Differential impact of the most extreme drought event over the last half century on growth and sap flow in two coexisting Mediterranean trees

Extreme climatic events such as intense droughts are becoming more frequent in Mediterranean regions, but our understanding of their impact on tree performance is still fragmentary. We analyzed growth and sap flow responses for a 3-year period including the most stressful drought over the last half century in the evergreen Pinus nigra and the deciduous Quercus faginea, two dominant tree species in the continental plateau of the Iberian Peninsula. Our aim was to quantify the differential impacts of this event on the performance of both species and their modulation by local microclimate. Growth was registered with digital dendrometers, and water use was assessed by continuously recording sap flow in 8-9 coexisting adult individuals of each species in two sites. Q. faginea spring growth rate decreased by 60 % during the dry year at the dry site, while the decrease in P. nigra was around 36 %. P. nigra exhibited larger sap flow reductions during the dry season and also larger decreases during the extreme year, but in contrast to Q. faginea, it was able to recover growth and sap flow values after the extreme drought. Minor microclimatic differences between sites had significant effects on growth and water use, with slightly more mesic conditions significantly attenuating the impact of drought on both species. Findings suggest that the study species were near to their tolerance thresholds, so that even moderate increases in the intensity and frequency of unusual droughts have important consequences for individual tree performance, and eventually species coexistence and ecosystem processes.