Concerns about loss of biodiversity and ecosystem complexity in managed forests have recently increased and prompted the need for new management strategies that restore or maintain ecosystem functions and allow for wood production. Variable Retention Harvest (VRH) systems, in which mature overstory trees are retained in various spatial arrangements across harvested areas, represent one potential approach to this problem. In the Great Lakes Region, VRH has been suggested as a management approach for Pinus resinosa (red pine) forests given this may more closely mimic historical disturbance regimes that resulted in mixed-species, multi-cohort forests; however, long-term evaluations of the effectiveness of this strategy at sustaining and restoring plant community complexity and diverse tree species do not exist. The objective of this thesis was to determine the long-term (10+ year) effects of overstory tree retention pattern and shrub competition on ground-layer community composition and tree regeneration in P. resinosa forests in Minnesota, USA. Long-term data from a large-scale manipulative study in which four overstory (control, small gap-aggregated, large gap-aggregated and dispersed) and two understory (ambient and reduced shrubs) treatments were replicated four times in 16 ha stands were used to address this objective. Changes in herbaceous community composition were apparent 11 years following harvest and increases in richness and diversity were driven by introduction and colonization of early successional species, while forest interior species continued to persist across treatments. Harvest resulted in immediate decreased cover by native forbs, but this result was not apparent in later sampling periods. All life forms responded positively to harvest with the exception of moss and clubmoss spp, which were more common in the control by the last sampling year. Retention harvests were successful at reintroducing hardwood species to the establishing cohort regardless of the spatial pattern of retention, and hardwood densities greatly outnumbered conifer regeneration in both regeneration size classes. Several mechanisms (disease, browse, and poor seedbed conditions) interacted to limit regeneration of P. resinosa. P. strobus densities were greater under an intact Corylus layer as well as in the large gap-aggregated treatment 11 years after harvest. In the case of both the herbaceous layer and natural tree regeneration, the presence of a dense and persistent shrub layer, likely a result of fire suppression, filtered the response to retention pattern. Overall, this work highlights the flexibility of VRH in attaining diversity goals in ground-layer plants but also reinforced the importance of understory competition control and seedbed preparation for ensuring natural regeneration of the dominant species in these systems.
University of Minnesota M.S. thesis. August 2015. Major: Natural Resources Science and Management. Advisors: Anthony D'Amato, Christel Kern. 1 computer file (PDF); viii, 87 pages.
Effects of variable retention harvesting on ground-layer plant communities and natural regeneration in Pinus resinosa (red pine) forests in northern Minnesota, USA.
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