Seasonal changes in photosynthesis, respiration and NEE of a mixed temperate forest

We analysed 6 years (1997–2002) of continuous eddy covariance CO2 fluxes over a mixed temperate forest in the Belgian Campine region and present the long-term mean seasonal changes in net ecosystem exchange (NEE), gross primary production (GPP) and total ecosystem respiration (RE). Seasonal changes in NEE, GPP and RE were tightly correlated, with extremes in July and December/January. Nonetheless, maximum rates of RE slightly lagged behind the maximum rates of GPP. We observed a substantial increase in NEE at optimum light conditions (reduced uptake) above 24 °C. Because this increase is unlikely to be related only to the increase in RE, we hypothesize that it also indicates a significant decrease in GPP under dry and warm meteorological conditions. Because this increase in NEE was better correlated with atmospheric vapour pressure deficit than with temperature, we further hypothesize that the observed change in NEE was predominantly related to the effect of high atmospheric evaporative demand on GPP via stomatal closure than to a negative-temperature response. Because dry conditions were rare in the maritime climate of this study site, its impact on the annual GPP or even on NEE was small (<1% and 10%, respectively).