Canopy Structure Stratification in Mature Mediterranean Cork Oak Forest Ecosystems and Its Impact on the Water Balance

Leaf area index (LAI) is the key functional variable in canopy structure that governs different water and carbon flows and tree growth. This important parameter was determined by hemispherical photography, in three populations of Quercus suber L. chosen according to an altitudinal gradient in the North-West of Tunisia. The impact of the different strata on the water balance was studied using a “CASTANEA” mechanistic model based on eco-physiological processes in which LAI is the key parameter. The results showed that the contribution of the different strata in the whole LAI ecosystem is about 70% for the tree stratum occupied by cork oaks and 30% for the understory stratum which is occupied by shrub and herbaceous species. Modeling has shown that whatever the year, the intra-annual and inter-annual variability and the duration of the stress are greater when taking into account total LAI than those taking into account only the LAI of each stratum. Below the 40% threshold of Relative Extracted Water (REW), water stress begins on average on day 179 (end of June) with a large extent ranging from day 142 (1992—water stress begins earliest in the period 1982–2009) to day 212 (2004—water stress begins at the latest). The shortest stress duration was 50 days and the longest was 161 days.