Production, respiration, and overall carbon balance in an old-growth Pseudotsuga-tsuga forest ecosystem

Ground-based measurements of stores, growth, mortality, litterfall, respiration, and decomposition were conducted in an old-growth forest at Wind River Experimental Forest, Washington, USA. These measurements were used to estimate gross primary production (GPP) and net primary production (NPP); autotrophic respiration (R-a) and heterotrophic (R-h) respiration; and net ecosystem production (NEP). Monte Carlo methods were used to calculate uncertainty (expressed as +/- 2 standard deviations of 200-400 calculations). Live carbon (C) stores were 39,800 g C m(-2) (34,800-44,800 g C m(-2)). The store of C in detritus and mineral soil was 22,092 g C m(-2) (20,600-23,600 g C m(-2)), and the total C stores were 61,899 g C m(-2) (56,600-67,700 g C m(-2)). Total NPP was 597 g C m(-2) y(-1) (453 to 741 g C m(-2) y(-1)). R-a was 1309 g C m(-2) y(-1) (845-1773 g C m(-2) y(-1)), indicating a GPP of 1906 g C m(-2) y(-1) (1444-2368 g C m(-2) y(-1)). R-h, including the respiration of heart rots in tree boles, was 577 g C m(-2) y(-1) (479-675 g C m(-2) y(-1)). Long-term NEP was estimated to be +20 g C m(-2) y(-1) (-116 to +156 g C m(-2) y(-1)), indicating this stand might be a small sink. These estimates contrast with the larger sink estimated at the same site using eddy-flux methods. Several hypotheses to explain this discrepancy were explored, including (a) undetected biomass increases, (b) underestimates of NPP, (c) unmeasured losses, and (d) a temporal mismatch between the two sets of measurements. The last hypothesis appears the most likely.