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 CO2 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.