Global relationships between plant functional traits and environment in grasslands

In order to understand how ecosystems are likely to respond to global anthropogenic change it is first necessary to identify general patterns and processes that can explain how they are created and maintained. Plant traits potentially provide a mechanistic explanation for the differences in growth and survival that explain a species niche that can scale up from individual to biome. In this thesis I investigate the relationships between grass functional traits and the biotic and abiotic environment, and test whether the predictions made by community ecological theory are more broadly applicable at continental and global scales.

I provide evidence that at the global scale soil nutrients is more important than climate in explaining the distribution of traits that reflect different strategies of resource use but that evolutionary history provides a stronger explanation for global trait distribution than contemporary environment. I then show the functional traits that are associated with gradients of grazing and fire and identify functional groups that have diverging responses to grazing across Sub-Saharan Africa. Finally I investigate species response to drought and identify traits which can explain a species hydrological niche.

The findings of this work provide evidence that trade-offs between carbon and nitrogen acquisition and use (leaf nitrogen content and C/N ratio) could provide a foundation for predicting plant responses to changes in climate, soil nutrients and disturbance at global scales. However, I also show that traits often used to reflect differences in leaf growth and longevity (ie. specific leaf area and leaf tensile strength) are not able to strongly predict response to either resource availability or disturbance at macro-ecological scales. This highlights the need to identify other axis of variation and organs beyond the leaf economic spectrum, for example root architecture, that are potentially important in explaining the differing aspects of a species niche and how vegetation may respond to global change.