Recovery Plan for the Northern Spotted Owl - DRAFT

Resultsstands were defined as being more than 200 years old in most studies. Maturestands in most of the studies were 80 to 200 years old and even-aged (or had'few canopy layers"), and had mean diameters of approximately 21 to 39inches. More recent work is summarized in the following sections.Several recent studies in California have analyzed habitats defined by meandbh of the stand. Some of these studies have used the following definitions:seedlings and saplings: less than 5 inches; pole timber: 5 to 10.9 inches; smalltimber 11 to 21 inches; mature and old-growth: more than 21 inches. Otherstudies have combined the two middle categories and have defined the followingcategories, referred to as seral stages: early successional (less than 5inches), mid successional (5 to 21 inches), and late successional (more than 21inches).Solis and Gutierrez (1990) and Blakesley et al. (1992) studied habitat use byowls during the breeding season at the Willow Creek study area. The sevenbirds studied by Solis and Gutierrez showed a clear preference for mature andold-growth stands (Table B. 1 1); 84 percent of the locations were in largediameter(more than 21 inches dbh) stands and small-diameter (less than 5inches) stands were almost never used. Blakesley et al. (1992) found thatvirtually all roost locations were in the 11 to 21 -inch dbh and more than 21 -inch dbh categories, and the number of locations per acre of habitat was nearlytwice as high in the more than 21-inch stands as in the smaller stands.During the nonbreeding season, the situation was more complex. Sisco (1990)found little or no preference for particular stands (Table B. 1). Only 56 percentof the observations were in mature and old-growth stands, and smalldiameter stands were used roughly in proportion to their abundance. Asshown in Tables B.2 through B.5, however, the average structural features ofsites used by these birds were similar in the breeding and nonbreeding seasons.It thus appears that during the nonbreeding season, the owls usedstands roughly in proportion to their availability but they selected sites withinthese stands that had larger-diameter trees. Blakesley et al. (1992) noted thatthe 11 to 21-inch dbh stands in this study area have resulted from naturalprocesses and are more variable than plantations of the same mean diameter.This study should not be taken as indicating that 11 to 21-inch dbh plantationsin this area would provide adequate habitat for owls during thenonbreeding season.Solis (1983) and Solis and Gutierrez (1990) also presented data on use ofhabitats categorized by overstory and understory canopy coverage. Stands inwhich both the overstory and understory canopy cover exceeded 70 percenthad more use than stands with a less well-developed overstory and much moreuse than stands with a poorly developed overstory and understory (Table B. 12).These birds thus concentrated their activities in stands having high canopycoverage in the overstory and understory.Zabel et al. (1991), working on the Six Rivers and Klamath National Forests,found only slight tendencies by owls on either forest to select particular standtypes, categorized by either mean dbh or canopy cover (Table B. 13). Only halfthe owls showed significant selection for any habitat (i.e., chi-square analysisrejected the hypothesis of random distribution), and patterns of use variedbetween owls. In the Klamath National Forest, owls showed slight preferencefor 21 to 36-inches trees, but no preference for trees more than 36 inches. Inthe Six Rivers National Forest owls showed no preference for 21 to 36-inchestrees and slight preference for more than 36 inches trees. Stands were alsodivided into those that did and did not meet the Forest Service definition of305