Net ecosystem production of a Douglas-fir stand for 3 years following clearcut harvesting ELYN R. HUMPHREYS, T. ANDREW BLACK, KAI MORGENSTERN, ZHONG LI and ZORAN NESIC Faculty of Agricultural Sciences, University of British Columbia, Vancouver, BC, Canada V6 T 1Z4 Abstract To investigate the variations in annual and seasonal net ecosystem production (F NEP ) during the development of a young forest, 3 years of continuous eddy covariance measurements of carbon dioxide (CO 2 ) fluxes were collected following clearcut harvesting and replanting of a coastal Douglas-fir stand on the east coast of Vancouver Island, BC, Canada. The impact of changing weather and stand structure on F NEP was examined by developing relationships between F NEP and variables such as light, temperature, soil moisture, and leaf area index (LAI). In all 3 years, the stand was a large source of CO 2 (620, 520, and 600 g C m 2 yr 1 in the first, second, and third years, respectively). During this period, the growth of pioneer and understory species resulted in an increase in maximum growing season LAI from 0.2 in the year the seedlings were planted to 2.5 in the third year. The associated increase in annual gross ecosystem production (P 5 F NEP R e , where R e is ecosystem respiration) from 220 g C m 2 yr 1 in the first year to 640 g C m 2 yr 1 in the third year was exceeded by an increase in annual R e from 840 to 1240 g C m 2 yr 1 . Seasonal and interannual variations in daytime F NEP and P were well described by variations in photosynthetically active radiation, temperature, and changes in LAI. Night-time measurements of R e exponentially increased with 2 cm soil temperature with an average Q 10 of 2 (relative increase in R e for a 10 1C increase in temperature) and R 10 (R e at 10 1C) that increased from 2.1 in the first year to 2.5 in the second year to 3.2 lmol m 2 s 1 in the third year. Although the re-establishment of vegetation in this stand had a major impact on both P and R e , interannual variations in weather also affected annual F NEP . Drought, in the summer of the third year, resulted in early senescence and reduced both P and R e . This resulted in more C being lost from the stand in the third year after harvesting than in the second year. Keywords: CO 2 , Douglas-fir, eddy covariance, forest harvesting, gross ecosystem production, leaf area index, net ecosystem exchange, net ecosystem production, respiration, succession Received 30 April 2004; accepted 30 August 2004 Introduction Stand-replacing disturbances such as harvesting, fire and insect damage have a major impact on the exchange of carbon dioxide (CO 2 ) between forested lands and the atmosphere. With the removal of the aboveground biomass, the amount of CO 2 taken up by photosynthesis is significantly reduced while the remaining belowground biomass and slash decompose, releasing CO 2 . Consequently, harvested lands tend to act as a net source of CO 2 . The duration and magnitude of this carbon (C) loss largely depends on the balance between the C taken up by photosynthesis and the C lost by respiration and the biophysical factors, which affect these processes. In the years following harvest- ing, the growth of planted and/or pioneer species gradually modifies the microclimate and the C balance. Currently about 1.02  10 6 ha of Canada’s 247  10 6 ha of commercial forestland are harvested annually (Kurz & Apps, 1999). It is estimated that 86  10 12 kg of C is stored in the vegetation and soils of Canada’s forests. Consequently, forest harvesting has the potential to significantly affect regional and national C budgets (Kurz & Apps, 1999) Correspondence: Elyn Humphreys, Geography Department, Trent University, 1600 West Bank Dr., Peterborough, ON K9J 7B8, Canada. tel. 1 1 705 748 1011 ext 1487, fax 1 1 705 748 1205, e-mail: elynhumphreys@trentu.ca Global Change Biology (2005) 11, 450–464 doi: 10.1111/j.1365-2486.2005.00914.x 450 r 2005 Blackwell Publishing Ltd