Environmental control over seasonal variation in carbon fluxes of an urban temperate forest ecosystem

Knowledge of surface-atmosphere CO2 exchanges in urban forests is imperative. We measured net ecosystem CO2 exchange of an urban forest in Beijing over a two-year period (2012-2013). The aim was to examine seasonal controls of environmental variables on ecosystem carbon (C) cycle assessed with eddy covariance technique. Net ecosystem production (NEP) was 30% less in 2012 (200 ± 27 g C m−2) than in 2013 (287 ± 35 g C m−2). Both years were warmer than the long-term average. Seasonal and annual ratios of ecosystem respiration (ER) to gross primary productivity (GEP) were higher in 2012 than in 2013. Dry conditions decreased GEP more than ER, while warm conditions increased ER more than GEP. Heat stress exerted controls over seasonal changes in NEP. Daytime NEP ceased to increase beyond 11 μmol CO2 m−2 s−1 and 13 μmol CO2 m−2 s−1 in 2012 and 2013, respectively, where mean photosynthetically active radiation was >700 μmol m−2 s−1 and air temperatures was greatest (>27 °C) in mid-summer days. The extremely strong precipitation day (176 mm) was recorded as having the greatest C discharge to the atmosphere (5.6 g C m−2). Intermediate-size precipitation events (>2 mm and ≤15 mm) acted to increase C-sink strength. The results highlight the roles of environmental stresses and their alleviation in regulating C fixation in the face of warmer climate and increasing intensity of extreme precipitation events.