An Assessment of Habitat Suitability in Old Growth and Secondary Growth Retention Area’s on Haida Gwaii

Abstract

In British Columbia, tree retention in harvested areas has become a standard forest management practice, however the ecological benefits that result from tree retention remain uncertain. Tree retention in the harvested landscape is thought to have the potential to ameliorate negative ecological effects of logging as it can provide refugia for elements of biological diversity, structural enrichment by retaining forest features, and enhance connectivity in the landscape between forested areas. Habitat quality in retention areas is important for avian species as it can influence suitable nest site structural features, predation levels, prey availability, competition rates, and nest-area microclimate. In the following study, old growth and second growth tree retention areas were analyzed on Haida Gwaii in order to examine structural habitat features. The structural features present in these retention types were then related to the published habitat characteristics of three old growth forest dependent avian species at risk on Haida Gwaii, in order to determine if suitable habitat is available in these retention areas. The three species analysed were the northern saw-whet brooksi owl (Aegolius acadicus brooksi), the marbled murrelet (Brachyramphus marmoratus), and the northern laingi goshawk (Accipiter gentilis /aingi). A total of 10 old growth and 11 second growth retention areas were surveyed in the Skidegate Plateau physiographic region of Haida Gwaii that were adjacent to cut blocks harvested within 3 years of 2015. The second growth retention areas surveyed in this study resulted from a harvest 50-65 years previously and most were mechanically thinned when 15-25 years old. Variable radius plots (prism sweeps) and snag and downed woody debris (DWD) transects were carried out in order examine structural features in retention stands. Vertical distribution of foliage was measured by estimating foliage percent cover in 6 height classes, while vertical canopy complexity was measured by estimating height differences between dominant and co-dominant canopy layers. Canopy gaps, salal (Gaultheria shallon) and Vaccinium (V. parvifolium, V. a/askaense, V. ovalifo/ium) abundances were estimated and dwarf mistletoe (Arceuthobium tsugense) and mossy platforms (> 18.5 cm dbh) were counted from plot centres. Old growth tree retention had significantly more diverse forest structure characteristics than second growth tree retention areas, however second growth tree retention areas contained some diverse features that most likely resulted from mechanical thinning. Results also indicated that old growth retention areas were significantly more suitable for all three avian species at risk with only a small proportion of the analyzed avian habitat characteristics present in second growth retention areas. Both retention types had edges exposed to harvested cut blocks, putting them at potential risk for forest edge related impacts, such as avian nest site predation and competition. My results indicate that second growth retention areas on Haida Gwaii lacked the structural complexity of old growth forests at age 50-65 years old. Strategies for improving habitats for old growth dependent avian species in such retention patches include: increased harvest rotation ages, strategic allocation in areas with large diameter trees and snags, and girdling or topping of suitable cavity nesting trees.