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The role of old forests and big trees in forest carbon sequestration in the Pacific NorthwestAuthor(s): Andrew N. Gray
Source: In: Stanton, Sharon M.; Christensen, Glenn A., comps. 2015. Pushing boundaries: new directions in inventory techniques and applications: Forest Inventory and Analysis (FIA) symposium 2015. 2015 December 8–10; Portland, Oregon. Gen. Tech. Rep. PNW-GTR-931. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station: 153-158.
Publication Series: General Technical Report (GTR)
Station: Pacific Northwest Research Station
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DescriptionForest ecosystems are an important component of the global carbon (C) cycle. Recent research has indicated that large trees in general, and old-growth forests in particular, sequester substantial amounts of C annually. C sequestration rates are thought to peak and decline with stand age but the timing and controls are not well-understood. The objectives of this study were to determine how the balance of tree growth, mortality, and dead wood decay vary by plant community type, site productivity, and stand age. We compiled remeasured tree and dead wood estimates from 8,767 inventory plots on Pacific Northwest Region National Forest lands and assessed changes by climax plant association zones (PAZs) and site productivity estimates of mean annual increment at culmination (MAI). Estimated maximum C density for old-growth stands (≥300 years old) varied significantly by MAI class within PAZ, but on average stands accumulated 66% of maximum stores by age 100. We did not see a decline in live tree production in older stands in moderate and low MAI classes, but a 33% reduction in high MAI classes. We found that mortality in undisturbed stands increased with stand age such that the net growth in live tree biomass, and the change in total C, was not significantly different from zero in stands over age 400 (0.15 ± 0.64 Mg/ha/yr for total C, 95% confidence interval). Mortality of large trees (>100 cm diameter) exceeded growth, but trees were growing into the larger size classes at a high-enough rate that a net increase in large tree C was seen across the region. Even though large trees accumulated C at a faster rate than small trees on an individual basis, their contribution to C sequestration was smaller on an area basis, and their importance relative to small trees declined in older stands compared to younger stands.
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CitationGray, Andrew N. 2015. The role of old forests and big trees in forest carbon sequestration in the Pacific Northwest. In: Stanton, Sharon M.; Christensen, Glenn A., comps. 2015. Pushing boundaries: new directions in inventory techniques and applications: Forest Inventory and Analysis (FIA) symposium 2015. 2015 December 8–10; Portland, Oregon. Gen. Tech. Rep. PNW-GTR-931. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station: 153-158.
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