Structural differences between precommercially thinned and unthinned conifer stands

Abstract

Effects of precommercial thinning (PCT) in young, high-density forest stands on the growth and yield of crop trees has been well-studied, but information about the response of habitat characteristics and structural attributes that are related to abundance and diversity of wildlife populations is deficient. We examined changes in habitat characteristics and forest structure that occurred with PCT and stand development in commercial spruce-fir stands within the Acadian forest of northern Maine. We selected 30 regenerating, herbicide-treated conifer stands (17 treated with PCT) of three development classes (1, 6, or 11 years post-PCT) and measured 27 variables that described the characteristics and structure of the understory, overstory, woody debris, or ground cover within these stands. The application of PCT accelerated some characteristics of stand development, resulting in a reduction of understory structure and complexity, which conflicts with previous studies of non-herbicide-treated forest that reported increases in understory complexity after thinning. Near-ground cover, overstory cover, and understory structure described >80% of the variation in vegetation structure between thinned and unthinned stands. Horizontal cover, an overstory to understory contrast, and a gradient of herbaceous vegetation described >75% of the variation that occurred as regenerating stands (thinned and unthinned) developed through time. The forest structure in regenerating stands treated with PCT may have negative effects on wildlife that are dependent on the structure of early successional forest, but may positively affect species that use more mature forest.