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Northwest Forest Plan—the first 25 years (1994–2018): status and trends of late-successional and old-growth forests

Informally Refereed
Authors: Raymond J. Davis, David M. Bell, Matthew J. Gregory, Zhiqiang Yang, Andrew N. Gray, Sean P. Healey, Andrew E. Stratton
Year: 2022
Type: General Technical Report (GTR)
Station: Pacific Northwest Research Station
Source: Gen. Tech. Rep. PNW-GTR-1004. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 82 p.


This is the fourth in a series of periodic monitoring reports on the status and trends of late-successional and old-growth (LSOG) forests since the implementation of the Northwest Forest Plan (NWFP) in 1994. The objective of this monitoring is to evaluate the success of the plan in reaching its desired amount and distribution of LSOG forest on federal lands within the range of the northern spotted owl (Strix occidentalis caurina) in the United States. We began our assessment in the years shortly preceding the NWFP, but primarily focused on how LSOG forests have changed as a result of disturbance and forest succession since 1993, the year of the assessment that led to the implementation of the NWFP. We developed an annual time series (1986–2017) of LSOG maps based on an “old-growth structure index” (OGSI) using two age thresholds: ≥80 and ≥200 years. These ages represent when forests commonly attain stand structure associated with late-successional forests (OGSI 80) and old-growth forests (OGSI 200) in this region.
Maps showed a slightly increasing trend in LSOG forests (OGSI 80) on federal lands with a 0.3-percent net gain between 1993 and 2017. Forest Inventory and Analysis plot data from two measurement/remeasurement periods (2000s and 2010s) were used to corroborate mapped estimates. For OGSI 80 and OGSI 200 forests, we estimated gross losses from wildfire at 6.2 and 6.9 percent, respectively; timber harvest losses at 1.9 and 2 percent, respectively; and loss from insects or other causes at 0.7 and 0.9 percent, respectively. This indicates that, at the NWFP scale, processes of forest succession compensated for losses. The NWFP anticipated a continued decline in LSOG forests for the first few decades until the rate of forest succession exceeds the rate of losses. Decadal gross losses of about 5 percent per decade from timber harvesting and wildfire (combined) were expected. Over the extent of the NWFP, observed losses from wildfire generally met expectations, but losses from timber harvesting were about one-third of what was anticipated. Results were consistent with expectations for OGSI 80 abundance, diversity, and connectivity outcomes for this period of time. For OGSI 200, these outcomes were slightly degraded. Given that we are only one quarter into a 100-year plan, nothing in these findings suggests that desired outcomes are unattainable over the next 75 years. However, observed increases in frequency and extent of large wildfires, and expected additional increases owing to climate change, provide reasons for concern.


Northwest Forest Plan, effectiveness monitoring, late-successional and old-growth forests, old-growth structure index, Gradient Nearest Neighbor imputation, GNN, Landscape Change and Monitoring System, LCMS, Forest Service, Bureau of Land Management, late-successional reserves, physiographic provinces.


Davis, Raymond J.; Bell, David M.; Gregory, Matthew J.; Yang, Zhiqiang; Gray, Andrew N.; Healey, Sean P.; Stratton, Andrew E. 2022. Northwest Forest Plan the first 25 years (1994 2018): status and trends of late-successional and old-growth forests. Gen. Tech. Rep. PNW-GTR-1004. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 82 p.