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

Temporal and spatial variability of soil respiration (Rs) 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 Rs measured during the two study years were 0.5–6.9 μmol CO2 m−2 s−1 for 2003–2004 (Year 1) and 0.4–6.8 μmol CO2 m−2 s−1 for 2004–2005 (Year 2). Mean annual Rs for the stand was the same for both years, 2.7 μmol CO2 m−2 s−1. Temporal variability of Rs was controlled mainly by soil temperature (Ts), but soil moisture had a confounding effect on Ts. Annual estimates of total soil CO2 emissions at the site, calculated using a simple empirical Rs–Ts relationship, showed that Rs can account for about 88 ± 27% of total annual ecosystem respiration at the site. The majority of soil CO2 emissions came from the upper 12 to 20 cm organic LFH (litter–fibric–humic) soil layer. The degree of spatial variability in Rs, along the measured transect, was seasonal and followed the seasonal trend of mean Rs: 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 Rs was found to be negatively related to spatial variability in the C:N ratio of the LHF layer at the site. Spatial variability in Rs was also found to depend on forest tree species composition within the stand. Rs 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, Rs in the mixedwood patch, having both deciduous and coniferous species, was dominated by broadleaf trees, reflecting changing physiological controls on Rs with seasons. Our results highlight the importance of discerning soil CO2 emissions at a variety of spatial and temporal scales. They also suggest including the LFH soil layer and allowing for seasonal variability in CO2 production within that layer, when modeling soil respiration in forest ecosystems.