Environment and tree size controlling stem sap flux in a perhumid tropical forest of Central Sulawesi, Indonesia

Abstract

We studied stem sap flux density in seven common tree species in a perhumid tropical rainforest at pre-montane elevation in Sulawesi with the aims (1) to analyse the among-species variation in flow patterns, (2) to search for an assumed convergence in the response to controlling environmental factors, and (3) to investigate the effect of tree size on xylem flux. Sap flux density was measured for 20 months in 39 tree individuals of seven species (from six of the most dominant families in the area). Synchronous sap flux density in the outermost xylem differed more than threefold among the species and was highest in the tall Fagaceae Castanopsis acuminatissima. Across the seven species, a tight exponential correlation was found between tree diameter (or tree height) and tree daily water use. Daily sap flux density correlated better with atmospheric vapour pressure deficit than with shortwave radiation, in spite of the permanently high atmospheric humidity. Soil moisture did not vary significantly and therefore showed no effect on mean daily sap flux density. The hysteresis in the diurnal plot of xylem flux density against vapour pressure deficit or radiation was larger for radiation supporting the close coupling of flux variation to vapour pressure deficit, which exists even in this perhumid climate. We conclude that the species of this perhumid forest show convergent patterns in the environmental control of sap flux. Largely different water consumption rates of coexisting trees were mainly caused by differences in tree size and the position of the tree within the canopy. Our extrapolated daily stand transpiration rates are low in comparison to other tropical forests which may be a consequence of the permanently high air humidity.