Sap flow in Bornean heath and dipterocarp forest trees during wet and dry periods.

Daytime xylem sap flow was measured by the compensation heat pulse velocity technique in dipterocarp forest trees growing on sandy loam (haplic acrisol) and in heath forest trees growing on bleached sand (albic arenosol) in Brunei during wet and dry periods. In dipterocarp forest trees, daily sap flow was stable during the dry period but declined on rainy days presumably as a result of reduced transpiration. At the heath site, daily sap flow of the small trees was stable during the dry period, whereas it declined in the large trees as the dry season progressed. In some trees at both sites, the hydroactive area of xylem was less during the dry period than during the wet period. Daily sap flow was linearly related to projected crown area, and a common regression fitted trees of different species from both forest types. The dry/wet period ratio of sap flow (about 0.7) was statistically equivalent for heath and dipterocarp forest trees, whereas it had been expected that trees on the freely draining arenosol would suffer greater water stress during drought. For well-watered trees >/= 0.2 m dbh (diameter at 1.3 m above ground) in both the dipterocarp and heath forests, stand water use was estimated to be 40 mm month(-1).