The Impact of a Severe Drought on Ecosystem CO2 and Water Fluxes in a Subtropical Forest

Southern China experienced an extreme drought in 2013, where the precipitation dropped 23% below the long-term norm and the abnormally high temperature lasted for 67 days. In this study, the impact of the severe drought on a mixed evergreen subtropical forest ecosystem was analyzed using eddy covariance measurements from a newly established flux tower at Ningxiang county of Hunan Province and simulations with the Community Land Model version 4.5 (CLM4.5) driven by high resolution forcing data from China Meteorological Administration Land Data Assimilation System and the in-situ meteorological data. When soil moisture reduced from 0.35 m3 m-3 to 0.12 m3 m-3, both vegetation productivity and evapotranspiration (ET) experienced a strong reduction in July-August period due to drought-induced stomatal closure. Results also showed that the ecosystem respiration (Reco) decreased together with gross ecosystem productivity (GEP), instead of accelerating with the increase of temperature under extremely dry soil condition (< 0.14 m3 m-3). As a balance of the carbon uptake and release, the magnitude of net ecosystem productivity also experienced a large reduction from 11.8 to 7.7 μmol m-2 s-1 and the ecosystem even switched to a net source of carbon. In addition, persistent soil water content deficit in summer limited the plant water uptake and led to a reduction of the stomatal conductance, which weakened the coupling relationship between carbon and water fluxes. Water availability severely limited the carbon sequestration of the targeted ecosystem with water use efficiency (WUE) plummeted during the drought period. With high quality of meteorological forcing data, CLM4.5 captured the variability of water fluxes during the growing season quite well with correlation coefficients of 0.95 for soil moisture and 0.82 for ET, except for an underestimation of GEP at the end of drought period. Further improvement in simulating the response to drought in subtropical forest ecosystem is required.