Drought and grazing impacts in semi-arid rangelands: Testing a novel trait-based conceptual framework

Drought and land use increase are among the most important global change drivers, particularly for dryland ecosystems. Semi-arid rangelands provide important ecosystem services and are predicted to be highly vulnerable to global change. Drought and grazing resistance are assumed to converge (Generalized Grazing Model), i.e. species/communities in drier areas are both grazing and drought resistant. However, the generality of this model has been called into question by newer studies in semiarid rangelands dominated by winter annuals.

We developed a novel trait-based conceptual framework that can resolve the apparent inconsistencies. Winter annuals display a substantially different strategy to cope with drought than perennials, they are drought escapers and survive the long summer drought in the seed bank. Associated with this distinct strategy are differences in underlying traits. Furthermore, even within winter annuals, there should be a wide range of trait expressions. We hypothesize a trade-off between traits associated with a drought escape strategy ('fast traits'), and traits for grazing tolerance and avoidance ('slow traits'), which lead to more negative impacts of grazing in annual species/communities in drier sites and more negative impacts of drought in wetter sites.

To test the proposed framework we will use a steep rainfall gradient in the Eastern Mediterranean as a model system. We will assess over 30 physiological, morphological and life history traits in 40 species in greenhouse experiments and combine them with in situ long-term ecosystem level survival and abundance data from previous experiments. This will allow to determine the key strategies for drought and grazing resistance and evaluate their relationships (convergence vs. trade-offs). Our study will for the first time 1) provide a comprehensive functional trait approach of winter annuals at the community level, and 2) explicitly link traits, short-term responses, long-term community responses, and distribution, filling an important gap in the basis of trait-based plant ecology. The proposed project will have the potential to transform our understanding of the interaction of drought and grazing, and substantially improve projections of consequences of global change with far reaching theoretical and applied implications.

Main Investigators: Katja Tielbörger (PI) & Mark Bilton

Collaborators:

Leonor Alvarez Cansino (PI), Functional and Tropical Plant Ecology, University of Bayreuth