Long-term impacts of variable retention harvesting on ground-layer plant communities in Pinus resinosa forests.
Concerns about loss of biodiversity and structural complexity in managed forests have recently increased and led to the development of new management strategies focused on restoring or maintaining ecosystem functions while also providing wood outputs. Variable retention harvest (VRH) systems, in which mature overstorey trees are retained in various spatial arrangements across harvested areas, represent one potential approach to this problem. However, long-term evaluations of the effectiveness of this strategy at sustaining plant community composition are needed as this strategy is increasingly applied in managed forest landscapes throughout the world. The forest ground layer plays a central role in forest ecosystem functioning, and we evaluated the long-term (11+ year) dynamics in ground-layer plant communities in response to VRH study in Pinus resinosa Aiton. forests. This large-scale, manipulative study included four overstorey (control, small gap-aggregated, large gap-aggregated and dispersed) and two understorey (ambient and reduced shrubs) treatments replicated four times in 16-ha stands. Changes in ground-layer community composition were apparent 11 years following harvest, regardless of live-tree retention pattern. Richness and diversity increased and were driven by introduction and colonization of ruderal species, while forest interior species continued to persist across treatments. All life-forms responded positively to harvest with the exception of moss and clubmoss species. The lack of effect of spatial pattern of retention on ground-layer plant communities was likely related to the presence of a dense and persistent shrub layer, a result of decades of fire suppression. In particular, the greatest responses to overstorey retention pattern occurred in areas receiving shrub reduction treatments, indicating this recalcitrant layer likely filtered response to retention pattern. Synthesis and applications. Overall, this work highlights flexibility in choosing a variable retention harvest approach when sustaining ground-layer plant community diversity and composition are goals, but altered disturbance regimes (e.g. fire suppression, timber harvesting) that have facilitated the presence or formation of recalcitrant understories, need to be considered. The legacy effects of historical land use and alterations to natural disturbance regimes on the understorey in northern temperate forests are of equal, if not greater, importance to overstorey retention patterns in eliciting desired responses to variable retention harvest and need to be more carefully considered in future applications of this method.