Abstract
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.
Highlights
Global climatic models predict a change in rainfall pattern in Tunisia, characterized mainly by a decrease in summer rainfall coupled with greater inter-seasonal and inter-annual variability (IPCC, 2007; Hulme et al, 2001)
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
Seasonal mean values of tree transpiration (Tr) and evapotranspiration of the undergrowth (Es) showed that maximum values were reached in spring being 1.4 mm/d and 1.3 mm/d, respectively (Figure 2)
Summary
Global climatic models predict a change in rainfall pattern in Tunisia, characterized mainly by a decrease in summer rainfall coupled with greater inter-seasonal and inter-annual variability (IPCC, 2007; Hulme et al, 2001) These previsions for the near future reveal an accentuation of the drought, which means that increasingly longer and more intense dry periods to be expected (Giannkopoulos et al, 2005). This forest has been an alarming deterioration, it occupies an area of 90 000 hectares against 140,000 hectares 100 years ago (Boudy, 1952) It still offers several goods and services to society mainly the photosynthesis carbon capture insured by these forming trees which are threaten by the expected drought and the alteration of water and carbon flows
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