Long-term Multifunctionality across Mediterranean-type ecosystems: Improving Restoration Outcomes through trait-based Modelling


Mediterranean-type ecosystems (MTEs) are among the global biodiversity hotspots most vulnerable to multiple factors of global change, such as climate warming and elevated atmospheric CO2. Global change is likely to exacerbate current ecosystem degradation in MTEs, with severe consequences for the long-term provision of multiple ecosystem services on which people depend, such as carbon sequestration, water and nutrient supply. Ecological restoration is increasingly aimed at counteracting the decline and improving the long-term provision of multiple ecosystem services. For successful restoration, however, ecologists require a fundamental understanding of the link between vegetation composition, related functions and services, and influencing environmental factors. Measurable traits of plant species such as specific leaf area have been recognized as a quantifiable link between vegetation composition and ecosystem functions underlying ecosystem services. Given this link, restoration can select plant species based on their traits in order to improve desired ecosystem functions. In this thesis, I aimed at assessing the linkage between plant traits and the provision of multiple ecosystem functions, as well as trade-offs among these functions under the influence of multiple changing environmental factors, to support the restoration of degraded MTEs towards multifunctionality world-wide. Through a literature review, I found that current trait-based research lacks a complete understanding of the combined effects of multiple environmental factors on the long-term provision of multiple ecosystem functions and a quantification of the trade-offs and synergies among these factors. To address this gap, I proposed a theoretical framework that complements trait-based empirical research with process-based simulation modelling. Based on this framework, I successfully developed the individual- and trait-based simulation model ModEST (Modelling Ecosystem Services and Functions based on Traits) that calculates the coupled dynamics of soil water, soil nutrients, and individual woody plants characterized by traits. ModEST allows quantification of ecosystem functions for a given planted trait composition over time under varying environmental conditions. As a first step, I supplemented a large-scale restoration project in Western Australia (called the Ridgefield Experiment) by evaluating all possible combinations of plant species locally available for restoration under both current and future climatic conditions using ModEST. The simulation results revealed that multifunctionality was increased by higher levels of planted species richness under current, but not under future climatic conditions. In general, multifunctionality could not be fully achieved because of trade-offs among functions that were attributable to sets of traits that affected the functions differently. Trade-offs and synergies among functions shifted with climate change as a result of differential direct and indirect impacts of environmental factors on functions. To understand how the link between plant traits and functions found in Ridgefield, as well as trade-offs and synergies among functions, can be applied to other Mediterranean-type ecosystems, I applied ModEST to several abiotic and biotic conditions found in Mediterranean-type ecosystems across the globe. Specifically, I simulated a full-factorial design of all possible combinations of six plant functional types, constructed from trait values of woody Mediterranean plant species, and various abiotic settings (i.e. mean annual precipitation, mean annual temperature, solar radiation, and soil texture) characteristic of the Mediterranean biome. I found that the maximization of multiple functions achieved by particular plant trait compositions as well as trade-offs among the maximized functions were dependent on the abiotic context. I could show that plant traits alone affected by the environment were not fully responsible for the differences in the functioning but that additionally abiotic conditions in interaction directly shaped ecosystem functioning. With this work, I have shown that there is a clear linkage between multiple environmental factors that directly and indirectly, via changes in plant traits, affect multiple ecosystem functions in MTEs, as well as trade-offs and synergies among them. My findings imply that restoration ecologists face a significant challenge in achieving their long-term multifunctionality goals in degraded MTEs across the world. I demonstrated that an integrated trait-based empirical and simulation modelling approach can unravel the complex multi-layered relationship of multiple plants traits on ecosystem functioning affected by the abiotic environment to support restoration towards multifunctionality in MTEs if not globally.