Interannual and seasonal variability of water use efficiency in a tropical rainforest: Results from a 9 year eddy flux time series

We used a continuous 9 year (2003–2011) eddy flux time series with 30 min resolution to examine water use efficiency in a tropical rainforest and determine its environmental controls. The multiyear mean water use efficiency ( W ue ) of this rainforest was 3.16 ± 0.33 gC per kg H 2 O, which is close to that of boreal forests, but higher than subtropical forests, and lower than temperate forests. The water vapor deficit ( V PD ) had a strong impact on instantaneous W ue , in the manner predicted by stomatal optimization theory. At the seasonal scale, temperature was the dominant controller of W ue . The negative correlation between temperature and W ue was probably caused by high continuous photosynthesis during low‐temperature periods. The V PD did not correlate with W ue at the interannual scale. No interannual trend was detected in W ue or inherent water use efficiency ( W ei ), either annually or seasonally. The fact that no increasing trend of W ei was found in the studied tropical rainforest, along with other evidence of CO 2 stimulation in tropical rainforests, requires special attention and data validation. There was no significant difference between W ue during a drought and the 9 year mean values in the forest we studied, but we found that dry season transpiration ( T r ) was consistently lower during the drought compared to the mean values. Finally, whether W ue increases or decreases during a drought is determined by the drought sensitivity of gross primary production ( G PP ).