age applied to all species. It looks at how the ecosystem functions as a whole and considers the condition of “related structural attributes ... [such as] tree size, accumulation of large dead woody material, number of canopy layers, species composition.” AR Tab 3, p. H-l. From these general characteristics, the Forest Service has adopted interim definitions for determining old growth forest and has identified about 180,-000 acres of the Forest that it considers “old growth.” AR Tab 1, p. 2-3. Under the Forest Service’s definition, the age of a stand is a function of “the prevailing situation in the stand” and not the age of the oldest tree in the area. AR Tab 23. The Forest Service relied on the CISC database and on a field team’s examination of the p 992 F.2d 977, 981 (9th Cir.1993) (<HOLDING>). The Forest Service’s method of determining
holding that deference to an agency interpretation is not appropriate where a statute is administered by more than one agency recognizing that courts accord significant deference to an agency decision where agency expertise is involved holding deference to agency methodology appropriate unless agency failed to address an essential factor holding that when an agency has not reached an issue the proper course is to remand to the agency in the first instance to address holding the agency is entitled to use its own methodology unless it is irrational