Holm oak fecundity does not acclimate to a drier world

Climate change might impact tree fecundity by altering the relative influences of meteorological and physiological drivers, and by modifying resource investment in reproduction. Using a 13-yr monitoring of Quercus ilex reproduction in a rainfall exclusion experiment, we analysed the interactive effects of long-term increased aridity and other environmental drivers on the inter-annual variation of fecundity (male flower biomass, number of initiated and mature fruits). Summer-autumn water stress was the main driver of fruit abortion during fruit growth. Rainfall exclusion treatment strongly reduced the number of initiated and mature fruits, even in masting years, and did not increase fruit tolerance to severe drought. Conversely, the relative contribution of the meteorological and physiological drivers, and the inter-annual variability of fruit production were not modified by rainfall exclusion. Rather than inducing an acclimation of tree fecundity to water limitation, increased aridity impacted it negatively through both lower fruit initiation due to changes in resource allocation, and more severe water and resource limitations during fruit growth. Long-term increased aridity affected tree reproduction beyond what is expected from the current response to inter-annual drought variations, suggesting that natural regeneration of holm oak forest could be jeopardised in the future.