Diagnosing plant water status as a tool for quantifying water stress on a regional basis in Mediterranean drylands

This study reports measurements of stomatal conductance, relative water content and water potential ( L ) from three dominant evergreens (Ceratonia siliqua L., Quercus coccifera L. and Olea oleaster Hoffmgg. et Link) growing in four coastal sites of Turkey. In particular, a fully vegetated site (H) was selected and compared for the above parameters to three degraded sites (D1, D2 and D3) with decreasing vegetation covers. From the integral of the diurnal time course of L , the water stress impact on each species (WSIS) was calculated. C. siliqua and Q. coccifera showed similar WSIS's, increasing significantly from H to D3. O. oleaster was sensitive both to summer drought and to increasing site degradation. The impact of water stress was scaled up from the species to the vegetation level (WSIV) as WSIV = WSIS S (1-f s ) where f s was the relative frequency of the species studied. WSIV was rather sensitive to the impoverishment of vegetation and was correlated to vegetation density as estimated both by field observations and remotely sensed Normalized Difference Vegetation Index. desertification / leaf water potential / Mediterranean evergreens / Normalized Difference Vegetation Index / water stress impact Rsum -Diagnostiquer l'tat de l'eau dans la plante : un outil pour quantifier le stress hydrique au niveau rgional dans les rgions sches mditerranennes. Cette tude rapporte les mesures de conductance stomatique, de la teneur relative en eau et du potentiel hydrique ( L ) d'arbres feuilles persistantes (Ceratonia siliqua L., Quercus coccifera L. et Olea oleaster Hoffmgg. Et Link) croissant sur 4 sites ctiers de Turquie. En particulier, un site totalement recouvert de vgtation (H) a t slectionn et compar, pour les paramtres ci-dessus, 3 sites dgrads (D1, D2 et D3) ayant une couverture vgtale de plus en plus faible. A partir de l'intgrale de L , pour le cycle diurne, l'impact du stress hydrique de chaque espce (WSIS) a t calcul. C. siliqua et Q. coccifera montrent des WSIS similaires, augmentant significativement de H D3. O. oleoaster a t sensible la fois la scheresse estivale et l'accroissement de la dgradation du site. Un changement d'chelle, du niveau de l'espce celui de la vgtation, a t ralis pour l'impact du stress hydrique (WSIV) par la transformation WSIV = WSISs (1-f s ) ou f s est la frquence relative de l'espce tudie. WSIV est particulirement sensible l'appauvrissement de la vgtation et est corrl la densit de la vgtation estime la fois par des observations aux champs et par l'indice normalis de diffrentiation de la vgtation par observation satellitaire.