CO2 exchange at the floor of a boreal forest

Net CO2 exchange at the forest floor in a mixed boreal spruce and pine forest in central Sweden was studied during 1996. Forest floor CO2 efflux was measured continuously by means of a ventilated open soil chamber, covering a surface area of 0.6 m2. The chamber was transparent and thus measured soil respiration by night, and soil respiration reduced by photosynthetic uptake by forest floor vegetation by day. Maximum nocturnal efflux rates were 0.2–0.7 and daytime rates were 0.05–0.2 mg m−2 s−1. Measured efflux rates were higher than reported in other studies, but have earlier been found to agree with nocturnal CO2 exchange of the forest ecosystem measured by eddy-covariance technique. Soil temperature at 5 cm explained 49% of the variation in nocturnal soil respiration, while moss and air temperature explained 29 and 17% of the variation, respectively. For the relationship with soil temperature at 5 cm, base respiration rate and effective Q10, derived from data over the seasonal course, were 0.04 mg m−2 s−1 and 4.75, respectively. Corresponding figures for the relationship with air temperature were 0.11 mg m−2 s−1 and 1.89. Over the 6 months (May–October) covered by measurements, August had the largest CO2 efflux, 0.89 kg m−2 and May the smallest efflux, 0.20 kg m−2. During daytime photosynthetic uptake by forest floor vegetation reduced potential soil efflux through respiration by ca. 20%. On an annual basis total forest floor respiration was estimated to be 4.5 kg CO2 m−2 and gross photosynthesis to be 0.7 kg CO2 m−2, resulting in a net efflux of 3.8 kg CO2 m−2.