Wood type influences tree responses to recurring hot and dry summer tree water deficits across European forests

Increasingly frequent summer droughts are intensifying stem dehydration across European forests, constraining tree growth and resilience. Even mild water-related stress can inhibit tree growth, although its impact is species-specific. Tree water deficit (TWD), a measure deduced from radial stem contraction during drought, reflects stem water status and can be used as a proxy for the physiological drought stress experienced by trees. Using high-resolution dendrometer measurements representing 29 tree species from 130 forest sites across Europe from 2003 to 2018, we examined how the length of TWD periods during summer is related to their recurrence within the same year, to TWD intensity, to climate factors, and to wood type (i.e., tracheid-based coniferous, ring-porous or diffuse-porous anatomy). A greater degree of incomplete rehydration, indicated by highest minimum TWD during TWD periods (70% increase of expected response for conifers and ring-porous species, 42% increase of expected response for diffuse-porous), together with warmer temperature (44% increase of expected response for all wood types) and stronger drought, prolonged the length of TWD periods. However, the recurrence of TWD periods only increased the length of the subsequent TWD period within the same year for diffuse-porous tree species (12% increase of expected response). Our results show that wood type and its associated functional traits influence tree responses to recurring stem dehydration periods. The greater sensitivity of diffuse-porous species to recurring TWD periods suggests that wood type anatomy may play an important role in determining how tree species cope with increasingly frequent summer droughts.