Schiller, G., Department of Field Crops, Volcani Center, Bet Dagan 50250, Israel
Cohen, Y., Dept. of Agricultural Meteorology, Volcani Center, Bet Dagan 50250, Israel

Transpiration (T), needle water stress (MPa) and the water balance of an Aleppo pine (Pinus halepensis Mill.) plantation, growing in an arid region at the edge of the Israeli Negev desert, were studied during 1 yr. The heat pulse technique for the measurement of the heat flow velocity was used for the estimation of the sap flow velocity, i.e., transpiration, in the stems of 16 trees (27% of the trees in a plot of 1000 m2). A pressure chamber was used to determine the needle water potential (MPa). Climatic parameters were measured in the forest for the computation of the potential transpiration (T(p)) by means of the Penman-Montheith equation. During the rainy period, the transpiration (T) rate was maintained at a level between 1 and 2 mm day-1, which then dropped after the last rain; the decrease of T in small trees (diameter at breast height [DBH] < 12 cm) was much faster than that in larger trees (DBH > 12 cm). In May, T dropped to a nondetectable rate (about 0.02 mm day-1) and remained at this level till the next significant rain in December. A maximum T/T(p) ratio of 0.3, which is nearly half of the ratio found previously in an Aleppo pine plantation under Mediterranean climatic conditions (Schiller and Cohen, 1995), was recorded in February. This difference between the two sites, in T/T(p) ratio, is attributed to the difference in the basal area of the trees between the two sites. Needle water potential at sunrise decreased from -0.8 MPa in the rainy period to more than -3.0 MPa during the dry period. The integrated T throughout the measurement period was used to estimate the total water uptake by the stand, which was 210.1 mm, i.e., 93% of the effective rainfall, or 80.2% of the total annual rainfall.