Forest stand characteristics and reproduction of northern spotted owls in managed north-coastal California forests

We monitored reproductive success of northern spotted owls (Strix occidentalis caurina) at 51 sites on Simpson Timber Company's (STC) managed, young-growth forests in northwestern California from 1991 to 1995. We compared habitat characteristics between sites with high and low fecundity at 5 spatial scales (concentric circles of 7, 50, 114, 203, and 398 ha), using 2 stratifications of annual reproductive success (upper 50% vs. lower 50%, and upper 25% vs, lower 75% of the proportion of years when greater than or equal to 1 owlet fledged). Habitat features included number of residual trees per hectare and mean proportion of 6 categories of stand age and 4 categories of basal area. Using the 50th percentile categories, we found there were higher proportions of age class 21-40 years and basal area classes 23-45 and 46-69 m(2)/ha (P less than or equal to 0.05), but lower proportions of recent clearcuts (0-5 yr) and basal area >69 m(2)/ha (P less than or equal to 0.05) at sites with high reproductive success. Using the upper 25% and lower 75% categories, we found there were higher proportions of basal area class 23-45 m(2)/ha, lower proportions of 61-80-year-old stands, and more residual trees per hectare at sites with high reproductive success (P less than or equal to 0.05). We also compared random sites to occupied sites via the above parameters. Spotted owl sites contained lower proportions of basal area class <23 m(2)/ha and greater proportions of class >69 m(2)/ha than did random sites (P < 0.05). There was less young forest (6-40 yr) and more forest of age class 41-60 years at spotted owl than at random sites (P < 0.05). Low prey abundance around spotted owl nest sites, roost sites, or both may explain why older stands with more basal area were found in higher proportions at sites with lower fecundity: Spotted owls that chose younger stands with smaller trees may have benefited from higher woodrat (Neotoma fuscipes) availability in young stands. Managing habitat by retaining residual trees and limiting clearcutting to at least 1.1 km beyond nest sites may prove useful in increasing reproductive success of northern spotted owls.