Vulnerability to forest decline in a context of climate changes: new prospects about an old question in forest ecology

International audience1 Why focus on vulnerability?Forest research has been particularly efficient in detecting the impacts of variability and long-term trends of climate. It is indeed in these ecosystems gathering perennial plants with a long lifespan that the first visible impacts of climate change have been convincingly documented. At first, positive changes in productivity were detected (Becker 1989; Becker et al. 1994; Badeau et al. 1995; Badeau et al. 1996). Improved growth conditions linked to warming, to nitrogen deposition and to increased atmospheric CO2 were perceived fairly early and were also heterogeneous among biogeographical areas due to local limiting factors (trophic or climatic). The first thoughts about the Dryade project were initiated during 2006. In France, the first simulations of potential impacts of available climate scenarios generated awareness, criticism and concerns among managers and foresters (Carbofor, Loustau et al. 2005; Badeau et al. 2010). In addition to climate trends, extreme events had already been identified as a major cause of forest dieback. All major drought events were historically followed by cycles of dieback (Innes et al. 1989; Jones et al. 1993; Landmann 1994; Beniston and Innes 1998; Thomas et al. 2002; Liang et al. 2003; Jurskis 2005). In France, forest dysfunctions induced by the drought events of 2003–2006 were mostly reversible (growth decrease, temporary degradation of crown conditions) but sometimes also irreversible (tree mortality). Several indicators of forest health surveys pointed to a degradation of forest health in France and Europe: abnormal increases in tree mortality (Pauly and Belrose 2005), decline of crown conditions (Lloret et al. 2004; Pauly and Belrose 2005; Belrose et al. 2004, 2006; Carnicer et al. 2011) and upsurge in biotic hazards (Nageleisen 2004; Piou et al. 2006; Rouault et al. 2006)