Grazing management or physiography? Factors controlling vegetation recovery in Mediterranean grasslands

Grazing intensification and abandonment are increasing the risk of degradation of Mediterranean grasslands. The development of techniques for monitoring grazing effects on herbaceous vegetation is an essential need for the management of these rangelands. However, the high variability of these systems make physiographical and management effects hard to disentangle and quantify. We present a methodology to support rangeland management and assess grazing effects on environmentally heterogeneous areas, and provide an example of its application in a Mediterranean rangeland in central Spain. We evaluated the difference in photosynthetically active vegetation cover between spring and summer using Spectral Mixture Analysis of very high spatial resolution (2.4m) Quickbird images. To analyze this difference, we developed Boosted Regression Trees models using grazing management (accumulated cost distance to points of livestock concentration and water points) and physiographical variables (slope, wetness, proximity to the closest tree and orientation). Results show that the main factor determining changes in vegetation cover is habitat type. The magnitude of this change was maximized at intermediate grazing pressures for humid habitats, suggesting the existence of an optimal level of grazing in this zones, while in dry habitats differences in vegetation cover increased consistently along with grazing pressure. Our models provided a valuable insight into how different variables and its interactions affect the observed recovery capacity of vegetation. Moreover, by identifying areas in which grazing-induced land degradation could be taken place, our methodology can be used as a powerful tool in the management of highly heterogeneous rangelands.