Recent studies have highlighted that higher species richness can increase the resistance and/or the resilience to disturbances and stresses. The present study quantifies the overall tree species richness effect on growth and intrinsic water-use efficiency (iWUE) in three target tree species (i.e. Fraxinus excelsior, Juglans spp. and Prunus avium) after drought in six deciduous plantations in the Friuli-Venezia Giulia plain, North-eastern Italy. Planting densities, management, climatic and soil characteristics were the same at all the plantations. Stands differed only for their total surface area and for their total tree species richness (3, 4, 6, 7, 8 and 9). We double-sampled 15 dominant trees for each of the three target species, we measured tree-ring width, and we removed age-related trends using a detrending function. We selected 2006 as the driest year at the sites and 2014 as the reference year using the De Martonne Index. For both years, we measured δ13C signature in tree rings to calculate iWUE. Tree species richness had a positive effect on the response to drought both in terms of normalized ring width and iWUE, but only at a lower number of consociated species (< 5), when facilitation and/or complementarity mechanisms prevailed. Instead, negative responses were typical at higher levels (≥ 5), when competition was the dominant process within the stand. Moreover, species richness had no effect on tree growth in 2014, maybe because either competition or complementarity processes did not occur, or these processes could cancel out each other, when environmental conditions were not limiting.
