Response of ecosystem carbon fluxes to drought events in a poplar plantation in Northern China
Poplar plantations are widely used for timber production and ecological restoration in northern China,a region that experiences frequent droughts and water scarcity. An open-path eddy-covariance (EC)system was used to continuously measure the carbon,water,and energy fluxes in a poplar plantation during the growing season (i.e., April–October)over the period 2006–2008 in the Daxing District of Beijing,China. We examined the seasonal and inter-annual variability of gross ecosystem productivity (GEP), net ecosystem exchange (NEE), and ecosystem respiration (ER). Although annual total precipitation was the lowest in 2006,natural rainfall was amended by flood irrigation. In contrast, no supplementary water was provided during a severe drought in spring (i.e.,April–June),2007, resulting in a significant reduction in net ecosystem production (NEP = NEE). This resulted from the combined effects of larger decrease in GEP than that in ER. Despite the drought–induced reduction in NEP, the plantation forest was a strong carbon sink accumulating 591± 62, 641 ± 71, and 929 ± 75 g C m -2 year-1 for 2006, 2007,and 2008, respectively. The timing of the drought significantly affected the annual GEP. Severe drought during canopy development induced a lasting reduction in carbon exchange throughout the growing season, while the severe drought at the end of growing season did not significantly reduce carbon uptake. Additionally, irrigation reduced negative drought impacts on carbon sequestration. Overall, this fast growing poplar plantation is a strong carbon sink and is sensitive to the changes in environmental conditions.