Understanding woody plant encroachment: a plant functional trait approach

The increase in the density of woody plants threatens the integrity of grassy ecosystems. It remains unclear if such encroachment can be explained mostly by direct effects of soil conditions and hydrology on woody plant growth or by indirect effects on fire regime and herbivory imposing tree recruitment limitation. Here, we investigate whether woody plant functional traits provide a mechanistic understanding of the complex relationships between these direct and indirect effects. We first assess the role of rainfall, soil fertility, texture, and geomorphology to explain variation in woody plant encroachment following anthropogenically-induced fire suppression across the Serengeti ecosystem. Second, we explore trait-environment relationships and how these mediate vegetation response to fire suppression. We find that woody plant encroachment is strongest in areas with high soil fertility, high rainfall, intermediate catenae positions, and fine soil textures. These conditions promote woody plant communities associated with small stature, small seed sizes, and high recruit densities (linked to a recruitment-stature trade-off). The positioning of species along this ‘recruitment-stature axis’ was found to be the most important predictor of recruitment limitation. Areas that support such plant communities - e.g. mid-catena position - were most sensitive to woody plant encroachment. These findings demonstrate the importance of trait-environment relationships in predicting the impact of human alterations on local vegetation change. Understanding how environmental factors directly (resources) and indirectly (legacy effects and plant traits) determine woody plant encroachment supports the development of process-based ecosystem structure and function models.