WATER RELATIONS OF EUROPEAN SILVER FIR (ABIES-ALBA MILL) IN 2 NATURAL STANDS IN THE FRENCH ALPS SUBJECT TO CONTRASTING CLIMATIC CONDITIONS

This paper reports on the diurnal and seasonal variations in water potential, stomatal conductance, and transpiration of twigs from silver fir in a mesohygrophilic stand of the external French Alps, and in a mesoxerophilic stand in the inner French Alps where this fir grows near its ecological limits. In both stands, predawn needle water potential was always 0.2-0.4 MPa below the potential of the driest soil layer. In the first one, it was maintained at about -0.4 MPa. Maximum stomatal conductance and maximum transpiration, which could reach 200 mmol/m2/s and 1 mmol/m2/s, respectively, occurred at the same time which corresponded to minimum leaf water potential. In the dry stand, predawn needle water potential never dropped below -1.14 MPa, yet a general browning of older needles was already observed. The decrease of predawn needle water potential was accompanied by the decrease of maximum stomatal conductance and transpiration to 15% of their highest value, which reached 150 mmol/m2/s and 1 mmol/m2/s, respectively, at this stand. Maximum stomatal conductance occurred in general before UT 07.00, and maximum transpiration 5-6 h later, irrespective of predawn needle water potential. Furthermore, in both stands, stomata closed at vapor pressure deficit value as low as 0.3 kPa. This extremely early reaction to water stress exhibited by European silver fir is consistent with its well-known sensitivity to atmospheric humidity and soil water availability. It indicates a strong avoidance strategy, which we have hitherto attributed only to species better adapted to drought.