Estimating water use by sclerophyllous species under east Mediterranean climate: II. The transpiration of Quercus calliprinos Webb. in response to silvicultural treatments

Abstract

The water use by a stand of Quercus calliprinos Webb. maquis, growing on Mount Carmel Israel, was estimated by means of the heat pulse method. Hourly heat pulse velocity (HPV) measurements on eight or 16 trunks, each being the main trunk in its stool, were done continuously for between 96 and 240 h, approximately every 3 weeks. The measured trunks were selected to represent the circumference distribution of the 240 trunks at the site. The HPV was measured with a probe carrying six thermistors along its axis, 8 mm apart. The six-thermistor probe was inserted into a hole drilled radially into the trunk exactly 15 mm above the heater, and the reference probe into a hole drilled at least 100 mm lower and aside from the heater. This kind of a probe facilitates measurement of the decline in the pulse velocity with the depth under the bark. Hourly and daily sap flow rates, i.e., transpiration were calculated by means of known procedures and equations. The highest average daily transpiration rate of about 28.1 l per tree per day occurred in May, and the lowest, of 5.0 l per tree per day in October. The estimated total yearly transpiration of the 0.1 ha plot containing 240 oak trunks in 47 stools ranged between 44 and 75% of the preceding winter rainfall, i.e., between 234 and 358 mm. In drier years the trees had used a higher proportion of the annual rainfall than in wetter years. The estimated water consumption by the oak trees in the early years after thinning which left only the 47 main trunks, one per stool, was only 13–20% of the preceding annual rainfall. Thinning caused increased transpiration of the single main trunk, but the estimated overall water use by the remaining trees (47 out of 240) was very much reduced. Thinning of most of the trunks in several stools enhanced the growth rate of the remaining trunks. The circumferences of the trunks in the thinned stools increased to 115.4% of their initial value in 3 years compared with to only 107% by those of the trunks in the non-thinned stools.