Wildlife Research and Management - Wildlife Research
Date Published: January 2004
Number of Pages: 36
Author(s): Matthew Vander Haegen, Susan Van Leuven and David Anderson
The western grey squirrel (Sciurus griseus) inhabits oak/conifer forests in California, Oregon, and Washington. In Washington, the western grey squirrel currently exists in only three locations (Puget Sound, Chelan and Okanogan Counties, and Klickitat County), its range severely reduced from historical times by loss of suitable habitat. This reduction in habitat combined with an uncertain future for the extant populations prompted the Washington Department of Fish and Wildlife to list the species as state-threatened in 1993.
Harvest of timber within the occupied range of the western grey squirrel has the potential to degrade habitat by removing mast-producing trees, destroying nests and potential nest sites, and decreasing the interconnected tree canopy that squirrels use to travel safely through their territories. Beginning in the mid-1980s, western gray squirrel habitat in south-central Washington has been logged at an accelerated rate due to a strong timber market and attempts to salvage beetle and drought-killed ponderosa pine (Pinus ponderosa). To address this threat, in 1996 the Washington Forest Practices Board established guidelines for commercial harvest within areas occupied by western grey squirrels. These guidelines were designed to protect existing nest trees and provide for retention of mast-producing trees and corridors to water sources within sites used by squirrels.
In spring of 1999 we began revisiting sites that had been harvested under approved forest practice applications for the purpose of documenting post-harvest nesting activity by western gray squirrels. Our objective was to address two questions of direct relevance to current nest protection guidelines: 1) does timber harvest affect nesting activity of western gray squirrels?, and 2) are operators complying with the current voluntary guidelines? Our approach was to resurvey sites that had been surveyed in prior years and document change in number of active nests. We resurveyed 10 sites that had been surveyed for western gray squirrels and subsequently harvested for timber, and 10 sites that had been surveyed but not harvested. All nest trees on post-harvest sites were evaluated for quality of protection according to nest protection guidelines. All sites were located within the Klickitat River drainage in south central Washington.
Examination of nests marked during pre-harvest surveys revealed that operators frequently were not complying with nest protection guidelines specified in individual forest practice permits. In some cases the violations appeared to represent disregard for the nest protection measures (e.g., removal of large pine trees in close proximity to nests), whereas in others the violations were less obvious. For example, fair or poor ratings for many of the nest trees on one site resulted from understory thinning of young trees within the 50-ft buffer. Situations such as this may have resulted from a misunderstanding on the part of the operator rather than a disregard for the guidelines. Regardless of cause, there obviously is much room for improvement in implementing current nest protection guidelines.
We found considerable change over time in the number of western gray squirrel nests on some sites, revealing the dynamic nature of nesting activity, and by association squirrel populations, on the landscape. The number of active nests changed substantially on some sites, but changes were not consistent in direction, either on the harvest or control sites. Resurveys were at least as rigorous as original surveys, so decreases in nesting activity noted in this study likely reflect real changes. All sites except one had active nests during the resurvey, indicating that western gray squirrels continued to use most sites at some level. No active nests were found during the resurvey on one harvested site (Squirrel #4), indicating possible extirpation of that population.
At the level of individual nest trees, our data from marked nest trees suggest that timber harvest had a negative effect on their continued use by gray squirrels. Specifically, nest trees that were provided poor protection were less likely to have active nests than those provided good protection. This suggests that current nest protection guidelines, when followed, are working to maintain at least some level of suitability of existing nest trees. Multivariate models of nest activity indicated that excluding harvest activity from within 50 ft of the nest tree might be the most important component of the existing nest-protection guidelines.
This study has enabled an initial examination of conditions on sites harvested under western gray squirrel protection guidelines and the findings should be considered preliminary. This was largely an observational study, as we lacked experimental control, either over the placement of stands or when stands were harvested. Future research should focus on a controlled study measuring the demography of the populations on each site and how it changes as a function of harvest patterns, with detailed measurements of annual survival and productivity, as well as immigration and dispersal.
Vander Haegen, M., S. Van Leuven, and D. Anderson. 2004. Surveys for western gray squirrel nests on sites harvested under approved forest practice guidelines: analysis of nest use and operator compliance. Wildlife Research Report. Washington Department of Fish and Wildlife, Olympia, Washington, USA.