Effect of inter-annual climate variability on evapotranspiration and canopy CO2 exchange of a tropical rainforest in Peninsular Malaysia

Abstract: We investigated inter-annual variation of canopy CO₂ exchange (NEE) and evapotranspiration during a 7-year period over a lowland Dipterocarp forest in Pasoh, Peninsular Malaysia, using the eddy covariance method. Annual rainfall fluctuated between 1,451 and 2,235 mm during this period. Annual evapotranspiration estimated by energy budget correction and gap filling using the relationship between latent heat and available energy was 1,287 ± 52 mm. Despite inter-annual variation in rainfall, annual evapotranspiration was stable, except for a slight decrease in the driest year (2009). Evapotranspiration was roughly related to the amount of available energy, but was regulated by stomatal closure to prevent excessive water loss at high vapour pressure deficit. Even during dry periods, no significant decrease in evapotranspiration occurred, as water was supplied from soil layers deeper than 0.5 m. Ecosystem respiration (RE) increased with soil water content. Daytime NEE was also stable during the 7 years, despite climate variability. Afternoon inhibition of canopy photosynthesis was seen every month. Daytime NEE did not become more negative with increasing solar radiation, or with increasing soil water content. During dry periods, gross primary production (GPP) and thus canopy gross photosynthesis decreased slightly, coupled with decreased daytime RE. In this forest, variability in rainfall pattern resulted in seasonal and inter-annual variability in micrometeorology; evapotranspiration, photosynthesis, and RE responded to these changes, and compensated for each other and/or other components of micrometeorology, resulting in rather stable annual evapotranspiration and NEE, even during a very dry year associated with an El Nino Southern Oscillation (ENSO) event.