Abstract
Elevation gradients present enigmatic diversity patterns, with trends often dependent on the dimension of diversity considered. However, focus is often on patterns of taxonomic diversity and interactions between diversity gradients and evolutionary factors, such as lineage age, are poorly understood. We combine forest census data with a genus level phylogeny representing tree ferns, gymnosperms, angiosperms, and an evolutionary depth of 382 million years, to investigate taxonomic and evolutionary diversity patterns across a long tropical montane forest elevation gradient on the Amazonian flank of the Peruvian Andes. We find 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. We suggest that 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. Overall our results support 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.
Highlights
Environmental gradients are commonly associated with trends in diversity, and the elevational diversity gradient (EDG) is one of the most widely discussed and enigmatic patterns in ecology (Humboldt and Bonpland, 1805; Rahbek, 1995; Guo et al, 2013)
Framed by the context of broad scale diversity trends predicted by the Tropical Niche Conservatism (TNC), Out of the Tropical Lowlands, and Environmental Crossroads hypotheses, our analyses aim to address three core questions: (1) What is the shape of the EDG within tropical montane forest? (2) Do elevational trends of taxonomic and phylogenetic diversity differ along this gradient? (3) Is there elevational variation in lineage age patterns along this gradient?
Our results demonstrate a discernible contrast between patterns of taxonomic and phylogenetic diversity of trees along an Amazon-to-Andes elevation gradient
Summary
Environmental gradients are commonly associated with trends in diversity, and the elevational diversity gradient (EDG) is one of the most widely discussed and enigmatic patterns in ecology (Humboldt and Bonpland, 1805; Rahbek, 1995; Guo et al, 2013). Diversity broadly declines with elevation, yet there is substantial variation amongst taxa and across systems, with many non-monotonic and non-linear diversity trends (Rahbek, 1995; Guo et al, 2013). While many investigations of diversity variation focus on taxonomic measures such as, species richness, interest has expanded to include quantification of evolutionary diversity (e.g., Faith, 1992; Dexter et al, 2019), providing potential for a new temporal perspective to the EDG. Possible contrasts between evolutionary diversity and measures of current taxonomic richness have yet to be fully explored in the context of the EDG. The interaction between the EDG and variation in the evolutionary ages of lineages at different elevation is still unclear
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