Past and future changes in water and carbon fluxes in temperate managed Pine forests from Southern France : attribution to climate, management and biophysical drivers

International audienceIntensification of forest management concerns an increasing fraction of temperate and tropical forests. The managed Pine forests in south-western Europe are submitted to increased soil preparation, fertilisation, drainage, thinning, clearcutting, whole tree - harvesting and rotation shortening and therefore provide a good example of such management changes. For the last 15 years, these forests were hit by a series of extreme climate events: two unprecedented storms in 1999 and 2009, severe soil droughts in 2002, 2005 and 2006 and heatwaves in 2003 and 2005. At flux tower sites located in a young stand following clearcut and mature stands respectively, the half-hourly fluxes of CO2, H2O vapour and energy as well as vegetation and soil carbon and water contents have been monitored during this period. Using data collected from flux tower sites and forest and soil inventories together with a process based model of forest growth, GO+, and geographic information, we analysed the impact of these events on the time series of forest canopy exchanges of water and CO2 and its interaction with management. The Bowen ratio of the forest was strongly enhanced and evapotranspiration decreased leading to a dramatic increase in water runoff and peak flows from the watersheds damaged by windstorms. Clearcutting following wind storms reversed the ecosystem from a net sink into a source of C-CO2 and that was not offset ten years later. Soil drought impacted mature forests through stomatal closure and leaf shedding, making their annual carbon balance almost neutral. Tree growth was however not affected to the same extent. Drought affected also dramatically the net carbon and water balances of young forest stands. However, at this stage, the effects of successive management operations (ploughing, vegetation burial, thinning) overtook climate impacts. Independent of stand age, the canopy photosynthesis was more sensitive to climate and management than the ecosystem respiration. A direct projection of these data allowed to assess the impact of the successive storms and droughts on the regional CO2, heat and water vapour fluxes at the sub-regional scale. The GO+ model applications to the prediction and analysis of climate scenarios impacts on southwestern European forests allowed to attributing the role of management alternatives, precipitation regime, CO2 concentration and atmospheric humidity in forest-atmosphere exchanges at larger spatial and temporal scales. Frequency of soil preparation operations and vegetation management at the juvenile stage and climate and rotation duration at the adult phase were the major drivers of the carbon and water balances, respectively. The model predicts that a drier and warmer climate will reduce productivity and deplete soil carbon stocks in forest from Southwestern Europe within decades, such effects being amplified by intensive management alternatives