Large herbivores and abiotic drivers jointly shape spatiotemporal grassland dynamics in a subalpine ecosystem

Managed grazing has a larger geographic extent than any other global form of land use. In subalpine grasslands, the spatiotemporal dynamics of vegetation during its growing season may depend on grazing intensity, the type of vegetation and abiotic drivers, as well as their complex interactions. Here, we examine how free-range grazing cows and abiotic factors (climate and topography) drive grassland dynamics over three growing seasons in a Pyrenean valley. We combine high spatial resolution satellite imagery to estimate our response variable (the Soil-Adjusted Vegetation Index (SAVI), a proxy of vegetation greenness and biomass that correlates with forage quantity and quality), with GPS-tracking of cows (80-85 cows per year), the primary herbivores in the valley. We generated a detailed vegetation map and used GPS accelerometer data to separate grazing from resting cow activities, the latter resulting in higher trampling and nutrient deposition on vegetation. Results indicate that cows showed a clumped spatial distribution, consistent with collective behaviour and a heterogeneous use of the available space. SAVI showed clear seasonal cycles, peaking in early summer and declining to winter dormancy. Grassland dynamics were influenced by climate variability, both seasonal (solar radiation and temperature) and non-seasonal (soil water content, precipitation, and wind). The other drivers were also selected in the best multiple regression model, which explained 46% of SAVI temporal variance. Both cow activities had negative impacts on vegetation, with lower SAVI values associated with resting; grazing showed non-linear negative effects on SAVI, varying synergistically with elevation, slope, and orientation. The nonlinearity resulted from a steep initial decline in SAVI with grazing, followed by a diminishing negative effect as grazing intensity increased. These findings reveal the complex interplay between biotic and abiotic drivers in SAVI dynamics, emphasising the role of large herbivores in the ecological processes in subalpine ecosystems under varying environmental conditions.