Stomata is a plant cell functioning as gate for both entry of carbon dioxide and out of water vapor. The carbon gain and water losses are two coupled processes through stomatal regulation. Water use efficiency is an index describes this trade-off. Several studies have used these large datasets to address ecosystem water use efficiency.

By using a continuous 9-year (2003-2011) eddy flux time series with 30 min resolution, Dr. TAN Zhenghong and his colleagues of Xishuangbanna Tropical Botanucal Garden (XTBG) of Chinese Academy of Sciences examined water use efficiency in a long-term ecological research site of Xishaungbanna tropical rainforest (101°15' E, 21°55' N) and determined its environmental controls.

The photosynthesis biochemistry suggests that rising of CO₂ concentration should lead to increase of water use efficiency which could be attributed by either increase of photosynthesis, decrease of transpiration, or both. This is called ‘CO₂ fertilization’ effect. The CO₂ fertilization has been tested by short time leaf level experiments. However, little evidence is available for long term ecosystem level observations, especially in tropical forests.  

The researchers tested the CO₂ fertilization hypothesis with their 9 year continuous dataset. They addressed the magnitude of tropical rainforest water use efficiency and asked whether it is higher than temperate and boreal forests. They then studied the daily, seasonal, and interannual patterns of tropical forest water use efficiency, how environmental factors controlled it and the responses of water use efficiency to severe drought.  

The study found that the water use efficiency of Xishuangbanna tropical forest is close to that of boreal forests, higher than subtropical forests, and lower than temperate forests. The instantaneous multiyear mean water use efficiency was strongly influenced by water vapor deficit. Water use efficiency was dominantly controlled by temperature seasonally. The water vapor deficit was not correlated with water use efficiency interannually. Either annually or seasonally, no interannual trend was detected in water use efficiency or inherent water use efficiency.   

No increasing trend of inherent water use efficiency was found in Xishuangbanna tropical rainforest. The researchers argued that the lack of increasing trend might be caused by the CO₂ fertilization effect covered by the confounding effect. CO₂ concentration and other environmental factors varied at the same time, which could exert stronger effect on water use efficiency than CO₂ does.  

The study entitled “Interannual and seasonal variability of water use efficiency in a tropical rainforest” has been on Journal of Geophysical Research: Atmospheres.