A b s t r a c t. Net ecosystem production reflects the potential of the ecosystem to sequestrate atmospheric CO 2 . Daily net ecosystem production of a mountain Norway spruce forest of the temperate zone (Czech Republic) was determined using the eddy covariance method. Growing season days when the ecosystem was a CO 2 source were examined with respect to current weather conditions. During the 2005, 2006, and 2007 growing seasons, there were 44, 65, and 39 days, respectively, when the forest was a net CO 2 source. The current weather conditions associated with CO 2 release during the growing seasons were: cool and overcast conditions at the beginning or end of the growing seasons characterized by a 3-year mean net ecosystem production of -7.2 kg C ha -1 day -1 ; overcast or/and rainy days (-23.1 kg C ha -1 day -1 ); partly cloudy and hot days (-11.8 kg C ha -1 day -1 ); and overcast and hot days (-13.5 kg C ha -1 day -1 ). CO 2 release was the highest during the overcast or/and rainy conditions (84%, average from all years), which had the greatest impact during the major production periods. As forests are important CO 2 sinks and more frequent weather extremes are expected due to climate change, it is important to predict future forest carbon balances to study the influence of heightened variability in climatic variables.K e y w o r d s: net ecosystem production, CO 2 source days, eddy covariance, weather conditions, Norway spruce
INTRODUCTIONThe net carbon budget of ecosystems consists in a fine balance between processes of carbon acquisition (such as photosynthesis, tree and plant growth, and carbon accumulation in soils) and carbon release (such as respiration of living biomass, tree mortality, microbial decomposition of dead biomass, oxidation of soil carbon, degradation and disturbance) (Malhi et al., 1999). The carbon budget expresses the ability of ecosystems to sequestrate atmospheric CO 2 . If acquisition prevails, ecosystems are considered carbon sinks. The differences among various types of ecosystems in their CO 2 sequestration potentials are large (Litton et al., 2007;Marek et al., 2011), as the aforementioned processes operate on a variety of time scales and are influenced by a number of factors. These factors include climate and meteorological parameters (amount and quality of radiation, temperature, and humidity); physiological state of the ecosystem (its age, structure, species composition, and history); water, nutrient, and substrate availability; and such ecosystem disturbances as diseases, insects, and thinning (Dore et al., 2012;Xenakis et al., 2012).Some ecosystem responses to environmental variations are immediate and direct, such as that of photosynthesis responding to light, temperature, soil moisture, and saturation deficit (Baldocchi et al., 1997;Chen et al., 1999) and respiration to temperature, soil moisture (Chen et al., 1999), and organic substrate availability (Kuzyakov and Gavrichkova, 2010). Others are indirect, mediated through such associated biophysical factors as leaf phenolo...
