Effect of stand age on whole ecosystem CO2exchange in the Canadian boreal forest

Abstract

One of the key steps to estimating the current and future contribution of boreal forests to the global carbon cycle is quantifying the role recovery from fire plays in stand level carbon dynamics. We used tower‐based eddy covariance to measure the CO 2 exchange above five black spruce stands in central Manitoba during the 1999 and 2000 growing seasons (June–September). Fluxes in the four youngest stands (11, 19, 36, and 70 year old burns) were measured using portable eddy flux systems stationed in each burn for 4 to 6 weeks. Fluxes in the oldest stand (∼1870 burn) were measured continuously throughout 1999 and 2000, providing a baseline for interpretation of the fluxes measured simultaneously in the younger stands. Light‐saturated CO 2 uptake was lowest in the 11 year old stand (−4.1 μmol m−2 s−1), high in the 19 year old stand (−8.9 μmol m−2 s−1), highest in the 36 year old stand (−10.1 μmol m−2 s−1), and moderate in the 70 and 130 year old stands (−6.3 and −7.1 μmol m−2 s−1, respectively). Whole‐ecosystem respiration was lowest in the youngest burn and consistently increased with stand age. Integrated daily carbon balance changed from a slight sink at the 11 year old stand (−0.20 g C m−2 d−1) to a modest sink at the 19 year old stand (−1.9 g C m−2 d−1) to a large sink at the 36 year old stand (−3.1 g C m−2 d−1) to a modest sink at the 70 year old stand (−0.53 g C m−2 d−1) to around zero at the 130 year old stand. The results from any single tower in a boreal region are therefore unlikely to be representative of the entire region. Reliable assessments of regional carbon balance will require an approach that incorporates information on the fractional coverage of stands in different age classes and measurements of ecosystem gas exchange by representative stands within each age class.