Intraspecific plant trait variation and grassland ecosystem function

The effects of global environmental change on ecosystem functions, such as carbon (C) and nitrogen (N) cycling, are in part mediated by changes in plant community composition, structure and productivity. Plant traits can serve as easily measurable proxies for plant function, useful for predicting vegetation responses to environmental change and effects of vegetation on ecosystem function. However, many trait-based studies do not take into account intraspecific trait variability (ITV) and it is unclear how much uncertainty this introduces. The overarching aim of this thesis was to improve understanding of the drivers that control ITV as well as the consequences of ITV for ecosystem functions related to C and N cycling in grassland ecosystems. To achieve this, key drivers of ITV including soil properties, neighbouring plants, N addition and drought stress were investigated, as well as consequences of ITV for ecosystem properties and function. A calcareous grassland field biodiversity experiment was used to investigate how neighbouring plants and soil properties affect ITV. A controlled outdoor mesocosm experiment was designed to investigate the effects of plant species interactions and N addition on ecosystem C and N cycling, and whether these effects were mediated by plant trait plasticity. A greenhouse drought experiment with a subsequent litter decomposition essay was conducted to investigate if drought-induced plasticity of root and shoot traits alters their decomposability. Overall, the results indicate that plant species interactions, soil properties, nutrient availability and drought stress contribute to controlling ITV in grasslands, but that the exact patterns of ITV are often species-specific. Phenotypic plasticity in response to these environmental drivers had either weak or no effects on ecosystem functions related to C and N cycling. This suggests that in contexts similar to the ones examined here it may be justified to ignore ITV in trait-based studies and focus on species means. However, particular species sometimes had disproportionate effects on ecosystem functions relative to their contribution to biomass, which might contribute to explaining why the explanatory power of plant traits for predicting ecosystem functions is often low