Evolutionary Diversity Peaks at Mid-Elevations Along an Amazon-to-Andes Elevation Gradient

However, focus is often on patterns of taxonomic diversity and interactions between diversity gradients and evolutionary factors, such as lineage age, are poorly understood. A study combining forest census data with a genus-level phylogeny representing tree ferns, gymnosperms, angiosperms, and an evolutionary depth of 382 million years, investigated taxonomic and evolutionary diversity patterns across a long tropical montane forest elevation gradient on the Amazonian flank of the Peruvian Andes. The study found that evolutionary diversity peaks at mid-elevations and contrasts with taxonomic richness, which is invariant from low to mid-elevation, but then decreases with elevation. This trend interacts with variation in the evolutionary ages of lineages across elevation, with contrasting distribution trends between younger and older lineages. For example, while 53% of young lineages (originated by 10 million years ago) occur only below ∼1,750 m asl, just 13% of old lineages (originated by 110 million years ago) are restricted to below ∼1,750 m asl. The study suggests that this trend supports an Environmental Crossroads hypothesis, whereby a mid-gradient mingling of distinct floras creates an evolutionary diversity in mid-elevation Andean forests that rivals that of the Amazonian lowlands. The Environmental Crossroads hypothesis proposes that intermediate conditions along environmental gradients should support the highest diversity due to coexistence of distinct evolutionary lineages from gradient extremes. The study's findings have implications for our understanding of the evolutionary dimension of the elevational diversity gradient of trees in tropical montane forests. The results provide a new perspective on the spatial organization of diversity and highlight the importance of considering the evolutionary dimension of diversity patterns. The study's findings also have implications for conservation efforts, as they suggest that mid-elevation Andean forests may be critical areas for preserving evolutionary diversity. The study's results are consistent with previous research on the elevational diversity gradient, which has shown that diversity patterns can vary depending on the dimension of diversity considered. The study's findings also highlight the importance of considering the evolutionary ages of lineages in understanding diversity patterns. The study's results provide a new perspective on the Environmental Crossroads hypothesis and suggest that it may be a key factor in shaping diversity patterns along environmental gradients. The study's methodology involved combining forest census data with a genus-level phylogeny representing tree ferns, gymnosperms, angiosperms, and an evolutionary depth of 382 million years. The study's results were based on an analysis of the phylogenetic relationships among the study's species and the distribution of evolutionary diversity across the elevation gradient. The study's findings have implications for our understanding of the evolutionary dimension of the elevational diversity gradient of trees in tropical montane forests and highlight the importance of considering the evolutionary dimension of diversity patterns.