Short- and Long-Term Results of Alternative Silviculture in Peatland Black Spruce in Minnesota, USA

Across the boreal forest in North America, the black spruce (Picea mariana) cover type is ecologically and economically important, occupying roughly 10 percent of Minnesota’s, USA 17.4 million acres (7.0 million hectares) of forestland. Traditionally managed through clearcut regeneration harvests, alternative silvicultural systems are being increasingly used in Canada. Here, we examine the 10- and 57-year effects of six silvicultural treatments (clearcut strips, clearcut patches, thinning, group selection, single-tree selection, shelterwood) on stand structure and dynamics in lowland black spruce. Treatments were installed in 1948 in northern Minnesota, and remeasured and re-treated 10 years later. A subset of the clearcut strips, clearcut patches, and shelterwood treatments were remeasured in 2017. After 10 years, diameter growth of residual stems varied by treatment, with the shelterwood experiencing the greatest growth, and basal area increased in all but the shelterwood treatment. Over the long term, the shelterwood exhibited larger diameters and heights and greater crown ratios, basal area, structural complexity, and compositional diversity than the clearcuts. Our results suggest that managers may consider using a shelterwood instead of traditional large clearcuts to achieve increased structural and compositional diversity, particularly when eastern spruce dwarf mistletoe (Arceuthobium pusillum) does not necessitate a traditional clearcut. Study Implications: This research suggests that alternatives to traditional clearcutting of black spruce stands in the United States Lake States have merit and may be used by managers to achieve stands that supply additional options in the future. In stands of older ages where advance regeneration is already present, a shelterwood treatment provides a means to achieving sufficient regeneration without the use of aerial seeding while also resulting in future stands that have larger-diameter trees, greater basal area, increased structural complexity and compositional diversity, and potentially a shortened rotation age. Additionally, a shelterwood increases diameter growth on residuals to make overstory removal more economically feasible. Consideration should be given to the amount of residual growing space to reduce mortality because of wind events over the short-term, possibly by aggregating residuals.