Water and Photosynthetic Rate Flows Under Drought Conditions in a Cork Oak (Quercus suber L.) Forest of Tunisia

Relationships between drought, carbon and water fluxes have been rarely studied in south Mediterranean forests. The present research focused on the determination of seasonal and annual water and carbon fluxes of Quercus suber L. forests in northern Tunisia. The methodology was based on the calculation of the standard precipitation index, measurements of trees sap flow and net photosynthesis. Estimations of photosynthesis and transpiration during the 1965-2003 period were used on crop coefficients and water use efficiency terms. Results indicate a wide evapotranspiration rates fluctuating from 354 mm y-1 to 784 mm y-1 with an average value of 553 mm y-1. Extreme values of the standard precipitation index were -2.4 and +2.7. The carbon flux ranged from 0.255 to 0.586 kg y-1 m-2 with a mean value of 0.448 kg y-1 m-2 while average water efficiency reached 0.8 gr C kg-1 H2O. Despite the fact, that there is a significant difference between the four studied sites and important annual variability of carbon fluxes, the correlations between water and carbon fluxes and drought index were very low. The results clearly indicate that deep transformations are occurring in the Quercus suber L. forests, as a result of carbon dioxide fertilization being cancelled by the drought effect.