Seasonal variation and meteorological control of CO2 flux in a hilly plantation in the mountain areas of North China

The carbon cycle of terrestrial ecosystems is an important scientific issue in global climate change research. Plantation forest plays an important role in terrestrial carbon budget in China. In this study, eddy covariance flux data measured at Xiaolangdi forest ecosystem research station (XLD) in 2007 and 2008 are used to analyze the seasonal variation and meteorological control of CO2 flux in a 30-yr-old mixed plantation. The plantation forest mainly consists of Quercus variabilis, Platycladus orientalis, and Robinia pseudoacacia. The results show that the seasonal variations of net ecosystem exchange of CO2 (NEE), gross primary production (GPP), and ecosystem respiration (R (e)) display single-peak curves. The maximum of carbon sequestration appears during May and June each year. The relative contribution of carbon release from ecosystem respiration to GPP varied slightly between 2007 and 2008. The relationship between NEE and photosynthetic active radiation (Q (p)) accords with the rectangular hyperbola model on diurnal scale, and shows a good linear correlation on monthly scale. The ecosystem photosynthetic parameters: the maximum photosynthetic rate (P (max)), the ecosystem photosynthetic photonyield (alpha), and the daytime ecosystem respiration (R (d)) exhibit seasonal variations. P (max) reaches the maximum in August each year, with small interannual difference. The interannual differences of alpha and R (d) are obvious, which is attributed to the changes of meteorological factors, such as solar radiation, vapor pressure deficit (D), precipitation, etc. Parameters R (e), GPP, and NEP (net ecosystem production) have obvious exponential relations with temperature on monthly scale. There is a hysteresis in the response of GPP and NEP to temperature, i.e., the carbon sequestration is not the maximum when the temperature reaches the peak value. The Q (10) values were 1.37 and 1.45 in 2007 and 2008, respectively. On monthly scale, R (e), GPP, and NEE increase as D increases, but rise slowly and even decrease when D is higher than 1.5 kPa.