Utilisation d'une chambre de transpiration portable pour l'estimation de l'évapotranspiration d'un sous-bois de pin maritime à molinie (Molinia coerulea (L) Moench)

Abstract

Water stress is frequent in stands of maritime pine in Southwest France and silvicultural practices should be optimized with this factor in mind. The consumption of available water by the vegetation of the understory has not been quantified, though it should be taken into account in management practices such as thinning or fertilization. This paper reports the results of measurements of hourly and daily evapotranspiration (ET) of a 19-yr-old maritime pine (Pinus pinaster Ait) stand understory. A portable closed transpiration chamber covering a projected area of 0.5 m2 was used to measure the ET from purple heath grass (Molinia coerulea (L) Moench) which accounted for more than 90% of the leaf area index of the understory. The ET was determined by enclosing tussocks and measuring the variations of dew-point temperature for 1 min. From these measurements, both the daily and hourly ET of the understory was estimated for 13 d in 1988 and 1989. The canopy conductance of the understory was also estimated. These results were compared with the transpiration of pine computed from sap flow measurements. When the total water content of the rooted zone was above 40 mm, the ET of the understory varied between 1 and 1.5 mm.day-1; it remained constant in relation to the Penman PET measured at 1 m above the ground. This correspond to 1/3 of the transpiration of the trees. Understory canopy conductance remained fairly constant throughout the day and fluctuated between 0.4 and 0.5 cm.s-1. When the soil water content dropped below 40 mm, the ET and the canopy conductance of the understory were reduced by a factor of 2-3. The transpiration of pines was more sensitive to soil water deficit and was reduced by a factor of 4 to 6 when the soil water content dropped below 55 mm.