Plant evolutionary assembly along elevational belts at Mt. Kilimanjaro: Using phylogenetics to asses biodiversity threats under climate change

摘要

Accumulating evidence suggests that species sensitivity to climatic change is strongly related to their ecological niches, which are somewhat evolutionary conserved. Insofar as this is true, climate change-driven extinctions may be non-random with respect to phylogeny, and entire branches of the tree of life might be lost. In mountainous regions, where macroecological conditions change abruptly with elevation, evolutionarily lineages may show deep phylogenetic turnover along elevational belts, and such "layering" of the phylogeny may lead to disproportionate losses of phylogenetic diversity. Here, we explored phylogenetic alpha and beta diversity patterns of plant assemblages among six elevational belts at Kilimanjaro (i.e. savanna woodland, lower montane forest, middle-montane Ocotea forest, upper-montane Podocarpus forest, Erica subalpine bushland and Helichrysum alpine scrubland). Specifically, we aimed at determining the degree of elevational turnover of plant evolutionary lineages as a diagnostic tool to assess phylogenetic diversity threats owing to shifting environmental conditions. We found relatively deep phylogenetic discontinuities that roughly matched the main modern vegetation types in East Africa, this is, subalpine-alpine vegetation, montane forests and savannas lowlands. However, the upper-montane Podocarpus forest was phylogenetically closer to subalpine-alpine vegetation than to middle (Ocotea) and lower montane forests, despite all these forest belts constituted a floristically integrated unit. Phylogenetic diversity followed a hump-shape relationship with elevation, with significantly high values in lower montane forest and lower than expected scores in savanna, subalpine and alpine belts. Overall, the phylogenetic layering of Kilimanjaro flora reported here suggests that a disproportionate amount of phylogenetic diversity in mountainous regions might be at risk, although the generality of this pattern is yet to be confirmed. In the particular case of Kilimanjaro, human population growth represents a pervasive threat to montane forest, and particularly to the phylogenetically singular remnants of lower montane forest. Besides, climate-change induced fires in the subalpine belt are causing over-riding shrinkage of the upper-montane Podocarpus forest from the timberline. Should these continuing pressures of global change persist, the montane forests of Kilimanjaro may experience substantial shrinking within the next decades, trapped between the "hammer and the anvil" of climate and land-use change.