Comportement hydrique du frêne (Fraxinus excelsior L) dans une formation montagnarde mésoxérophile

Abstract

Summary — Water relations of ash (Fraxinus excelsior L) in a stand. This paper reports on the diurnal and seasonal variations in water potential, stomatal conduc- tance, and transpiration of the leaves of an ash tree in a mesoxerophilic mountain summer drought the pre-dawn water potential and minimum water potential of leaves may fall as low O) m the intermediate zone of the north-western Alps. The pre-dawn water potential is close to the soil water potential at 30 cm mesoxerophilic mountain stand situated in depth and is closely correlated with the latter. In the case of pronounced as -4.8 MPa and -5.5 MPa, respectively. Under good water supply conditions the maximum stoma- tal conductance may reach 200 mmol (H alluvial forests. The maximum stomatal conductance holds for only a short time, as stomata partially close at noon 2, a value much lower than that of ash leaves in -2 .s -1 mum stomatal conductance decreases as the soil dries up. Accordingly ash water uptake is less in mountain stands than in alluvial forests. The survival of ash during drought is, however, attributed to tolerance strategy rather than to avoidance strategy.