Sap flow estimates of Quercus suber according to climatic conditions in North Tunisia

The determination of oak tree transpiration could be a key factor in understanding the species response to climate change and especially to drought events. This experiment was carried out on four cork oak trees using a thermal dissipation probe (TDP) for measuring the sap flow density (SFD) during the 2008 vegetative season in Aïn Snoussi forest (North Tunisia). The natural thermal gradient (NTG) seemed to exert a minor effect on computing the sap flow. The maximum SFD varied from 0.7 to 3.1 L dm _-2 h _-1. The average SFD was positively related with climatic factors, such as air temperature (Ta), vapor pressure deficit (VPD) and photosynthetic active radiation (PAR). The general behavior indicated that SFD increased proportionally with increasing temperature, VPD or PAR for values of those parameters. However, SFD tended to a threshold value when temperature, VPD and PAR demand continued to increase. Finally, sap flow followed the pattern of the reference evapotranspiration estimated by the Penman-Monteith formula on a diurnal basis.