et al ., 2022).…”

“…The studied vegetation can be broadly described as being part of the Caatinga domain (de Lima Araújo et al, 2007) with the physical and chemical properties of soil sampled and analysed as in Quesada et al (2011), yielding a World Reference Base (IUSS, 2015) soil classification of 'Haplic Lixisol (Loamic, Hypereutric, Ochric and Magnesic)'. The Brazilian Caatinga is recognised as the largest and most species-rich forests of the Seasonally Dry Tropical Forest biome in the New World (Pennington et al, 2000;Fernandes et al, 2022).…”

The evolution of extant South American tropical biomes

“…As with savannas, many XF lineages evolved a long time before the much more recent expansion of XFs; for instance, Cactaceae originated during the late Eocene but most species‐rich clades originated in the late Neogene (Arakaki et al ., 2011). A similar pattern is observed for the Caatinga (Fernandes et al ., 2022); for example, some Caatinga bombacoids date to the late Eocene, and many Caatinga taxa are derived from migrations toward the Caatinga, but since the Pliocene, in situ diversification has been the dominant mode of species accumulation, with a peak of in situ radiations during the Pleistocene (Fernandes et al ., 2022).…”