Acaulescence promotes speciation and shapes the distribution patterns of palms in Neotropical seasonally dry habitats

Rainforests have been a source of lineages to open and seasonally dry
habitats throughout Angiosperm evolution, especially in the Neotropics.
However, the underlying mechanisms that allow such shifts remain poorly
understood at large spatial scales. Here, we test whether acaulescence (an
underground stem or a very short stem concealed in the ground) has
affected the colonization and speciation in Neotropical seasonally dry
habitats by cocosoid palms (Cocoseae). Acaulescent species maintain their
growth underground, which increases their chances of survival from
prolonged seasonal dry season and frequent fires. We use an integrative
approach based on trait‐dependent diversification models, phylogenetic
comparative methods, and ecological niche models. We found that shifts
towards acaulescent growth form were accompanied by evolutionary
transitions to seasonally dry habitats. Acaulescent lineages had higher
speciation rates than non-acaulescent ones. However, the interaction
between acaulescence and seasonally dry habitats had no significant effect
on Cocoseae speciation rates. Acaulescent palms are primarily distributed
in Neotropical seasonally dry habitats and non-acaulescent palms are
concentrated in Amazonian rainforests. Our results suggest that an
underground stem, with high carbohydrate and water storage capacity, is a
preadaptation by which rainforest lineages were able to colonize and
diversify in new fire-prone, increasingly seasonal and drier adaptive
zones. The projected global expansion of dry seasonal habitats requires an
understanding of how drought-avoidance functional traits evolve and how
they are linked to seasonally dry habitats. Our results are, thus, a step
forward in determining plant response mechanisms to drier and seasonal
conditions.