Water regime of a pine forest under a Mediterranean climate

Abstract

The transpiration ( T) of an Aleppo pine (Pinus halepensis Mill.) forest was studied during 1 year to evaluate soil water storage and tree response to water stress in a Mediterranean climate. The heat pulse method was used for continuous measurement (excluding rainy days) of sap flow in the stems of eight trees at hourly intervals. Climatic parameters were measured in the forest for computation of potential transpiration ( T p) using the Penman formula. Extrapolating T of sampled trees to stand T was found to be difficult because of a poor correlation between T and stem diameter. During the rainy season the average daily rate of T was linearly related to T p and the ratio T T p was 0.62, independent of T p rate. After the last rainfall, T T p decreased steadily, reaching 0.05 approximately 30 days later; the leaf water potential ( ψ 1) at sunrise also decreased, from −0.8 to −2.9 MPa. The drop in T and ψ 1 indicates that most of the available water in the main root zone had been extracted during less than 30 days after the last rain. Decreasing values of wood thermal diffusivity with the progress of the dry season show that trees may use internally stored water when soil water stress is intense. The integrated T throughout the measurement period was used for estimation of total water uptake by the stand and for evaluation of the amount of water which was available to the trees following the last rain.