ESTIMATING THE AGE AND STRUCTURE OF FORESTS IN A MULTI-OWNERSHIP LANDSCAPE OF WESTERN OREGON, USA

Forests of the Pacific Northwest region of the U.S.A. are part of an o ngoing political debate that focuses on the trade-offs between commodi ty and non-commodity values. A key issue in this debate is the locatio n and extent of closed canopy mature and old-growth forest remaining i n 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 l eaf area indices. Thus, most incident solar energy is absorbed, making these forests difficult targets for discrimination of classes. Additi onally, spectral characteristics can be affected more by the effects o f steep topography than condition of the closed canopy forest. Experim enting with a number of techniques, we estimated and mapped forest age and structure in 1988 over a 1237482 ha area on the west side of the Oregon Cascade Range with an overall accuracy of 82 per cent. Unsuperv ised classification enabled several forest classes to be defined in te rms 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 per cent is compris ed of non-conifer species), and closed conifer (> 85 per cent, of whic h less than 10 per cent is non-conifer). These classes represented nea rly distinct spectral groups. Within the closed canopy conifer class, between two and three age and structural classes could be distinguishe d using regression analysis (e.g., young, mature, and old-growth). Def ining more classes seriously degraded map accuracies. The Tasseled Cap wetness index was not sensitive to topography, and yielded more accur ate results in closed canopy conifer stands than Tasseled Cap brightne ss or greenness, even when regression models using these indices were based on solar incidence angle stratification. The multi-ownership stu dy 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 mat ure or old-growth state. Forests administered by the USDI Bureau of La nd Management (BLM) and the USDA Forest Service, but protected by cong ressional and administrative mandates from harvest, were 10 per cent o f the total forestland. Of the protected category, only 60 per cent wa s mature and old-growth forest. Unprotected BLM and Forest Service lan ds 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 the ir holdings in a pre-canopy closure and young conifer condition. Thirt y-five per cent of the forestland was privately owned, and consisted o f 73 per cent pre-canopy closure and young conifer forest stands. Of a ll 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 Fores t Service land, and 15 per cent on protected land.