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 wate r stress because of high precipitation during the vegetative period. S ince 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 preda wn water potential was between -0.29 and -1.0 MPa with moderate differ ences 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) dur ing a mild drought period. L. decidua showed the mean daily maximum sa p flux density (about 3.3 dm(3) dm(-2) h(-1)) while mean maximum value s 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 var iations 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 drough t avoidance strategies. L. decidua maintained a relatively high transp iration 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 sa ving behaviour. These different responses should be taken into account when considering the effects of global change on timberline trees. (( C) Inra/Elsevier, Paris.).