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 con tent and water potential (Psi (L)) from three dominant evergreens (Ceratoni a siliqua L., Quercus coccifera L. and Olea oleaster Hoffmgg. et Link) grow ing in four coastal sites of Turkey. In particular, a fully vegetated site (H) was selected and compared for the above parameters to three degraded si tes (D1, D2 and D3) with decreasing vegetation covers. From the integral of the diurnal time course of Psi (L), the water stress impact on each specie s (WSIS) was calculated. C. siliqua and Q. coccifera showed similar WSIS's, increasing significantly from H to D3. O. oleaster was sensitive both to s ummer drought and to increasing site degradation. The impact of water stres s was scaled up from the species to the vegetation level (WSIV) as WSIV = S igma WSISS (1-f(s)) where f(s) was the relative frequency of the species st udied. WSIV was rather sensitive to the impoverishment of vegetation and wa s correlated to vegetation density as estimated both by field observations and remotely sensed Normalized Difference Vegetation Index.