Aridity induces nonlinear effects of human disturbance on precipitation-use efficiency of Iberian Quercus ilex woodlands

Abstract

The effects of land degradation are pervasive worldwide, particularly under the present context of global changes in climate and land uses, which are affecting biodiversity and landscape functioning at an unprecedented rate. How ecosystems undergo environmental change is recognized as one of the main frontiers in ecology and environmental sciences. Land degradation may take place in either a smooth gradual way or a more abrupt manner. Theoretical studies and empirical evidences increasingly suggest that drylands are particularly prone to develop abrupt changes in their structure and functions in response to climate variations and human disturbance. Precipitation-use efficiency (PUE) represents the ratio of vegetation production to annual precipitation, and provides an excellent evaluation tool for the assessment of human and climate impacts on landscape functionality. Holm oak (Quercus ilex) forests are one of the most conspicuous dry woodlands in the Iberian Peninsula and the Mediterranean basin, and show a variety of conservation status, due to their very long history of human use. We study the response of holm oak woodlands to human disturbance along an aridity gradient (i.e. semiarid, dry-transition and sub-humid landscapes; annual precipitation to potential evapotranspiration aridity ratio 0.40-0.50, 0.50-0.65, and >0.65, respectively) across a 20,000 km2 region in eastern Spain, using remote-sensing estimations of PUE from enhanced vegetation index (EVI) observations of the Moderate Resolution Imaging Spectroradiometer (MODIS). PUE decreases from semiarid to sub-humid climate conditions in the studied holm oak landscapes, suggesting that as aridity and water stress decrease, other factors (e.g. nutrient and light availability) may control vegetation production. Overall, our results indicate that aridity largely impacts the response of PUE to human disturbance, assessed as a function of the integrity of vegetation conditions. PUE decreases linearly with the degradation of tree cover in subhumid holm oak woodlands. Differently, semiarid and dry-transition landscapes show little change in PUE by increasing disturbance intensity until a threshold is passed in vegetation conditions (30%-50% cover), where PUE decreases abruptly. The impact of disturbance on PUE is larger for dry years than for wet years, and increases with climate aridity from sub-humid to dry-transition and semiarid woodlands. Therefore, aridity may also interact with land degradation in Mediterranean holm oak woodlands by intensifying the loss of the ecosystem ability to buffer large changes in vegetation production caused by climate variability.