Evolutionary constraints on tree size and above-ground biomass in tropical dry forests

Abstract

The extent (or lack) of phylogenetic signal for key ecological traits reveals the role of evolutionary processes on present-day ecosystem function and hints on future ecological trends under climate change scenarios. This approach has been applied to South American tropical moist forests, but not to the highly threatened seasonally dry tropical forests (SDTF), despite the acknowledgement of their unique evolutionary history. To fill this knowledge gap, we investigated the legacy of evolutionary processes on vital ecological characteristics among SDTF trees: regional dominance, tree size and soil preference. We used tree community data on 313 plots of SDTF (12.52 ha) and locally collected soil data in central-eastern Brazil. For each assessed trait (three for regional dominance, three for tree size and nine for soil preference), we investigated the legacy of evolution using two different approaches: calculating the extent of phylogenetic signal and comparing the fit of four different models of evolution. Above-ground woody biomass and tree size showed strong phylogenetic signal. Most of the SDTF biomass stock was concentrated on a few large-sized and closely related tree genera. Among the soil preference variables, only phosphorus displayed significant, albeit weak, phylogenetic signal. Synthesis. Our study is the first to show that evolutionary constraints related to tree size significantly determine regional biomass stocks of seasonally dry tropical forests (SDTF) in a few closely related tree lineages. This suggests that even isolated SDTF fragments with low taxonomic and phylogenetic diversity can play an important role in the global carbon cycle, storing disproportionally large amounts of carbon in trees that belong to high-biomass lineages. Whether these lineages also share climate change-induced mortality risk deserves future investigation, as they are largely responsible for the maintenance of regional SDTF biomass stocks.