Agricultural and Forest Meteorology 101 (2000) 1–14 CO 2 exchange at the floor of a boreal forest Ann-Sofie Morén a,∗ , Anders Lindroth b,1 a Department for Production Ecology, Swedish University of Agricultural Sciences, P.O. Box 7042, SE-750 07 Uppsala, Sweden b Department of Physical Geography, Lund University, Sölvegatan 13, SE-223 62 Lund, Sweden Received 26 February 1999; received in revised form 3 November 1999; accepted 16 November 1999 Abstract Net CO 2 exchange at the forest floor in a mixed boreal spruce and pine forest in central Sweden was studied during 1996. Forest floor CO 2 efflux was measured continuously by means of a ventilated open soil chamber, covering a surface area of 0.6 m 2 . 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 CO 2 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 5cm, base respiration rate and effective Q 10 , 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 CO 2 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 CO 2 m −2 and gross photosynthesis to be 0.7 kg CO 2 m −2 , resulting in a net efflux of 3.8 kg CO 2 m −2 . ©2000 Elsevier Science B.V. All rights reserved. Keywords: Soil respiration; Net assimilation; Chamber system; Scots pine; Norway spruce 1. Introduction Soils throughout the world, and boreal forest soils in particular, are currently attracting the attention of the scientific community. One reason for this is that global circulation models (GCM), indicate for a range of CO 2 emission scenarios, a continuous increase in temperatures, the largest increases being expected ∗ Corresponding author. Tel.: +46-018-67-2559; fax: +46-018-67-3376. E-mail addresses: ann-sofie.moren@spek.slu.se (A.-S. Mor´ en), anders.lindroth@natgeo.lu.se (A. Lindroth). 1 Fax: +46-046-2224011. at high latitudes. Recent simulations for the period 1990−2050 predict a global warming of 2–3 ◦ C in winter and 1–2 ◦ C in summer for most of the boreal region (Greco et al., 1994). Soil respiration and soil organic matter (SOM) decomposition are most sen- sitive to a temperature increase in areas where soil temperatures are low, as is the case in tundra and boreal forests (Lloyd and Taylor, 1994; Kirschbaum, 1995). Therefore, the combination of a large carbon pool in boreal soils (Dixon et al., 1994) and increas- ing temperature, will inevitably increase soil respi- ration rates and SOM decomposition, which might transform many boreal forests into carbon sources (Kirschbaum, 1995). Both Goulden et al. (1998) and 0168-1923/00/$ – see front matter ©2000 Elsevier Science B.V. All rights reserved. PII:S0168-1923(99)00160-4