Late-successional and old-growth (LSOG) monitoring characterizes the status and trends of older forests to answer such questions as: How much older forest is there? Where is it? How much has changed and from what causes? Is the Northwest Forest Plan (the Plan) maintaining or restoring older forest ecosystems to desired conditions on federal lands in the Plan area? This assessment is the second in a continuous monitoring cycle. We initially reported on LSOG status and trends from 1994 to 2003 in the “10-year report.” This document, the mid-cycle “15-year report,” updates the assessment to 2006 in Washington and Oregon and to 2007 in California. The next major assessment will be the 20-year report.

We used maps of forest vegetation and change and regional inventory plot data to assess the distribution and trends of LSOG on federal and other lands in the Plan area over the monitoring periods 1994 to 2007 in California and 1996 to 2006 in Washington and Oregon. We used statistical mapping techniques to develop maps of forest composition and structure at the two monitoring cycle endpoints (“bookend” maps), and yearly maps of forest disturbance. From the two bookend maps we assessed changes in the amount and distribution of LSOG (defined as average diameter of overstory conifers >20 in and conifer canopy cover >10 percent) over time. We used the disturbance maps to characterize the agents of change (harvest, wildfire, and insects/disease) associated with areas mapped as LSOG loss from the bookend maps. To corroborate the mapped information, we estimated LSOG area from two successive forest inventories from which such data were available (Forest Service and Oregon Bureau of Land Management lands), and compiled the first Plan-wide estimates of LSOG on all ownerships from a regionally consistent inventory design.

The bookend maps suggested a slight net loss (-1.9 percent) of LSOG from federal lands in the Plan area, from 33.2 percent of federal forest to 32.6 percent (from 7.3 to 7.1 million ac). Trends varied by province, but in all cases, the net changes were small relative to the sources of error and uncertainty in the estimates, which limit our ability to estimate the precise amount of LSOG change. Nevertheless, strong evidence suggests that >200,000 ac of LSOG were lost to stand-replacing disturbance (mostly wildfire) on federal lands. Almost 90 percent of the loss of federal LSOG was from reserves.

The losses apparently were roughly balanced by recruitment, although recruitment is much more difficult to estimate than disturbance with available data and technology. Recruitment was most likely through incremental stand growth over the 20-in threshold, or from understory disturbances that eliminated smaller diameter trees and increased average stand diameter. Increases in the area of forests of much larger and older trees are unlikely to occur over the 10- to 14-year monitoring period. Use of a more restrictive definition of LSOG (larger average tree size or denser canopy) likely would increase the estimate of LSOG loss and decrease the estimate of LSOG gain. The small net decrease in LSOG was corroborated by successive forest inventories, but the plot-based estimates of LSOG change were not statistically significant.

The results support the assumption made in the Plan that the primary responsibility for maintaining or restoring LSOG and related habitats in the Pacific Northwest would fall to public lands. Federal lands contained less than half of the total forest land, but the federal share of total LSOG increased from 65 to 67 percent over the monitoring period. Harvesting removed about 13 percent (approximately 491,000 ac) of LSOG on nonfederal lands. Loss of LSOG on federal lands resulting from harvest was less than 0.5 percent (approximately 32,100 ac). Wildfire was the most significant change agent for LSOG on federal lands over the Plan area, and will continue to be a key consideration for policies affecting older forests, associated species, and watershed conditions.

Keywords: Old growth, forest monitoring, Gradient Nearest Neighbor imputation, LandTrendr change detection, Pacific Northwest