Regeneration patterns of key pine species in a mixed-pine forest indicate a positive effect of variable retention harvesting and an increase in recruitment with time

Abstract

Abstract Background Many fire-dependent forests have experienced significant declines in species, structural, and functional diversity. These changes are attributed in part to traditional management approaches that were dominated by even-aged regeneration methods such as clearcutting. Variable retention harvesting (VRH) is an ecologically based forestry practice that involves retention of some mature overstory trees and other biological structures in the postharvest stand to emulate the effects of natural disturbance events. In this study, we examined the effect of a VRH treatment on recruitment of historically dominant pine species and understory vegetation two- and six years after its implementation in a naturally regenerated mixed pine forest in the Upper Peninsula of Michigan, USA. Results We found greater regeneration of red pine (Pinus resinosa L.) and eastern white pine (Pinus strobus L.) in VRH stands compared to the unharvested controls. Although red pine recruitment was very low during the first 2 years, both species increased significantly from year two to year six. Recruitment responses for the two species did not differ significantly between the aggregated and dispersed retention treatments. We also found high recruitment of deciduous tree species, consisting primarily of sprouts, and red pine exhibited a negative correlation with these sprouts. Treatment, time, and the interaction of treatment and time were significant factors in red pine recruitment. Time had a strong effect on the understory vegetation, with significant decreases in shrubs and ferns over time. Conclusions The results suggest that VRH enhanced recruitment of red pine and eastern white pine and that this recruitment increased with time. We identify competition from sprouts and unfavorable seedbed conditions as the main factors that limited recruitment of red pine during the first 2 years. To reduce competing vegetation and improve seedbed conditions, we suggest following the VRH with a prescribed fire, mechanically removing sprouts, and implementing herbicide treatments. At a time when changes in global climate exacerbates the effects of traditional drivers of forest degradation, and with the need to maintain biodiversity, new ecologically based forest practices such VRH have the potential to facilitate regeneration of native trees and enhance the resilience of many fire-dependent forests.