Responses to drought of two Mediterranean ring-porous, deciduous species: Searching for climate smart trees and shrubs

Drought-tolerant tree species with high growth rates and a good capacity for carbon storage in woody tissues (dense wood) are searched for due to aridification. Deciduous, ring-porous tree and shrub species could show such drought tolerance and growth traits, thus representing good candidates for climate-smart rewilding. However, we still do not know the long-term growth rates of these species and how they respond to drought, particularly in climate change hotspots such as the Mediterranean Basin. We analysed these issues at the site and individual levels in two ring-porous, deciduous species (Pistacia terebinthus, Celtis australis) using dendroecology and wood anatomy. The ring width, earlywood vessel diameter, vessel density (VD) and area (%) were measured in two focal sites, one per species, and then growth data were compared with two secondary sites to test if site-to-site synchrony changed through time. Ring-width indices (RWI) and the hydraulic diameter (Dh) of earlywood were calculated. Growth rates (ring width), Dh and vessel area were higher in C. australis (1.03−2.26 mm, 269 μm, 33.9 %) than in P. terebinthus (0.57−0.72 mm, 146 μm, 21.5 %). Consequently, VD was higher in P. terebinthus than in C. australis (104 vs. 61 vessels mm⁻²). The ring width and Dh were more coupled in P. terebinthus (r = 0.43) than in C. australis (r = 0.32). RWI series of the focal and secondary sites have been synchronized since the 1990s as temperatures rose. Precipitation during the growing season (May, June) enhanced growth and VD of both species. P. terebinthus was more responsive to a drought index than C. australis. The two study species show high growth rates and tolerate drought being thus suitable candidates for climate-smart rewilding.