Recovery Plan for the Northern Spotted Owl - DRAFT

and numbers of trees, and percent of them that were hardwoods, as shown inTable B. 10. We could then determine which fraction of the utilized sites ineach study were in this category and which fraction were not.In analyses of this sort, the parameter of interest must be specified with care,and the estimation procedure must take proper account of the field methodsused to collect the data. The definition of 'site" could have a substantial effecton the outcome of the study. Suppose, for example, that the acre centered onutilized sites always conformed to the description in Table B. 10. If a few smallplots were placed within the acre surrounding each site and means per plotwere calculated, then many sample values would not conform to the descriptionsin Table B. 10 due solely to sampling error. This is a nontrivial problem(i.e., the errors could be quite substantial), and obtaining unbiased estimatesof the proportion of acres meeting the definition would be a difficult statisticaltask. The analysis would be much simpler if plots and sites were defined to bethe same size. Different studies, however, have used different numbers andsizes of plots so such an analysis cannot be performed using existing data.Information on the structure of utilized sites also will be of value in guidingsilvicultural efforts to develop or maintain suitable habitat and in minimizingadverse effects of harvest in the matrix. All of the cautions discussed earlierapply to such efforts, and it must remembered that these structural measurementsprovide little information about whether the sites provide suitablehabitat because they do not relate structure to reproductive success of theowls. Even less basis is provided by these data for deciding whether otherenvironments sharing some of the structural features might be suitable.Nonetheless, data on structure of utilized stands are an important first step indesigning silvicultural programs that will be effective in helping protect owls.Amount of Habitat in Home RangesResultsThomas et al. (1990) reviewed studies reporting territory or home range size,and amount of older forest within home ranges. This information is presentedin section II of this plan (Tables 2.1 and 2.2) and will not be repeated here, butresults from a few other recent studies are presented.Several recent studies in California provide additional information on theamount of one or more types of habitat within home ranges or circles centeredon the nest or activity sites. These studies report the acreage within territoriescovered by stands with large (more than 21" dbh) trees. Solis and Gutierrez(1990) found that eight summer home ranges in the Willow Creek study area(Six Rivers National Forest) contained a mean of 481 acres (range: 208 to 979)of this habitat. Zabel et al. (1991) reported that annual home ranges containedan average of approximately 900 acres of this habitat in the Six Rivers NationalForest (Mad River District, 10 owls), 1,400 acres of this habitat in the KlamathNational Forest (Ukonom District, nine owls), and 1,600 acres of this habitat inthe Siskiyou National Forest (Chetco District, two owls). Self and Brown pers.comm.) reported that 21 pairs of owls had a mean of 503 acres of stands inwhich the mean dbh was more than 24 inches within 1.3 miles of their nest.Home ranges were not mapped by Self and Brown, but the home rangesstudied by Solis and Gutierrez (1990) were almost entirely within 1.3 miles ofthe nests, so the home ranges studied by Self and Brown probably had lessthan 503 acres. Not all nests studied by Self and Brown were in this habitattype and some home ranges contained no acres of this type.302