Carbon sequestration in Pacific Douglas-Fir stands following harvesting

The seasonal and interannual variability of gross ecosystem photosynthesis (P_g) and ecosystem respiration (R_e), and their relationships to environmental variables and stand characteristics were used to explain the variation of eddy-covariance-measured net ecosystem productivity (F_NEP) of three different-aged Douglas-fir stands located on the east coast of Vancouver Island in British Columbia, Canada. During the 9-year period, 1998-2006, which included a strong El Niño/La Niña event, the near-end-of-rotation stand (DF49, 57 years old in 2006) was a moderate carbon (C) sink for CO₂ with annual F_NEP ranging from 267 to 410 g C m^-2 y^-1 (mean ± SD, 357 ± 51 g C m^-2 y^-1). The pole/sapling stand (HDF88, 18 years old in 2006) was a weak C source (F_NEP = -64 ± 75 g C m^-2 y^-1) and the recently-harvested stand (HDF00, 6 years old in 2006) was a large C source (F_NEP =-515 ± 88 g C m^-2 y^-1 during 2002-2006. Irrespective of stand age, all sites responded quite similarly to changes in environmental variables during each year. Daily total values of P_g and R_e were highest in July-August in all three stands while daily F_NEP peaked during April-June at DF49, May-June at HDF88 and June-July at HDF00. Reductions in root-zone soil water content decreased both P_g and R_e especially during the dry period from May to September and this effect was more pronounced in the younger stands. Evapotranspiration and dry-foliage surface conductance also decreased with decreasing root-zone soil water content whereas water-use efficiency appeared to be conservative, especially at DF49. Increasing spring temperature had a positive effect on annual P_g and R_e but caused a slight decrease in annual F_NEP. During the summer to autumn transition period, increases in soil water content resulted in a greater increase in R_e than P_g causing a reduction in F_NEP The interannual variation in the C balance was determined mainly by the interannual variation in R_e for the near-end-of-rotation stand and P_g for the two younger stands. The results indicate that regardless of the stand age, interannual variability in the C balance was mainly determined by year-to-year variability in spring temperature and water availability in late summer

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• Krishnan, P., Black, T. A., Jassal, R.S., B. Chen and Z. Nesic, Interannual variability of the carbon balance of three different-aged Douglas-fir stands in the Pacific Northwest, J. Geophys. Res., 114, G04011, doi:10.1029/2008JG000912, 2009 [PDF]