A third constraint was that after 40 years (the end of the deciduous phase), the rate
of C accumulation slowed following an exponential function and with a relaxation time
of 75 years. This ensured that after 150 years, the carbon balance of the forest was close
to steady state, and consistent with observations from other older black spruce stands
(14). A slowing of the carbon accumulation rate of this magnitude is also consistent with
first-order decomposition and accumulation rate constants derived from radiocarbon
S5
measurements in other boreal forest stands with feathermoss ground cover (15).
Trumbore and Harden (15) predicted accumulation rates of approximately 12 ± 13 g C m2
yr
-1 and 4 ± 2 g C m2
yr
-1 for feathermoss and sphagnum cover, respectively for an
approximately 150 year old black spruce (OBS) site in Canada. With the NEP curve
shown in fig. S3A, we estimate NEP to be 7 g C m2
yr
-1 at 150 years after fire. We
recognize that our accumulation rates are lower than other reported trajectories in the
boreal forest (16, 17) and attribute this to the well-drained soils and lack of permafrost at
our sites that probably slowed rates of both vegetation and soil carbon accumulation. We
assumed that other carbon gain/loss pathways, including leaching and organic C losses
were negligible so that NEP was equal to net ecosystem carbon balance (NECB) (18).