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 ecotone 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 minimum 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 demonstrated 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 different drought avoidance strategies. osmotic adjustment / Pinus cembra / Picea abies / sap flow / pressure-volume curves Rsum -vidence d'un ajustement osmotique chez Larix decidua la limite de l'tage forestier alpin et rponses compares la disponibilit en eau de deux espces de conifres co-existantes. On a tudi les stratgies hydriques de trois espces de conifres (Larix decidua, Picea abies et Pinus cembra) qui poussent dans un cotone la limite suprieure de l'tage forestier dans le sud des Alpes. On a mesur le flux de sve et le potentiel hydrique du xylme pour deux individus des espces cites prcdemment pendant une partie de l't 1997. Durant la priode de vgtation, L. decidua a montr une diminution progressive du potentiel hydrique minimum tandis que les variations de P. abies et de P. cembra taient corrles en grande partie aux conditions atmosphriques. Le flux de sve journalier aussi tait plus important chez L. decidua que dans les deux autres espces. Les courbes pression-volume tablies pour L. decidua au mois de juillet 1997 montrent que l'espce est en mesure de diminuer ultrieurement son potentiel hydrique minimum travers un ajustement osmotique. Pendant les priodes de scheresse, les autres espces ont manifest une rduction visible du flux de sve suggrant ainsi un comportement adapt pour limiter la consommation d'eau. Les trois espces semblent donc avoir dvelopp des stratgies diffrentes pour viter la scheresse.