Spring and Fall Conditions Explain the Interannual Variability in Carbon Exchange in a Boreal Peatland

M.-C. Bonneville, I. Strachan

Fluxes of carbon dioxide were measured in a boreal peatland using the eddy covariance technique for two consecutive growing seasons and one winter. The carbon uptake period was 143 days in 2008 and 127 days in 2009, and cumulative net ecosystem exchange of CO2 (NEE) from May to October was -89.5 ± 8.8 g C m-2 and -78.3 ± 9.0 g C m-2 in 2008 and 2009
143 days in 2008 and 127 days in 2009, and cumulative net ecosystem exchange of CO2 (NEE) from May to October was -89.5 ± 8.8 g C m-2 and -78.3 ± 9.0 g C m-2 in 2008 and 2009, respectively. For this same period, both ecosystem respiration (ER) and gross ecosystem productivity (GEP) were higher in 2008 than in 2009, with cumulative ER of 217.4 ± 7.1 g C m-2 (2008) and 190.6 ± 9.7 g C m-2 (2009), and cumulative GEP of 307.0 ± 14.4 g C m-2 (2008) and 268.9 ± 16.2 g C m-2 (2009). Integrated over one complete year, the results show that this peatland was a net sink of -53.8 ± 4.9 g C m-2 yr-1, with 238.8 ± 8.1 g C m-2 yr-1 emitted as ER and 292.7 ± 11.9 g C m-2 yr-1 assimilated as GEP. Water table depth (WTD) was a better predictor of ER in 2009 (r2=0.59) than in 2008 (r2=0.06). GEP was found to be higher at high temperature and at low WTD, even after the effect of seasonality was taken into account. Overall, the larger cumulative NEE in 2008 is the result of a combination of earlier onset of the growing season due to warmer temperatures and earlier snowmelt, and of greater light availability and higher temperatures in the fall. The peak growing season months showed similar NEE in both study years, so that most of the May to October difference in NEE is indeed related to differences in spring and fall fluxes resulting from hydro-meteorological conditions. This study suggests that the prevailing conditions in the shoulder periods (spring and fall) may be crucial in determining the annual carbon balance of peatlands.

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