Agricultural and Forest Meteorology 126 (2004) 15–31 Seasonal changes in photosynthesis, respiration and NEE of a mixed temperate forest Arnaud Carrara 1 , Ivan A. Janssens, Jorge Curiel Yuste, Reinhart Ceulemans ∗ Research Group of Plant and Vegetation Ecology, Department of Biology, University of Antwerpen (UA), Universiteitsplein 1, B-2610 Wilrijk, Belgium Received 9 October 2003; received in revised form 27 April 2004; accepted 13 May 2004 Abstract We analysed 6 years (1997–2002) of continuous eddy covariance CO 2 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). © 2004 Elsevier B.V. All rights reserved. Keywords: Carbon balance; Net ecosystem exchange; Eddy covariance; CO 2 exchange; Atmospheric evaporative demand; CARBOEUROFLUX 1. Introduction Forest carbon balance studies have become a ma- jor research activity because of increasing awareness of rising atmospheric CO 2 concentration and its major influence on climate and vegetation functioning. Net- works of tower stations performing long-term, contin- ∗ Corresponding author. Tel.: +32 3 820 2256; fax: +32 3 820 2271. E-mail addresses: arnaud@ceam.es (A. Carrara), Reinhart.Ceulemans@ua.ac.be (R. Ceulemans). 1 Present address: Fundacion CEAM, Parque Tecnologico, Calle Charles R. Darwin, Paterna (VALENCIA) SP-46980, Spain. uous direct measurements of forest atmosphere CO 2 fluxes (using the eddy covariance method) constitute a powerful tool to improve the understanding of these CO 2 exchanges between forests and the atmosphere (Baldocchi et al., 2001). Within the context of a larger European network (CARBOEUROFLUX; Valentini et al., 2000), measurements of net ecosystem CO 2 flux were performed at the Belgian Brasschaat forest site from August 1996 to March 2003. The characterisation of seasonal timing and am- plitude of the net ecosystem carbon flux (F NEE ), as well as of the component fluxes corresponding to assimilatory (gross primary production, F GPP ) and respiratory (ecosystem respiration, F RE ) processes, is 0168-1923/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.agrformet.2004.05.002