Transpiration, canopy conductance and the decoupling coefficient of a lowland mixed dipterocarp forest in Sarawak, Borneo: dry spell effects

Tropical rainforests are among the most important biomes in terms of annual primary productivity and water cycling. However, the materials-exchange properties of Southeast Asian forests in their significant functions are less studied. We investigate key factors controlling materials-exchange, such as canopy (gc) and aerodynamic conductance (ga) and the decoupling coefficient (Ω) in an intact lowland mixed dipterocarp forest in Sarawak, Borneo. We operated an eddy covariance system above the canopy to measure canopy transpiration rate (Ec) using an 80-m-tall canopy crane, and made concurrent meteorological measurements. In this region, solar radiation and air temperature have small seasonal variations, and annual rainfall is distributed evenly throughout the year, with some unpredictable intra-annual dry spells. We define three specific periods A, B and C as medium dry, heavy dry and wet periods, respectively, for analyses of the environmental control on Ec, according to their dryness level. The mean daily Ec, gc and Ω were higher in the wet period C than in the dry periods A and B. Also, each gc in its diurnal variation was much higher in the wet period C than in the dry periods A and B, where diurnal variations of gc were almost the same. Both the value of gc and its sensitivity to environmental variables were higher in the wet period C than in the dry periods A and B. The peak of Ω was higher in the wet period C than in the dry periods A and B because of a higher value of gc in period C. However, the higher values of gc in the wet period C did not cause a higher Ec compared to in the dry periods A and B.