Cork oak biomass productivity in a damaging sequence of cytisus cork oak forest in Kroumirie (Tunisia)

The net primary productivity (PNI) of cork oaks was evaluated in six plots representing variable damage stages of cytisus cork oak forest in Kroumirie region using biomasses of twelve sampled trees. The counting of woody rings on an independent tree sample allowed to correlate the age and radial growth with the diameter at breast height (DBH). Allometric relationships were established to estimate the productivities at individual and stand levels. At individual level, the DBH is a highly predictive parameter of reproduction cork; its production ranges from 0.24 to 4.12 kg DM.tree(-1).yr(-1) for a DBH varying between 8.7 and 60 cm. The total biomass increment sharply increased with tree age rising from 2.1 to 14.6 kg DM.tree(-1).yr(-1) between 53 and 303 years (8.7 and 60 cm of DBH). At the same time, the relative growth rate (RGR) decreased from 2.14 to 0.34%.yr(-1) and the contributions of reproduction cork and trunk wood increased at the expense of other compartments. At stand level, basal area, which is a variable closely related to the canopy covering and very highly predictive of PNI, appears as a quantitative criterion of forest damaging. So, PNI reaches 5.2 t DM.ha(-1).yr(-1) (2341 kcal.m(-2).yr(-1)) in the best preserved plot and only 0.7 t DM.ha(-1).yr(-1) (325 kcal.m(-2).yr(-1)) in the most damaged one. The increments of the different compartments showed a continuous decrease along the whole damaging sequence, ranging from 1376 to 51, 892 to 70 and 263 to 44 kg DM.ha(-1).yr(-1) for boles, crowns and roots, respectively. Simultaneously, reproduction cork decreased from 456 to 5 kg DM.ha(-1).yr(-1) and litterfall from 2.85 to 0.56 t DM.ha(-1).yr(-1). At the advanced damaging stages, a significant decrease of the relative allocation of assimilates to reproduction cork, boles and reproductive organs occurred and inversely an increased mobilization of assimilates by the twigs and the roots.