Temporal and spatial variability of soil respiration in a boreal mixedwood forest

Abstract

Temporal and spatial variability of soil respiration ( R s) was measured and analyzed in a 74-year-old, mixedwood, boreal forest in Ontario, Canada, over a period of 2 years (August 2003–July 2005). The ranges of R s measured during the two study years were 0.5–6.9 μmol CO 2 m −2 s −1 for 2003–2004 (Year 1) and 0.4–6.8 μmol CO 2 m −2 s −1 for 2004–2005 (Year 2). Mean annual R s for the stand was the same for both years, 2.7 μmol CO 2 m −2 s −1. Temporal variability of R s was controlled mainly by soil temperature ( T s), but soil moisture had a confounding effect on T s. Annual estimates of total soil CO 2 emissions at the site, calculated using a simple empirical R s– T s relationship, showed that R s can account for about 88 ± 27% of total annual ecosystem respiration at the site. The majority of soil CO 2 emissions came from the upper 12 to 20 cm organic LFH (litter–fibric–humic) soil layer. The degree of spatial variability in R s, along the measured transect, was seasonal and followed the seasonal trend of mean R s: increasing through the growing season and converging to a minimum in winter (coefficient of variation (CV) ranged from 4 to 74% in Year 1 and 4 to 62% in Year 2). Spatial variability in R s was found to be negatively related to spatial variability in the C:N ratio of the LHF layer at the site. Spatial variability in R s was also found to depend on forest tree species composition within the stand. R s was about 15% higher in a broadleaf deciduous tree patch compared to evergreen coniferous area. However, the difference was not always significant (at 95% CI). In general, R s in the mixedwood patch, having both deciduous and coniferous species, was dominated by broadleaf trees, reflecting changing physiological controls on R s with seasons. Our results highlight the importance of discerning soil CO 2 emissions at a variety of spatial and temporal scales. They also suggest including the LFH soil layer and allowing for seasonal variability in CO 2 production within that layer, when modeling soil respiration in forest ecosystems.