Tree water relations and climatic variations at the alpine timberline: seasonal changes of sap flux and xylem water potential in Larix decidua Miller, Picea abies (L.) Karst. and Pinus cembra L.

Trees growing at the alpine timberline very seldom undergo severe water stress because of high precipitation during the vegetative period. Since trees are adapted to moist conditions, moderate water deficit may lead to a strong reduction in transpiration. Transpiration and xylem water potential were measured in two individuals each of Pinus cembra, Larix decidua and Picea abies growing at the timberline (2 080 m a.s.l.) in the north-eastern Italian Alps. From June to October 1996 predawn water potential was between -0.29 and -1.0 MPa with moderate differences among species. Throughout the growing period L. decidua showed a progressive decrease in the minimum water potential (from -0.45 to -1.93 MPa); in P. abies and P. cembra variations were more correlated to weather conditions with minima (-1.2 and -1.49 MPa, respectively) during a mild drought period. L. decidua showed the mean daily maximum sap flux density (about 3.3 dm 3 dm-2 h-1) while mean maximum values in P. abies and P. cembra were about 0.9 and 0.7, respectively. High daily fluctuations of sap flow were observed in relation to rapid variations in weather conditions, particularly in L. decidua. Regardless of species a very high stomatal sensitivity to vapour pressure deficit was recorded. The three species seem to have evolved different drought avoidance strategies. L. decidua maintained a relatively high transpiration even during moderate water deficit periods because of its high water uptake capacity. During the same drought period P. abies and P. cembra showed an evident reduction in sap flux, suggesting a water saving behaviour. These different responses should be taken into account when considering the effects of global change on timberline trees.