Implications of spotted owl management for landbirds on National Forests of the Sierra Nevada, California.
Ryan D. Burnett, Nadav Nur, and Christine A. Howell
PRBO Conservation Science
3820 Cypress Drive #11
Petaluma, CA 94954
Mail proofs to the following address:
Ryan Burnett
PO Box 634
Chester, CA 96020
(530) 258 – 2869
Abstract
The Sierra Nevada ecosystem supports a diverse avian community that occupies a broad range of habitat types and conditions. In recent years, National Forest management has shifted focus towards California spotted owl (Strix occidentalis occidentalis) and other late seral associated wildlife species. As a result – in concert with a lack of natural disturbance – stand density and canopy cover are increasing on National Forests in the Sierra Nevada. In order to understand the implications of these habitat changes on the rest of the avian community, we compared abundance indices of the 20 most commonly detected landbird species in the study area, five species of conservation interest, and several community and nesting guild metrics, inside and outside of spotted owl Home Range Core Areas on the Lassen and Plumas National Forests in northeastern California. We surveyed 1164 point count stations in both 2005 and 2006 across a 96,542 ha study area. Ten of the 25 species were significantly less abundant within owl Home Range Core Areas as were the Shannon Index of Diversity and total bird abundance. Five species were significantly more abundant inside of Core Areas. Where an association with Core Areas was found, the effect was generally consistent among years and across geographic sub-regions. The majority of the species negatively associated with spotted owl Core Areas are tied to early successional and other disturbance dependent habitats, with many experiencing significant population declines in the Sierra Nevada. Forest heterogeneity, including early successional and edge habitats, is an important component of the Sierra Nevada ecosystems and should be managed for to ensure the long-term persistence of biological diversity.
Key words:
Disturbance dependent, indicators, landbirds, Sierra Nevada, spotted owl, umbrella species
1. Introduction
Sustaining biodiversity across large and complex landscapes that are increasingly impacted by human activities is an enormous challenge currently facing land managers. With limited resources available to monitor the myriad species, surrogates such as indicator, focal and umbrella species have been used to inform management decisions (Noss, 1990; Lambeck, 1997; Robergé and Angelstam, 2004; Geupel and Chase 2005). However, there are a number of caveats to employing surrogates that if not addressed, may limit their effectiveness (Landres et al., 1988; Simberloff, 1999; Wiens et al., 2008). An implicit assumption for the use of surrogates is that actions intended to benefit the surrogate will provide for the needs of some pre-defined group of other species or ecosystem elements (Landres et al., 1988; Roberge and Angelstam, 2004; Wiens et al., 2008). Though this appears fundamental to using surrogates to preserve bio-diversity across large and complex ecological systems, little empirical evidence exists examining how other species respond to surrogate guided management, especially umbrella species (Simberloff 1998; Roberge and Angelstam, 2004). An understanding of the species that will benefit or be negatively effected by management for a surrogate can help identify gaps leading to selection of complimentary surrogates and land management approaches that ensure the needs of a greater number of species are addressed.
In the Sierra Nevada Mountains of California, National Forest management is heavily influenced by concern over the California spotted owl (Strix occidentalis occidentalis). The owl has recently been petitioned, but denied listing under the U.S. Endangered Species Act (USFWS, 2006). It is a Forest Service sensitive and management indicator species in the Sierra Nevada, and a California species of special concern (SNFPA, 2001; SNFMIS, 2007; Shuford and Gardali, 2008). Both of the plans currently guiding management of Forest Service land in the Sierra Nevada focus considerable attention on promoting late seral forest conditions including using a system of small reserves to protect areas occupied by the owl (HFQLG, 1999; SNFPA, 2004).
The composition and structure of Sierra Nevada and southern Cascade forests have been altered by timber harvest and fire suppression over the past century. Along with the loss of late seral forest there has been an increase in shade tolerant conifer species and tree densities (Vankat and Majors, 1978; Parsons and DeBenedetti, 1979; McKelvey and Johnston, 1992; Minnich et al., 1995; Taylor, 2000; Beaty and Taylor, 2008), with a concomitant decrease in shade intolerant plant assemblages (Vankat and Majors, 1978; Nagel and Taylor, 2005; Rogers et al., 2007). With a significant reduction in the role of fire, in combination with spotted owl management and other measures intended to protect old forest associated species, the amount of closed canopy habitat is predicted to continue to increase on National Forest land in the Sierra Nevada under the current management plans (HFQLG, 1999; SNFPA, 2001, 2004).
Understanding the composition of the avian community in the context of spotted owl management can provide insight into which species needs may or may not be met under such a management approach. Though the spotted owl is technically a management indicator species for late seral forest, with the considerable resources devoted to its study and its current influence on forest management decisions, we suggest it is in practice being employed as broad reaching umbrella species in the Sierra Nevada. While it has been suggested as a good umbrella for old growth forest, little empirical evidence exists to suggest what species are afforded protection from management for the owl (Franklin 1994; Simberloff, 1998). To evaluate the ability of spotted owl influenced management to provide habitat for other members of the avian community, we compared the abundance of twenty five landbird species and several avian community indices inside and outside of spotted owl Home Range Core Areas on National Forest land across a 96,542 ha area of the Northern Sierra Nevada Mountains of California.
2. Methods
2.1 Study Location
The study occurred within the Plumas-Lassen Administrative Study Area (study area) in the Plumas and Lassen National Forests at the intersection of the Sierra Nevada and Cascade mountains in Plumas County, California, USA (Figure 1). The study area was previously defined and originally established in order to investigate the effects of forest management on various ecosystem components within the boundaries of the Herger-Feinstein Quincy Library Group (HFQLG) Forest Recovery Act Pilot Project (Stine et al. 2002). The study area was divided into study units comprised of three to six adjacent California Interagency Planning Watersheds each of which ranged in area from 1280 – 6324 ha (CalWater, 1999). We investigated the effects of spotted owl management areas within the five western most study units each of which encompassed between 15,074 and 22,341ha for a total area of 96,542 ha. Each of the units lies within the mixed conifer or true-fir zone with survey sites ranging in elevation from 944 to 2140 m and encompassing considerable variation in topography; with slopes ranging from 0 to 65 degrees and a full range of aspects.
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2.2 Defining Spotted Owl Management Areas
Home Range Core Areas (Core Areas) are a Forest Service designation that corresponds to the best available 405 ha (1000 acres) of habitat associated with each known spotted owl nest site or territory (SNFPA 2004). The Core Area as we refer to it is comprised of a 121 ha Protected Activity Center (PAC) and an additional 284 ha surrounding the PAC. Core Areas are afforded protection from major anthropogenic disturbance that would significantly reduce canopy cover or otherwise alter habitat suitability for spotted owl (SNFPA 2001). The Forest Service defines the best available habitat as areas with two or more tree canopy layers, trees in the dominant crown class having an average diameter at breast height over 61 cm (24 inches), canopy cover exceeds 50%, and trees over 7.3 m (24 feet) crown diameter are present (SNFPA 2001). Core Areas are designated after the presence of a nesting or roosting owl is confirmed and they are retained administratively, unless specifically petitioned for removal following major habitat alteration (e.g. stand replacing fire). Thus, not all Core Areas were necessarily occupied by spotted owls during our study, but they had been at some point between 1986 when designation of them began, and when our study concluded. Conversely, since spotted owls have been intensively monitored across each of our five study units since at least 2002 (Blakesely 2001, Stine et al. 2002), there were very few if any breeding owl territories that had not been detected and assigned Core Areas.
In the Herger Feinstein Quincy Library Group Forest Recovery Act (HFQLG) portion of the Northern Sierra, which includes all of our study area, only the 121 ha PAC portion of the Core Area has restrictions on timber harvest. The additional 284 ha portions have been designated, but are not currently under the same restrictions for silvicultural treatments as elsewhere in the Sierra Nevada. Since little, if any, active management had occurred in the 284 ha portions of the Core Areas by the conclusion of this study, we felt they were the best representation of the habitat conditions being protected and promoted for spotted owl management across National Forests in the Sierra Nevada.
In the HFQLG managed area of the northern Sierra Nevada, an older designation known as Spotted Owl Habitat Areas (SOHA) are also recognized. We chose not to include SOHA’s in our analysis so results would be more directly comparable to elsewhere in the Sierra Nevada. Thus, our results are not a direct comparison of the avian community in relation to current areas being managed for spotted owl in the HFQLG project, but we believe the results from this study are directly relevant to the HFQLG area for several reasons. There is a large overlap in the landbase designated as SOHA’s and Core Areas, few Core Areas have been subject to recent timber harvest in our study area, and the HFQLG managed portion of the Sierra Nevada appears to be experiencing trends in tree density and canopy cover similar to the rest of the Sierra Nevada (HFQLG, 1999).
2.3 Site Selection
Three point count transects were established in each planning watershed using a random starting point generated in a GIS environment (ArcView 3.2a). For each random site selected, 11 additional sites were added using a random compass bearing from the random site and spaced at approximately 250 m intervals. If transects could not be established using a random bearing due to inaccessible areas being encountered (e.g., private property, steep topography) we attempted a non-random bearing; if they still could not be established we placed the transect on or adjacent to the secondary road nearest the random site. A total of 1092 sites along 91 transects were established in this manner across the 26 planning watersheds in the study area. We added an additional 72 sites in clusters of two to four surrounding known spotted owl nesting or roosting sites within Core Areas to increase our sample size of spotted owl management areas. These 72 sites were established by randomly selecting a minimum of four Core Areas in each of the five study units with priority given to Core Areas with confirmed recent nesting activity. The total sample size was 1164 sites with 608 outside and 556 inside of Core Areas, each was surveyed twice in both 2005 and 2006.
2.4 Survey Protocol
Standardized five-minute multiple distance bin point count censuses (Ralph et al., 1993, 1995) were used to sample the avian community. All birds detected (visual or auditory) at each count station were recorded within one of six bins based on estimated distance from observer. Each transect was visited twice between May 15 and June 30 in both 2005 and 2006. Counts were completed within four hours of sunrise and did not occur during inclement weather.
2.5 Analysis
Existing digitized polygons of Core Areas provided by the Plumas and Lassen National Forest were used to delineate each point count locations as either inside or outside Core Areas in ARC GIS 9.2 (ESRI, 2006).
Dependent variables included an index of abundance for each of the twenty most abundant species; Shannon Diversity index; total bird abundance (the summed abundance of all individuals), and the summed abundance of all individuals within each of the three primary nesting guilds: tree, shrub, and cavity (Table 1). Since we were also interested in understanding how a spotted owl focused management approach would impact some of the rarer bird species of greatest conservation interest in the Sierra Nevada we also included species that met the following four criteria: sample size sufficient to allow for meaningful statistical analysis (Nur et al. 1999), adequately censused using the point count method, breeds primarily in upland habitat in the study area (non-riparian), and has a significant negative population trend in the Sierra Nevada according to the Breeding Bird Survey (Sauer et al., 2008; Table 1). Using these criteria we included five additional species: pileated woodpecker (Dryocopus pileatus), olive-sided flycatcher (Contopus borealis), western wood-pewee (Contopus sordidulus), chipping sparrow (Spizella passerina), and Cassin’s finch (Carpodacus cassinni).