Avian Responses to Residual Tree Retention in Managed Pine Forests of the Lake States Region

Abstract

Ecologically sustainable forest management has been recommended as an alternative to traditional timber harvest practices as a way to balance economic objectives while maintaining composition and structure of wildlife habitats. Retention of live trees after even-aged timber harvest, commonly referred to as green tree retention (GTR), is increasingly recommended as a method to maintain and develop habitat structural complexity within regenerating clearcuts. Although GTR is commonly applied throughout North America, responses of wildlife species and communities to GTR in the Lake States region remains poorly understood. This study sought to determine if avian species and communities responded to differing levels of green tree retention within stands of red pine (Pinus resinosa) and jack pine (Pinus banksiana) in Wisconsin and Minnesota. We compared avian species and communities two years before and two years after retention of varying levels of GTR in 24 managed forest stands in Minnesota and Wisconsin: 12 jack pine stands in Minnesota and 12 red pine stands in Wisconsin. Each stand was ≥ 10 hectares. Stands were monitored using 50-m fixed-radius point counts at three separate visits between May and July of each year before (2008 and 2009) and after harvest (2010 and 2011). Four treatment levels were randomly assigned to stands: reference (100% stand area), high retention (7% stand area), low retention (2.5% stand area), and clearcut (0% stand area). Intermediate treatments consisted of 2-3 patches of overstory trees with diameters reflective of the larger stand. We adjusted raw counts for species and groups using time-of-removal analysis, and tested for differences in species richness, total abundance, diversity, and corrected abundance among treatments over time using a repeated measures analysis of variance. We also examined avian-habitat relationships using stepwise linear regressions of stand and patch-level habitat characteristics to quantify predictors of variation in avian abundance. We found no difference among treatments or years in avian diversity for either stand type. Richness and total abundance increased with retention level, but did not significantly differ between 2.5% and 7% retention. Richness was significantly greater in jack pine reference stands than clearcut stands in 2010, and lower in post-harvest clearcut and 2.5% retention stands than pre-harvest stands in both red pine and jack pine forest types. Total abundance was significantly greater in jack pine reference stands than all harvested treatments in 2010, and lower in post-harvest clearcut (2010 and 2011) and 2.5% (2011) stands in both forest types. Four of 10 species groups and 9 of 22 species exhibited significant interactions among treatment or years. However, no species or group responded to levels of retention. Instead, responses were most often related to extremes (harvested vs. unharvested stands). Overall, individual species and groups were influenced by variation in stand and patch characteristics not explicitly tied to level of aggregate retention. For example, abundances of long-distance migrants and early successional species were often associated with variables resulting from harvest and subsequent succession (e.g., shrub stem count and percentage of woody debris). However, many individual species and groups were associated with stand and patch-level variables tied to retention level. For instance, ovenbirds (Seiurus aurocapilla) and hermit thrushes (Catharus guttatus) were positively associated with greater basal area and patches per stand, respectively, reflecting these species’ association with mature forests. We suggest that levels and quality of retention evaluated were not effective for maintaining forest species in recently harvested red pine or jack pine stands, and recommend that managers consider higher levels and more complex features of retention to support avian biodiversity.