Cascading effects of shrub encroachment in sub-alpine grasslands from a trait-based perspective

In the context of climate change, shrub encroachment is expected to advance in temperature-sensitive ecosystems such as arctic and alpine grasslands. In mountain regions, shrub encroachment is further triggered by grassland management changes (i.e., land abandonment or extensification). Shrub encroachment is expected to significantly impact ecosystem properties and functions like carbon stocks in both aboveground and belowground compartments, nutrient concentrations, and to slow down biogeochemical cycles. While studies of shrub encroachment processes and their effects on plant and soil functioning have increased our understanding of underpinning processes, there is still a lack of integrated studies and knowledge gaps on the interaction between plant-soil changes and their cascading effects. Our study focuses on understanding these effects on plant community traits and soil properties, and whether these changes are linear. We took herbaceous, shrub and soil samples along gradients of encroachment in sub-alpine grassland communities at two sites in the Alps: Lautaret (France) and the Stubai Valley (Austria). We used a trait-based approach to analyze hypothesized nonlinear functional changes in communities, using community-weighted means (CWM) to scale herbaceous and shrub functional traits and plant allometries. Structural equation models (SEM) support our hypothesis that changing CWM with increasing shrub biomass flows on to changes in soil properties. Dwarf shrub encroachment leads to more conservative and less nutrient-rich plant communities, resulting in an accumulation of recalcitrant organic matter and nutrient-poor soils. Nevertheless, contrary to our expectations, decreased nitrogen in plant communities along the encroachment gradient did not lead to decreased soil available nitrogen. Our results generally suggest it is possible to characterize shrub encroached ecosystems in the Alps using well-studied traits of the global plant economic spectrum, like nitrogen content or dry matter content. With these findings, we are confident that well-researched trait-based models are also applicable for dwarf shrubs, allowing to scale-up from plant traits to the delivery of ecosystem services. This research provides a novel understanding of shrub encroached ecosystems and is a first step in understanding the patterns and mechanisms underpinning their provision of ecosystem services.