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Keywords:

  • CO2;
  • Douglas-fir;
  • eddy covariance;
  • forest harvesting;
  • gross ecosystem production;
  • leaf area index;
  • net ecosystem exchange;
  • net ecosystem production;
  • respiration;
  • succession

Abstract

To investigate the variations in annual and seasonal net ecosystem production (FNEP) during the development of a young forest, 3 years of continuous eddy covariance measurements of carbon dioxide (CO2) 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 FNEP was examined by developing relationships between FNEP and variables such as light, temperature, soil moisture, and leaf area index (LAI). In all 3 years, the stand was a large source of CO2 (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=FNEPRe, where Re 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 Re from 840 to 1240 g C m−2 yr−1. Seasonal and interannual variations in daytime FNEP and P were well described by variations in photosynthetically active radiation, temperature, and changes in LAI. Night-time measurements of Re exponentially increased with 2 cm soil temperature with an average Q10 of 2 (relative increase in Re for a 10°C increase in temperature) and R10 (Re at 10°C) that increased from 2.1 in the first year to 2.5 in the second year to 3.2 μmol 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 Re, interannual variations in weather also affected annual FNEP. Drought, in the summer of the third year, resulted in early senescence and reduced both P and Re. This resulted in more C being lost from the stand in the third year after harvesting than in the second year.