Restoration of woody plant communities in red pine plantations on abandoned agricultural land in northern Minnesota, USA

Abstract

While tree plantations have increased globally, there is growing interest in restoring plantations to native ecosystems. Restoration efforts may be more successful when tree species native to original ecosystems are planted instead of species that are either introduced or native, but off site.Plantations of red pine (Pinus resinosa Ait.) in the western Great Lakes region of North America were commonly established on red pine sites, often after agricultural clearing and abandonment. Red pine is the only species in the overstory, compared to 10 or more species in red pine woodlands. Plantations differ in structure from woodlands, being single-aged versus multi-cohort and having high and spatially-uniform density versus lower and variable density.Our aim was to contrast changes in structure and woody species composition over a 15-year period in response to two silvicultural approaches in red pine plantations on the Red Lake Wildlife Management Area in northwestern Minnesota, USA. Approaches included: 1) uniform thinning followed by a dispersed retention harvest (dispersed treatment) and 2) variable density thinning followed by an expanding gap irregular shelterwood (aggregated treatment).Basal area of trees ≥10 cm dbh was reduced similarly in both treatments to 9 m2 ha−1, but there was high within-stand variability (coefficient of variation) in the aggregated treatment (>100%) compared to the dispersed treatment (20%). Understory woody species abundance increased with both treatments, including sapling, large regeneration, and small regeneration layers, with >100% increases. In the sapling layer, the increased abundance (to 2.5 m2 ha−1) reflected new recruitment into this layer, as there were few saplings before treatment (<0.5 m2 ha−1). By year 15, the majority of saplings were canopy capable species, while lower regeneration layers contained mixtures of canopy trees, sub-canopy trees, and shrubs.Composition in the understory changed, but was similar between treatments, including increases in woody shrubs in the small regeneration layer and increases in red pine regeneration in the large regeneration layer. The sapling layer had increased importance of several native tree species, especially in the aggregated treatment.Both of our silvicultural approaches moved structurally simple, species poor red pine plantations toward a condition reflecting native red pine woodlands, which had mixed-species composition and variably open canopy cover. The treatment using variable density thinning and an irregular shelterwood resulted in greater spatial variability of structure and greater importance of shade-intolerant tree species, both characteristics of native woodlands. While focused on red pine, this approach should have application to conifer restoration more generally.