Temperate forest trees and stands under severe drought: a review of ecophysiological responses, adaptation processes and long-term consequences

The extreme drought event that occurred in Western Europe during 2003 highlighted the need to understand the key processes that may allow trees and stands to overcome such severe water shortages. We therefore reviewed the current knowledge available about such processes. First, impact of drought on exchanges at soil-root and canopy-atmosphere interfaces are presented and illustrated with examples from water and CO 2 flux measurements. The decline in transpiration and water uptake and in net carbon assimilation due to stomatal closure has been quantified and modelled. The resulting models were used to compute water balance at stand level basing on the 2003 climate in nine European forest sites from the CARBOEUROPE network. Estimates of soil water deficit were produced and provided a quantitative index of soil water shortage and therefore of the intensity of drought stress experienced by trees during summer 2003. In a second section, we review the irreversible damage that could be imposed on water transfer within trees and particularly within xylem. A special attention was paid to the inter-specific variability of these properties among a wide range of tree species. The inter-specific diversity of hydraulic and stomatal responses to soil water deficit is also discussed as it might reflect a large diversity in traits potentially related to drought tolerance. Finally, tree decline and mortality due to recurrent or extreme drought events are discussed on the basis of a literature review and recent decline studies. The potential involvement of hydraulic dysfunctions or of deficits in carbon storage as causes for the observed long term (several years) decline of tree growth and development and for the onset of tree dieback is discussed. As an example, the starch content in stem tissues recorded at the end of the 2003's summer was used to predict crown conditions of oak trees during the following spring: low starch contents were correlated with large twig and branch decline in the crown of trees. drought / water balance / time lag effect / hydraulic properties / dieback Rsum -Arbres et peuplements forestiers temprs soumis scheresse : une revue des rponses cophysiologiques, des processus d'adaptation et des consquences long terme. La scheresse exceptionnelle de 2003 a t l'occasion de faire le point de nos connaissances sur les mcanismes cophysiologiques permettant aux arbres de traverser un tel vnement climatique extrme. L'analyse a t conduite l'chelle de l'arbre et du peuplement, tandis que l'intensit de la scheresse a t quantifie l'aide d'un calcul de bilan hydrique sur neuf sites forestiers europens contrasts du rseau CARBOEUROPE. Le rle du couvert dans les changes avec l'atmosphre est rappel puis intgr dans l'analyse des rductions de bilan d'eau et de carbone en 2003 dus la rgulation stomatique. Les caractristiques du complexe sol-racine, important la fois pour l'accs la ressource et l'efficience de son absorption, constituent un des premiers traits d'adaptation la scheresse. La rponse et les adaptations des espces ont surtout t analyses en termes de diversit inter-spcifique de fonctionnement hydraulique et du couplage entre proprits hydrauliques et rgulation stomatique. Enfin, nous discutons l'hypothse selon la quelle les dysfonctionnements hydrauliques ou les dficits de mise en rserve sont impliqus dans les ractions diffres de croissance, de dveloppement, d'induction de dprissement. Par exemple, des mesures de rserves glucidiques dans les troncs de chnes menes en fin d't 2003 ont permis de prdire l'tat des couronnes des arbres au printemps 2004. Les faibles taux d'amidon taient associs une forte mortalit de branches et de jeunes pousses.