Free access
Issue
Ann. For. Sci.
Volume 48, Number 1, 1991
Page(s) 29 - 45
DOI http://dx.doi.org/10.1051/forest:19910103
Ann. For. Sci. 48 (1991) 29-45
DOI: 10.1051/forest:19910103

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)

D Loustau, H Cochard, M Sartore and M Guédon

INRA, Recherches forestières, laboratoire d'écophysiologie et nutrition, domaine de l'Hermitage BP 45, 33610 Cestas, France

Résumé - Une chambre de transpiration légère, close, couvrant une surface au sol de 0,5 m2 a été utilisée pour mesurer l'évapotranspiration (ET) de touffes de molinie en sous-bois d'un peuplement de 19 ans de pin maritime. L'ET horaire et journalière du sous-bois et sa conductance de couvert ont été estimées au cours de 13 journées de mesure réalisées sur 2 saisons de croissance en 1988 et en 1989. Ces mesures sont comparées avec la transpiration des arbres estimée à partir de mesures de flux de sève. Quand le stock hydrique du sol est supérieur à 40 mm, l'évapotranspiration de la molinie est comprise entre 1 et 1,5 mm/j. Elle correspond à 1/3 de celle des arbres et représente une fraction constante de l'ETP à 1 m. La conductance de couvert du sous-bois est globalement constante pendant la journée et varie entre 0,4 et 0,5 cm.s-1. Au cours de l'été, quand le stock hydrique diminue en dessous de 40 mm, l'évapotranspiration et la conductance de couvert de la molinie diminuent d'un facteur 2-3. La transpiration du pin maritime présente un comportement différent : elle est réduite dès que le stock descend en dessous de 55 mm, et dans des proportions plus importantes.


Abstract - Use of a portable transpiration chamber for estimating the evapotranspiration of the understory of a maritime pine stand. 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 corresponded 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.


Key words: canopy conductance / soil water deficit / leaf area index / seasonal change

Mots clés : conductance de couvert / sécheresse édaphique / indice foliaire / variation saisonnière