Faster tree speciation in wet versus dry habitats: Evidence from a rainfall gradient in the Panama Canal watershed

Background/Question/Methods

Identifying the mechanisms that generate species diversity patterns has been a central issue in ecology for a long time. In recent years, phylogeography has been an important tool in investigating the origin and maintenance of biodiversity from local to global scales. Most phylogeographical studies focus on phylogenetic relatedness or distance in relation to community structure or environmental gradients. Few explore how the phylogenetic age structure changes along an environmental gradient and its macroevolutionary implications. Here, using forest plot survey data from the Panama Canal watershed and molecular phylogenetic reconstruction, we investigate how average phylogenetic age changes along a rainfall gradient and its possible explanation and potential implications. 

Results/Conclusions

Data from 48 forest plots in the watershed show a significant negative relationship between average phylogenetic age and precipitation (ordinary least square (OLS) regression R²=0.2419, P<0.001) for tree-sized species (breast height diameter≥10cm). In addition, the fraction of species with ages less than 10 MY significantly increases with rainfall (R²=0.2414, P<0.001). Other age thresholds produce similar qualitative results. This decreasing phylogenetic age (or increasing fraction of young species) towards wetter habitats suggests a significant niche difference in macroevolutionary processes and faster speciation in wet versus dry sites. Differences in tree mortality rate and, consequently, community turnover rate could be major drivers of the speciation rate difference in wet versus dry sites.