Estimating the age and structure of forests in a multi-ownership landscape of western Oregon,U.S.A.

Forests of the Pacific Northwest region of the U.S.A. are part of an ongoing political debate that focuses on the trade-offs between commodity and non-commodity values. A key issue in this debate is the location and extent of closed canopy mature and old-growth forest remaining in the region. Remote sensing can play a major part in locating mature and old-growth forests, but. several challenges must be overcome to do so with acceptable accuracy. Conifer forests of the region have high leaf area indices. Thus, most incident solar energy is absorbed, making these forests difficult targets for discrimination of classes. Additionally, spectral characteristics can be affected more by the effects of steep topography than condition of the closed canopy forest.

Experimenting with a number of techniques, we estimated and mapped forest age and structure in 1988 over a 1 237 482 ha area on the west side of the Oregon Cascade Range with an overall accuracy of 82 per cent. Unsupervised classification enabled several forest classes to be defined in terms of per cent cover: open (0-30 per cent), semi-open (30-85 per cent), closed mix (> 85 per cent, of which at least 10 percent is comprised of non-conifer species), and closed conifer (> 85 per cent, of which less than 10 per cent is non-conifer). These classes represented nearly distinct spectral groups. Within the closed canopy conifer class, between two and three age and structural classes could be distinguished using regression analysis (e.g., young, mature, and old-growth). Defining more classes seriously degraded map accuracies. The Tasseled Cap wetness index was not sensitive to topography, and yielded more accurate results in closed canopy conifer stands than Tasseled Cap brightness or greenness, even when regression models using these indices were based on solar incidence angle stratification.

The multi-ownership study area consisted of 76 per cent forestland. Of the total forestland, 70 per cent was closed canopy conifer, with 42 per cent being in a mature or old-growth state. Forests administered by the USD1 Bureau of Land Management (BLM) and the USDA Forest Service, but protected by congressional and administrative mandates from harvest, were 10 per cent of the total forestland. Of the protected category, only 60 per cent was mature and old-growth forest, Unprotected BLM and Forest Service lands accounted for 53 per cent of the forestland in this study (8 and 45 per cent, respectively). Of the unprotected category, the BLM had 63 per cent, and the Forest Service had 49 per cent, respectively, of their holdings in a pre-canopy closure and young conifer condition. Thirty-five per cent of the forestland was privately owned, and consisted of 73 per cent pre-canopy closure and young conifer forest stands. Of all mature and old-growth forest, 22 per cent was found on private land, 7 per cent on unprotected BLM land, 55 per cent on unprotected Forest Service land, and 15 per cent on protected land.