Agricultural and Forest Meteorology 126 (2004) 15–31
Seasonal changes in photosynthesis, respiration
and NEE of a mixed temperate forest
, 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
We analysed 6 years (1997–2002) of continuous eddy covariance CO
ﬂuxes over a mixed temperate forest in the Belgian
(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 observedchange 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
exchange; Atmospheric evaporative demand;
Forest carbon balance studies have become a ma-
jor research activity because of increasing awareness
of rising atmospheric CO
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: email@example.com (A. Carrara),
Reinhart.Ceulemans@ua.ac.be (R. Ceulemans).
Present address: Fundacion CEAM, Parque Tecnologico, Calle
Charles R. Darwin, Paterna (VALENCIA) SP-46980, Spain.
uous direct measurements of forest atmosphere CO
ﬂuxes (using the eddy covariance method) constitute
a powerful tool to improve the understanding of these
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
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
well as of the component ﬂuxes corresponding to
assimilatory (gross primary production, F
respiratory (ecosystem respiration, F
) processes, is
0168-1923/$ – see front matter © 2004 Elsevier B.V. All rights reserved.