Theoretical models suggest that drylands are particularly prone to suffer critical transitions with abrupt non-linear changes in their functional dynamics in response to climate variation and human disturbance. How drylands undergo functional changes has become an important issue in ecology which needs empirical data to validate theoretical models.
We aim at determining the response of Mediterranean holm oak woodlands to human disturbance in three different climatic areas in order to detect thresholds of changes in the functionality of these ecosystems.
We hypothesize that aridity largely influences this response, with semiarid and dry-transition landscapes being more prone to suffer abrupt changes than subhumid ones.
We selected 138 holm oak sites within a 20,000 km2 region in eastern Spain (Fig 1) distributed within semiarid, dry-transition and subhumid climatic conditions (annual precipitation to potential evapotranspiration ratio: 0.35-0.50, 0.50-0.60 and 0.65-0.75, respectively). They were homogeneous in lithology (calcareous), topography (flat) and soil type (Mollic Haploxeralfs), but affected by different disturbance intensities (estimated as an inverse function of tree cover, Fig.2). In each site, precipitation use efficiency (vegetation production/ annual precipitation) was estimated by remote-sensing from enhanced vegetation index (EVI) observations (MODIS) and soil (organic matter, enzymatic activity) and vegetation (functional groups) parameters were sampled in ten 10 x 2 m subplots.
Results/Conclusions
Although no threshold of abrupt change is observed, important differences in the functional response of holm oak woodlands to disturbance exist between the three climatic zones. In subhumid woodlands, the structural change from a woody- to a herbaceous-dominated landscape enhances the recovery of the functional state of the ecosystem at high-disturbed intensities after an important loss of functionality at low disturbance intensities. Semiarid and dry-transition woodlands suffer a more accelerated loss of functionality at high-disturbed intensities (than subhumid ones) that cannot be counterbalanced by the woody-dominated vegetation.