Drought assessment through actual evapotranspiration in Mediterranean vegetation dynamics

The increasing dryness of the environment is due to the rapid rise in temperature and changes in precipitation patterns. This has made it even harder for the soil to retain moisture, causing greater water stress in terrestrial ecosystems. Mediterranean ecosystems are especially vulnerable to this problem because they face a higher risk of heat waves and prolonged droughts during summer. This has far-reaching ecological consequences, such as changing the composition of plant communities and impacting ecosystem functionality. To understand the impact of aridity on vegetation, it is crucial to consider the broader context. This work aims to propose an integrated analysis using the Standardized Precipitation Evapotranspiration Index (SPEI) to characterize the influence of aridity on three vegetation types (trees, shrubland, grassland) at the following study sites within the NL4DL project (LIFE 20/PRE/IT/000007): Palo Laziale wood (Italy), Alta Murgia National Park (Italy), Delta of the Nestos River (Greece). This study compares SPEI based on potential evapotranspiration (Et0) and SPAEI based on actual evapotranspiration (Eta). Actual evapotranspiration is computed using the NDVI-cws model, which combines meteorological data from weather stations and satellite-derived NDVI vegetation data. The respective SPEI/SPAEI values are aggregated on a monthly (SPEI1/SPAEI1) and quarterly (SPEI3/SPAEI3) basis to observe seasonal variability over the considered period (2000-2022). The results show that in Palo Laziale wood, no increase in aridity is observed, as confirmed by both SPEI3 based on Et0 (slope = 0.16 Qrt- 1) and SPAEI3 based on vegetation types. In contrast, for the Alta Murgia National Park, SPEI3 indicates an intensification of aridity (slope = -7.00E-4 Qtr- 1), which is not in agreement with SPAEI3 based on Eta for shrubland (slope = 3.20E-4 Qtr- 1). A similar pattern is observed for the Delta of the Nestos River, where a negative SPEI3 value is recorded (slope = -1.90E-4 Qtr- 1), while the SPAEI3 for shrubland is positive (slope = 3.50E-4 Qtr- 1). These observations provide insight into the impact of aridity on Mediterranean vegetation composition. The response of shrubland in comparison to trees and grassland types affirms its better adaptability to drought.