Effects of Commercial Thinning on Breeding Bird Populations of Western Hemlock Forests

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ABSTRACT.-Bird populations and habitat structure were compared between three commercially thinned and three unthinned western hemlock (Tsuga heterophylla) stands to assess short-term effects of commercial thinning on breeding bird communities. Thinning reduced the density of small trees and snags (<=30 cm dbh), but did not affect the density of large trees or snags (>30 cm dbh). The overstory canopy was more open and cover of forbs, grasses and seedlings was higher in thinned than unthinned stands. Winter wrens (Troglodytes troglodytes), dark-eyed juncos (Junco hyemalis), chestnut-backed chickadees (Parus rufescens) and red-breasted nuthatches (Sitta canadensis) were more abundant in thinned than unthinned stands, but total bird density did not differ between thinned and unthinned stands. Commercial thinning thus enhances habitat conditions for some bird species while having minimal effects on other bird species.

INTRODUCTION

Commercial thinning is a forest management practice increasingly being applied to dense stands of timber in the Pacific Northwest (McNeel and Dodd, 1996). Commercial thinning is used to regulate densities of growing trees and provide an early financial return from managed stands. Usually, smaller trees are removed during thinning, permitting the site's growth potential to be transferred to larger, more vigorous trees and often resulting in improved timber quality for future harvests (Smith, 1986). In addition to improving timber quality, commercial thinning has been recommended as a management practice for enhancing wildlife habitat (Hall et al., 1985). Thinning promotes growth of understory vegetation (McConnell and Smith, 1970; Thomas et al., 1999), thereby increasing forage and cover for deer, elk, small mammals and ground-nesting birds (Larson et al., 1986; Hayes et al., 1997). Thinning also may accelerate development of old-growth forest characteristics such as large trees and branches, thus improving habitat for wildlife species dependent on these characteristics (Hall et al., 1985). However, thinning may have negative effects on wildlife; routine removal of snags during thinning for logistical and safety reasons, for example, may reduce habitat suitability for cavity-nesting birds (Zarnowitz and Manuwal, 1985; Hayes et al., 1997).

Despite the potential for thinning to be used to meet management objectives such as habitat enhancement, information on wildlife response to commercial thinning is limited. The objective of my research was to assess the short-term effects of commercial thinning on breeding bird populations in western hemlock (Tsuga heterophylla) forests. I compared habitat characteristics and bird populations between commercially thinned and unthinned stands and identified habitat features associated with each.

METHODS

Six study sites were located in the Kapowsin Tree Farm, Pierce County, Washington. This tree farm encompasses over 40,000 ha of second-growth forest, consisting of a mosaic of stands of different age classes and silvicultural treatments. Three sites were in commercially thinned stands, whereas the other three sites were in unthinned stands (Fig. 1). The study sites were selected based on similarities in vegetation, site quality, slope, aspect and elevation. The study sites ranged from 7 to 9 ha, with study site boundaries located at least 25 in from logging roads to minimize edge effects. The size of the study sites was limited by the size of the stands within the tree farm and by the presence of logging roads subdividing the stands (see Fig. 1). Forest cover on the study sites was dominated by 45 to 55 y old western hemlock. This forest composition was the result of natural regeneration after clearcutting in the 1930s. Commercial thinning treatments had been applied 3 to 5 y before data collection. The thinning prescription specified removal of trees <=30 cm dbh (diam at breast height, 1.4 m aboveground).

I measured habitat characteristics in 15 circular plots (0. …