Effects of variable retention harvesting on natural tree regeneration in Pinus resinosa (red pine) forests

Concerns over loss of ecosystem function and biodiversity in managed forests have led to the development of silvicultural approaches that meet ecological goals as well as sustain timber production. Variable Retention Harvest (VRH) practices, which maintain mature overstory trees across harvested areas, have been suggested as an approach to balance these objectives; however, long-term evaluations of outcomes of VRH strategies do not exist for most forest types. The objective of this study was to determine the 11-year effects of overstory tree retention pattern and shrub removal on regeneration in P. resinosa forests in Minnesota, USA using a large-scale manipulative study in which four overstory (control, small gap-aggregated, large gap-aggregated, and dispersed) and two shrub (ambient and reduced shrubs) treatments were applied. Hardwood regeneration greatly outnumbered conifer regeneration and several mechanisms (disease, browse, and seedbed conditions) likely interacted to limit P. resinosa regeneration across treatments. The presence of recalcitrant shrub layers filtered response to retention with regeneration of P. strobus L. being greater under an intact Corylus layer irrespective of overstory conditions. This work reinforced the importance of accounting for shrub competition when designing VRH to secure natural regeneration.