Drought effects on water relations in beech: The contribution of exchangeable water reservoirs

Abstract

The exceptional soil drought and heat wave that occurred in Europe in summer 2003 provided a good opportunity to analyze the response of forest ecosystems to extreme climatic conditions. This work aimed at studying the functioning of exchangeable water reservoirs of beech trees under climatic and edaphic constraints, in a 37-year-old beech stand located in north-eastern France. We characterised the impact of drought on seasonal variations in water fluxes at the tree (sap flow measurements) and the forest (eddy covariance measurements) scales and estimated (i) the daily water storage capacity of beech trees as the difference between stand-scaled sap flow and water vapour flux over the stand and (ii) the contribution of exchangeable water in tree reservoirs to the total tree transpiration, under non-limiting soil water (summer 2002) or severe soil drought conditions (summer 2003). In parallel, daily variations in trunk circumference were analyzed to estimate the contribution of elastic tissues as a compartment for water storage to tree transpiration. Stand transpiration was strongly reduced by soil water shortage (up to 80% at the peak of drought in August 2003). From the beginning of August 2003, we observed daytime contraction of stem circumference, with only partial, or even no night recovery, suggesting that trees were less and less able to refill the elastic and strongly depleted reservoirs. Even if those elastic reservoirs were active throughout the season, the corresponding volume of water withdrawn for tree transpiration remained very low (maximum 1% of the daily transpiration). Thus, elastic tissue reservoirs play a minor role in the total water budget of beech trees. The amount of water depleted daily from the whole reservoirs was much higher than the water extracted from elastic tissues. Furthermore, the contribution of total exchangeable water within trees to the transpiration stream increased during the dry period (from a few percent to 67% at the peak of the drought) and was positively correlated with soil water shortage. Our results thus clearly demonstrated the strong sensitivity of beech to both climate and drought and the major role of whole tree water reservoirs to maintain leaf transpiration under severe drought.