Effect of Dichrostachys cinerea encroachment on plant species diversity, functional traits and litter decomposition in an East‐African savannah ecosystem.

Questions: Woody encroachment is increasingly affecting biodiversity and ecosystem functioning of savannah ecosystems worldwide, yet the direction and magnitude of these impacts often seem context‐dependent. Here, we investigated the potential of a trait‐based framework to understand the effect of the encroaching shrub Dichrostachys cinerea on plant species diversity and litter decomposition in a savannah system. Location: Savannah plains of Nech Sar National Park, South‐Ethiopian rift valley. Methods: We conducted a three‐month litter burial experiment using two standard 'tea bag index' litter types and three local litter types across 45 vegetation plots, equally spread across unencroached, sparsely encroached and densely encroached savannah locations. We additionally quantified the specific leaf area, leaf dry matter content, leaf nitrogen content and leaf area of both D. cinerea and all common savannah species in each plot, and surveyed soil characteristics and species diversity and composition at the plot level. Using this set‐up we explored the potential of the four recorded plant traits to predict changes in litter decomposition following D. cinerea encroachment, in addition to changes in soil characteristics and savannah species diversity and composition. Results: Dichrostachys cinerea encroachment introduced novel functional trait values that reoriented community level resource use towards a more acquisitive strategy. These trait changes were partly associated with a reduction of decomposition rates within encroached areas compared to unencroached areas. Unexpectedly, encroachment resulted in increased, rather than decreased, herbaceous species diversity in sparsely invaded plots, but not in densely invaded plots. Species composition nonetheless shifted strongly following encroachment, indicating a strong species turnover, likely towards more shade‐tolerant species. Conclusions: Functional traits can partly help explain effects of encroachment on litter decomposition, but other environmental and biotic drivers nonetheless seem to additionally impact decomposition rates. [ABSTRACT FROM AUTHOR]