Effect of elevated CO2 on water-use efficiency of a scrub-oak ecosystem

Ecosystem carbon and water cycles are closely linked through water-use efficiency (WUE). It is essential to know how WUE would change in the predicted global climate change scenarios. The Florida scrub-oak ecosystem was exposed to elevated atmospheric CO2 (ambient plus 350 µmol mol-1 CO2) in large open-top chambers (eight ambient, eight elevated) from 1995 to 2007, The objectives of this study were (i) to quantify water-use efficiency, which is defined as the ratio of net ecosystem CO2 exchange (NEE) to evapotranspiration (ET), at ambient and elevated CO2, and (ii) to determine the mechanisms underlying the CO2 effect. Open-top chambers were used as gas exchange cuvettes to measure NEE and ET about 10 days each month. At normal ambient CO2, NEE, ET and WUE measured using this method were not different from simultaneous measurements using eddy covariance on the same stand. But at elevated CO2, WUE was doubled due to increasing in NEE and reduced ET. Increased NEE resulted from enhanced LAI and leaf photosynthesis while reduced ET was caused by closure of stomata.