Abstract:
Phylogenetic niche conservatism (PNC) is the tendency of species within a clade to retain ancestral traits and
to persist in their primary ecological niches on geological time scales. It links evolutionary and ecological processes and has
been hypothesized to explain patterns of species richness and the composition of species assemblages. Decreasing patterns
of species richness along latitudinal gradients were often explained by the combination of ancient tropical climates, trait
retention of tropical lineages and environmental filtering. PNC also predicts decreasing phylodiversity and family age with
decreasing tropicality and has been invoked to explain these patterns along climatic gradients across latitudinal as well as elevational
gradients. However, recent studies on tree assemblages along latitudinal and elevational gradients in South America
found patterns contradicting the PNC framework. Our study aims to shed light on these contradictions using three different
metrics of the phylogenetic composition that form a gradient from recent evolutionary history to deep phylogenetic
relationships. We analyzed the relationships between elevation and taxonomic species richness, phylodiversity and family
age of tree assemblages in Andean rainforests in Ecuador. In contrast to predictions of the PNC we found no associations
of elevation with species richness of trees and increasing clade level phylodiversity and family age of the tree assemblages
with elevation. Interestingly, we found that patterns of phylodiversity across the studied elevation gradient depended especially
on the deep nodes in the phylogeny. We therefore suggest that the dispersal of evolutionarily old plant lineages with
extra-tropical origins influences the recent composition of tree assemblages in the Andes. Further studies spanning broader
ecological gradients and using better resolved phylogenies to estimate family and species ages are needed to obtain a deeper
mechanistic understanding of the processes that drive the assembly of tree communities along elevational gradients.
