Retention of canopy trees as biological legacies for balancing woody biomass production and biodiversity in managed aspen forests of the Great Lakes Region

Abstract

With increasing interest in using forests as a source for bioenergy feedstock, aspen (Populus spp.) forests may play an important role in meeting this new demand. To balance forest biomass harvesting with the habitat requirements of wildlife species, green-tree retention under the conceptual framework of ecological forestry provides potential for meeting both needs. We examined the effects of retained trees as biological legacies (“legacy trees”) on above-ground live woody biomass and bird assemblage species richness, diversity, abundance, and composition in 27 aspen-dominated forest stands in northern Wisconsin. We selected nine stands in each of two legacy tree retention treatments (conifer retention and hardwood retention) and clearcuts (no retention) across a chronosequence (4-36 years post-harvest). Hardwood retention stands and clearcuts had similar above-ground live woody biomass for regenerating trees, legacy trees, and all trees combined with minimal suppression of regenerating trees. Conifer retention stands had greater legacy tree and all tree biomass but lower regenerating tree biomass than clearcuts. Coniferous but not hardwood legacy trees appeared to suppress regenerating tree biomass. Bird species richness was greatest in the hardwood retention treatment particularly early in stand development likely reflecting the important compositional and structural role of the large-diameter northern red oak (Quercus rubra) trees in this system. Six conservation priority bird species (Blackthroated Green Warbler, Setophaga virens; Brown Thrasher, Toxostoma rufum;Common Yellowthroat, Geothylpis trichas; Golden-winged Warbler, Vermivora chrysoptera; Mourning Warbler, Geothylpis philadelphia; White-throated Sparrow, Zonotrichia albicollis) were indicators of legacy tree retention or clearcuts with at least one species in each treatment suggesting that a mixture of these management strategies across the landscape may be necessary to aid declining species collectively. Legacy tree retention increased bird species richness with benefits to a group of conservation priority species, increased aspen forest biomass until 30 years post-harvest, and potentially maintained greater carbon stocks in young stands which could dampen carbon ecosystem fluxes