Intraspecific Trait Variation Drives Functional Shifts in Grass Communities Under Grazing in the Scottish Uplands

Questions: Shifting grazing regimes across Europe are altering temperate grasslands, affecting biodiversity and ecosystem services. While grazing influences the structure of these grasslands, and thus the distribution of functional traits, the relative role of species turnover versus intraspecific variation (ITV) in governing trait variation of temperate grasslands is poorly understood. We investigated the impacts of grazing on grass community composition and structure within a 20-year landscape-level grazing experiment in the Scottish uplands to determine if differences in grass community functional composition are driven by ITV. Location: Glen Finglas, Scotland, UK. Methods: We exploited the design of the experiment to contrast three levels of sheep grazing across three plot locations and two drainage positions (upslope and downslope). Grass composition and seven grass functional traits representative of grazing adaptations were measured. Multivariate abundance models assessed the effect of grazing treatment, location and plot drainage position on grass community composition, while phylogenetic linear mixed models (PLMMs) tested for trait differences between plots. Grass community trait variation was then decomposed to quantify the relative contributions of species turnover and ITV in trait responses to grazing and environmental variables. Results: Compositional differences between plots were largely driven by plot location, with a marginal effect of grazing treatment. Increased grazing shifted grasses towards combinations of avoidance (e.g., thick leaves), resistance (e.g., small stature) and tolerance traits (e.g., high SLA). ITV accounted for most trait variation in response to grazing, indicating that community-level functional trait changes occur primarily through within-species trait adjustments rather than species turnover. Conclusions: The high ITV observed in our results suggests that these grasslands exhibit community resilience to grazing disturbances. Recognising the role of ITV in grass community responses will improve predictions of how changes in grazing management impact Scottish grassy ecosystems, informing grassland management and conservation strategies.