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.