The impact of an unprecedented hotter drought on oak growth was related to climate of origin

Future forests will have to face compound climate events such as hotter droughts as climate keeps warming. A better understanding of tree growth resilience to such extreme droughts is key to better inform future forests capacity to provide ecosystem services. Arboreta provide a suitable setting to assess the capacity of tree species from different origins to respond to drought under similar climatic conditions. However, the comparison of growth trajectories and growth resilience to drought of arboretum trees are still scarce, particularly for temperate oaks, a major group of trees with remarkable ecological and economical values across the northern Hemisphere. We propose that the comparison of the post-drought growth resilience of oaks co-occurring in an arboretum along with bioclimatic variables, and functional (wood and leaf) traits can help to identify vulnerable species in sight of climate change. To this end, we studied 20 oak species originating from different regions around the world, planted in the Iturrarán botanical garden located in Northern Spain. Dendrochronology was used to calculate growth statistics and resilience indices during the exceptionally hot 2022 drought. These indices were related to: bioclimatic variables, tree diameter, leaf (leaf area, leaf mass per area) and xylem traits (wood density, hydraulic diameter, conductive area, Huber value). Additionally, we compared resilience and drought legacies for eight oak species sampled in the arboretum and in the field, respectively. All oak species showed marked growth reductions in the year 2022. Diameter and mean ring width covaried with leaf traits and wood density since wider rings were observed in species with lower leaf mass per area. Resilience to the 2022 drought was related to the precipitation of origin of each species. The resilience data of the eight oak species sampled in the arboretum and in the field showed a weak positive, but not significant relationship. Among-tested variables, precipitation-of-origin explains the largest share of the variation in resilience to drought. Tree-ring data from arboreta trees provide relevant information which can be used to better preserve and manage temperate forests subjected to more arid conditions.