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Northwest Forest Plan–the first 20 years (1994-2013): status and trends of late-successional and old-growth forests


Raymond J. Davis
Matthew J. Gregory
Zhiqiang Yang
Heather M. Roberts



Publication type:

General Technical Report (GTR)

Primary Station(s):

Pacific Northwest Research Station


Gen. Tech. Rep. PNW-GTR-911. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 112 p.


This is the third in a series of periodic monitoring reports on LSOG or latesuccessional and old-growth (older) forest status trends on federally administered lands since implementation of the Northwest Forest Plan (NWFP or the Plan) in 1994. The objective of this monitoring is to determine if the NWFP is providing for conservation and management of older forests as anticipated. This report focused on the amount, distribution, and spatial arrangement of older forests across the NWFP landscape, and how these have changed as a result of disturbance and ingrowth starting with the year of the NWFP analyses in 1993.
We developed maps of older forest using three definitions. The first definition was the same as used to define LSOG for the prior monitoring reports (10- and 15-year reports). The other two definitions were based on 80- and 200-year thresholds using an “old-growth structure index” (OGSI) developed in this report to represent the continuum of forest succession. The 80-year threshold (OGSI-80) represented forests that had achieved structure commonly associated with mature, late-successional, and old-growth forests in this region. The OGSI-200 threshold represented forests that had progressed past maturation and had achieved structure found in the later stages of succession commonly associated with old growth in this region. The OGSI threshold estimates bracketed the LSOG estimates using the previous definition, and changes in forest structure were better interpreted when using them. Thus, we terminated the use of the previous LSOG definition for use of the newer OGSI-based definitions.
We developed older forest maps for the beginning and ending periods of our analyses (1993 and 2012) and called these “bookend” maps. From these bookend maps, we assessed changes in the amount and distribution of older forests over time. We also used an annual time series of forest disturbance maps to characterize the agents of change (harvest, wildfire, and insects/disease) associated with areas mapped as older forest loss. To corroborate the mapped information, we estimated older forest area from Forest Inventory and Analysis plots, and older forest change from two successive forest inventories from which such data were available (Forest Service and Oregon Bureau of Land Management lands).
The maps showed net changes in amount of older forests on federal lands managed under the NWFP have been small (a 2.8 to 2.9 percent net decrease). This occurred despite gross losses from wildfire (4.2 to 5.4 percent), timber harvest (1.2 to 1.3 percent), and from insects or other causes (0.7 to 0.9 percent), suggesting that processes of forest succession have compensated for some of the losses resulting from disturbance. The Plan anticipated a continued decline in older forests for the first few decades until the rate of forest succession exceeds the rate of gross losses. Decadal gross losses of about 5 percent per decade as a result of timber harvesting and wildfire were expected. Observed losses from wildfire were about what was expected, but losses from timber harvesting were about one quarter of what was anticipated. Results were consistent with expectations for older forest abundance, diversity, and connectivity outcomes for this period of time. Nothing in the findings suggests that attainment of desired outcomes over the next few decades is not feasible; however, we noted some portions of the NWFP federal landscape that had been set back from those outcomes, particularly resulting from large wildfires in the fire-prone portions of the NWFP area.


Davis, Raymond J.; Ohmann, Janet L.; Kennedy, Robert E.; Cohen, Warren B.; Gregory, Matthew J.; Yang, Zhiqiang; Roberts, Heather M.; Gray, Andrew N.; Spies, Thomas A. 2015. Northwest Forest Plan–the first 20 years (1994-2013): status and trends of late-successional and old-growth forests. Gen. Tech. Rep. PNW-GTR-911. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 112 p.


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