Diversity of tree-related microhabitats on Mt. Kilimanjaro

Tree-related microhabitats (TReMs) have been proposed as important indicators of biodiversity guiding forest management. However, their application has been limited mostly to temperate ecosystems and information on how the diversity of TReMs varies along environmental gradients is largely unknown.

In this study, we assessed the diversity of TReMs on 180 individual trees and 48 plots alongside a large environmental gradient on Mount Kilimanjaro, Tanzania. We adjusted a previous typology to suit tropical forests and developed a tree-climbing protocol to obtain quantitative information on TreMs on large trees and dense canopies. We computed the effective diversity of TReMs for each individual tree and plot and tested how TReM diversity was associated with properties of individual trees and environmental conditions in terms of climate and human impact. We used non-metric multidimensional scaling (NMDS) to investigate the composition of TReM assemblages alongside the environmental gradients.

We found that diameter at breast height and height of the first branch were the most important determinants of TReM diversity on individual trees. At plot level, we found that TReM diversity increased with mean annual temperature and decreased with human impact. Composition of TReMs showed a high turnover across the gradients in climate and human impact.

Our study demonstrates a first application of the concept of TreMs to tropical ecosystems. The importance of tree size and architecture in fostering microhabitat diversity underlines the importance of protecting large veteran trees. Diversity and composition of TReMs are sensitive to climate and land-use effects, suggesting that they can be used to monitor consequences of global change on forest biodiversity.