GAS-EXCHANGE AND SAP FLOW MEASUREMENTS OF SALIX-VIMINALIS TREES IN SHORT-ROTATION FOREST .1. TRANSPIRATION AND SAP FLOW

Simultaneous field measurements of transpiration and sap flow were performed on short-rotation Salix viminalis trees ranging in diameter from 1.5 to 3.5 cm (2-year-old shoots on 8-year-old stumps). Transpiration was measured using an open-top ventilated chamber enclosing the whole foliage of a tree. Sap flow was measured using a tree-trunk heat balance (THB) technique with a constant temperature difference and variable heat input. Both the instantaneous and daily values of water flux measured by the two absolute techniques agreed well with a difference of up to about 5%. In July, the hourly transpiration reached a maximum of about 0.2 kg m(-2) (leaf area) or 0.45 kg tree(-1), whereas maximum daily integrals reached 4 kg tree(-1). The response of sap flow rate to abrupt flux change when inducing emboli by cutting-off the stem was very rapid: the registered signal dropped by 85% within 10 min for a specimen with a projected leaf area of 2 m(2). For S. viminalis trees, transpiration was linearly correlated with stem cross-sectional area and with leaf area.