PhD defence Isabelle Buyens
Isabelle Buyens (ConsEco)
Promotores: Prof. C. Smit, Prof. P.C. le Roux (University of Pretoria)
From patches to processes
Linking grazing, facilitation, and trait variation in a grazed salt marsh
This thesis investigated how plants, grazers, and the environment interact to shape vegetation patches and intraspecific plant traits. The research was conducted on the cattle-grazed and non-grazed salt marsh of the island Schiermonnikoog (the Netherlands) where visible patches of the grazer-defended rush Juncus maritimus and grazer-intolerant grass Elytrigia atherica occur. First, a conceptual framework was developed to describe cyclical vegetation patch dynamics in rangeland systems. This framework integrates theories of cyclical succession, self-organization, and multi-trophic feedbacks, proposing a four-phase cycle of patch initiation, establishment, expansion, and degeneration driven by interactions among grazer-defended and grazer-intolerant plants, large grazers, and bioturbation by soil fauna. Second, a transplant experiment assessed how plant-plant interactions between J. maritimus and E. atherica vary with grazing, abiotic stress, and E. atherica ecotype differentiation. The experiment found that survival and growth of E. atherica were strongly impacted by J. maritimus patches, grazing and inundation, with E. atherica ecotype playing no significant role. Third, an observational study examined how grazing, inundation stress, and plant interactions interactively influence structural and reproductive trait variation of E. atherica. Patches of J. maritimus buffered E. atherica against grazers and inundation stress. However, in the non-grazed salt marsh, the plant-plant interactions shifted towards competition, particularly under low inundation stress. Overall, this thesis demonstrated that vegetation patchiness and intraspecific plant trait variation emerge from context-dependent interactions among grazers, environmental stress, and plant-plant interactions, with implications for our understanding of vegetation structure and rangeland management.