The Imprint of Droughts on Mediterranean Pine Forests

Triggered by frequent high temperatures and scarce precipitation, droughts are a recurrent phenomenon in the Mediterranean Basin, causing significant impacts on forests. We analyzed the effects of drought intensity, duration, and seasonality on tree growth by investigating the relationship between the Standardized Precipitation-Evapotranspiration Index (SPEI) at different time scales and tree-ring width (TRW) in three pine species (Pinus halepensis Mill., P. sylvestris L, and P. uncinata Ramond ex A.DC) throughout a dense dendrochronological network in the Mediterranean Basin. We used generalized linear mixed models to predict such values over the entire distribution of the analyzed species. Furthermore, in areas where the species coexist, we analyzed the same parameters to highlight differences in their responses to similar climatic conditions. Our results showed that the maximum intensity of drought-affected conifers occurred more in the southern areas of the Spanish Mediterranean coast, especially P. halepensis, with maximum values of r = 0.67, while in the rest of the study area, the intensity decreased with elevation; we obtained maximum values of r = 0.40 and r = 0.33 for P. sylvestris and P. uncinata, respectively. This spatial distribution was also related to the duration of the drought impacts, showing that areas with lower intensity had shorter durations (2–4 months). We observed a latitudinal pattern in the seasonality of the drought impacts, with earlier growing seasons at high elevations (June–August) and later ones in the semi-arid Mediterranean. Similar intensity impacts were found in P. halepensis and P. sylvestris coexistence zones, although P. halepensis had a much longer duration and an earlier beginning of seasonality. Higher intensity, duration, and seasonality of drought effects were observed for P. sylvestris in areas where P. sylvestris and P. uncinata are distributed sympatrically. Understanding the plasticity and climatic response of these common southern European species to different types of droughts is crucial in the context of climate change where droughts are increasing in frequency and intensity.