The seasonal and interannual variability of gross ecosystem photosynthesis (Pg) and
ecosystem respiration (Re), and their relationships to environmental variables and stand
characteristics were used to explain the variation of eddy-covariance-measured net
ecosystem productivity (FNEP) 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 Nin˜o/La Nin˜a event, the near-end-of-rotation
stand (DF49, 57 years old in 2006) was a moderate carbon (C) sink for CO2 with annual
FNEP ranging from 267 to 410 g C m2 yr1 (mean ± SD, 357 ± 51 g C m2 yr1
).
The pole/sapling stand (HDF88, 18 years old in 2006) was a weak C source (FNEP =
64 ± 75 g C m2 yr1
), and the recently harvested stand (HDF00, 6 years old in 2006)
was a large C source (FNEP = 515 ± 88 g C m2 yr1
) during 2002–2006. Irrespective
of stand age, all sites responded quite similarly to changes in environmental variables
during each year. Daily total values of Pg and Re were highest in July–August in all three
stands, while daily FNEP peaked during April–June at DF49, May–June at HDF88,
and June–July at HDF00. Reductions in root-zone soil water content decreased both Pg
and Re 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 Pg and Re but caused a slight decrease in
annual FNEP. During the summer to autumn transition period, increases in soil water
content resulted in a greater increase in Re than Pg causing a reduction in FNEP. The
interannual variation in the C balance was determined mainly by the interannual variation
in Re for the near-end-of-rotation stand and Pg 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.
