Vegetation response components to drought regimes attributes in the Mediterranean Basin

Climate models project increasing frequency and intensity of droughts in the Mediterranean Basin, increasing the threat to Mediterranean ecosystems. The lack of water may result in plant wilting and cavitation, reduced resistance to disease and pests, stronger competition between species, and increased wildfire frequency, among many other ecological processes that might be affected. Water-limited ecosystems, like those in the Mediterranean Basin, although adapted to water scarcity, may be particularly vulnerable to extreme droughts. 

The objective of this research is to examine the impact of drought regimes on the response and resilience of Mediterranean ecosystems. We expect to detect a nonlinear relationship between drought regimes and vegetation response as successive drought events cumulate on stronger impacts on ecosystem resilience. To test this hypothesis, we employed an event-based approach to drought regime analysis, for which at each event we measured duration, intensity, severity, and time since the last event as drought attributes. Droughts are detected using the Standardized Evapotranspiration-Precipitation Index (SPEI) at different time scales (3, 6 and 12 months), with precipitation and potential evapotranspiration data retrieved from global downscaled re-analyses of the CHELSA database. We have analyzed the response of vegetation to drought events by extracting the temporal components of resistance, recovery, and resilience. The vegetation response is evaluated using the Normalized Difference Vegetation Index (NDVI), Enhanced Vegetation Index (EVI), Normalized Difference Water Index (NDWI) and Near-infrared Reflectance of Vegetation (NIRV) spectral indices from the MODIS multispectral sensor as proxies of vegetation functioning.  

We examined the 2001-2018 time series for several ecoregions in the Mediterranean Basin to detect the functional shape of the vegetation response curve for this region. Our preliminary results suggest that vegetation response components and drought regime attributes can characterize different aspects of the two variables. Furthermore, the distribution of the vegetation response over drought regimes exhibits multimodal patterns, thereby supporting the hypothesis of a nonlinear relationship. This suggests that the drought response modelling approach used is challenging but promising.