Phylogenetic diversity and structure above the tree line in the central Chilean Andes in the light of competing macroecological hypotheses

The “Tropical Niche Conservatism” (TNC) and “Out of the Tropics” (OTT) hypotheses were developed originally in relation to latitudinal species richness gradient. To determine whether the predictions of these theories play out across elevational gradients in the central Chilean Andes, we determined species richness (SR), Relative Phylogenetic Diversity (PD_SES), Mean Pairwise Distance (MPD_SES) and Mean Nearest Taxon Distance (MNTD_SES) at 100 m elevational intervals for five plot sizes over two above tree line gradients embedded in the same regional flora. We assessed the effect of phylogenetic resolution using two species-level phylogenies resolved to different degrees. After an initial gradual increase followed by a gradual decline, both SR and PD decreased with elevation. MNTD_SES transitioned from phylogenetic overdispersion in the subalpine to clustering in the upper alpine with unclear elevational trends at the smaller spatial scales. Results for MPD_SES were equivocal. Although results for the two phylogenies were closely correlated, lower phylogenetic resolution favored decreased site-level PD at all spatial scales and a trend in the direction of phylogenetic overdispersion for PD_SES and MNTD_SES at the smaller spatial scales. Overall, TNC provides a more coherent explanation for our results in the Chilean Andes than OTT. Lower SR and lack of phylogenetic clustering at lower elevations on the two gradients are attributed to contingencies of the Miocene uplift of the Andes leading to a more arid climate and depressed treeline. Spatial scaling effects call for well resolved species-level phylogenies in studies seeking to relate phylogenetic structure to local ecological effects. However, the degree of phylogenetic resolution becomes less critical when the focus is on broad macroecological trends.