Evidence of osmoregulation in Larix decidua at Alpine treeline and comparative responses to water availability of two co-occurring evergreen species

The water relations of three coniferous species (Larix decidua, Picea abies and Pinus cembra) growing at a treeline eco- tone in the Southern Alps were investigated. Sap flux and xylem water potential were measured in two individuals of each of the above-mentioned species during part of summer 1997. Throughout the growing period, L. decidua showed a gradual decrease in min- imum water potential while for P.abies and P.cembra, variations were more correlated to actual weather conditions. Daily sap flux was also higher in L.decidua than in the two other species. Pressure-volume curves derived for L.decidua during July 1997 demon- strated that the species further diminished its minimum water potential through osmoregulation. During dry periods the other species showed an evident reduction in sap flux suggesting a water saving behaviour. The three species seem therefore to have evolved dif- ferent drought avoidance strategies.