Do Quercus ilex woodlands undergo abrupt non-linear functional changes in response to climatic variation and human disturbance?

Drylands are experiencing major alterations as a result of complex interactions between climatic fluctuations and disturbances caused by human activities. Theoretical models suggest that these ecosystems are particularly prone to suffer critical transitions with abrupt non-linear changes in their structure and functions in response to climate variation and human disturbance. How drylands undergo environmental change has become an important issue in ecology which needs empirical data to validate theoretical models. We studied the functional response of holm oak (Quercus ilex) woodlands to human disturbance along an aridity gradient (from semi-arid to sub-humid conditions) in eastern Spain, using (a) remote-sensing estimations of precipitation-use-efficiency (PUE) from enhanced vegetation index (EVI) observations performed in 231x231 m plots of the Moderate Resolution Imaging Spectroradiometer (MODIS) and (b) soil parameter determinations (extracellular soil enzyme activity: phosphatase, ß-glucosidase and urease, and associated nutrient cycling processes) from soil sampled in the same plots. We evaluated and compared the shape of the relationship between ecosystem functionality (in terms of PUE and soil parameters) and disturbance intensity for our holm oak sites along the aridity gradient by fitting the experimental data to linear vs. non-linear model equations. Overall, our results demonstrated that climatic conditions along the aridity gradient played a relevant role in the shape of the functional response of Quercus ilex woodlands to human disturbance. Whereas sub-humid areas showed a linear decrease with an increasing disturbance intensity, semi-arid areas were characterized by non-linear dynamics with the existence of abrupt changes at critical disturbance intensity values.