Ecosystem CO 2 flux over two growing seasons for a sub - Boreal clearcut 5 and 6 years after harvest

We measured the ecosystem-level growing season CO2 fluxes for a 6-year-old vegetated sub-Boreal clearcut from 24 May to 20 September 2000, and compared the results to CO2 fluxes from the same clearcut in 1999 (27 June–3 September). Two independent approaches were used to measure ecosystem CO2 flux for both years. A Bowen ratio energy balance (BREB) method was contrasted with a second approach using component fluxes. The Component model approach was based on scaling up from regressions relating in situ CO2 flux measurements for conifer seedlings (Picea glauca × engelmannii), as well as representative herbaceous (Chamerion angustifolium), woody (Lonicera involucrata) plant species and soil surface CO2 efflux to microclimate conditions. The BREB method and Component model estimated the clearcut to be a source of CO2 6 years post-harvest (24 May–20 September 2000) in amounts of 142 g C m−2 (1.4 t ha−1) and 103 g C m−2 (1.0 t ha−1), respectively. The positive net ecosystem CO2 fluxes over the growing season resulted from a large soil surface CO2 efflux (686 g C m−2) that surmounted the photosynthetic CO2 uptake for the clearcut. The photosynthetic CO2 uptake partially compensated for the soil surface losses: the conifer seedlings, herbaceous plants and woody shrubs were estimated to uptake −96, −315, −172 g C m−2, respectively. The results of 2000 contrasted with those of 1999. Over a comparable period of measurement (27 June–3 September), the clearcut was a sink for carbon in 1999 (−22.4 g C m−2 using the BREB method and−85 g C m−2 using the Component model) and a source in 2000 (65 g C m−2 using the BREB method and 44 g C m−2 using the Component model). The growing seasons of 1999 and 2000 experienced similar photosynthetic uptake over this same interval (−423 and −422 g C m−2, respectively). The main difference between the two field seasons was an increase in the soil surface CO2 efflux from 1999 to 2000. In 1999, the soil surface CO2 efflux was 338 g C m−2 and in 2000 the flux was 38% higher (466 g C m−2) for the same period. The results indicate that while there was notable inter-annual variation in CO2 fluxes, particularly the soil surface CO2 effluxes, this young regenerating sub-Boreal forest (≤6 years after harvesting) is a net source of CO2 when the entire growing season is considered. © 2002 Elsevier Science B.V. All rights reserved.