Net ecosystem production of a Douglas-fir stand for 3 years following clearcut harvesting

摘要

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 (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 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 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=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 degrees C increase in temperature) and R-10 (R-e at 10 degrees C) that increased from 2.1 in the first year to 2.5 in the second year to 3.2 mu mol 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.