Understory dynamics in response to forest management

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Alexa S. Wagner, Biology, Case Western Reserve University, Cleveland, OH and Katharine Stuble, Holden Arboretum, Kirtland, OH

Background/Question/Methods
Forest management can be used to accelerate the trajectories of secondary growth forests from low diversity, low productivity, and highly invaded stands toward mixed-aged stands harboring diverse native species. Both overstory thinning and invasive species removal are important component of understory management. Understanding the mechanisms driving demographic shifts in plant communities as a result of forest management can help us determine forest trajectories. We explored the impacts of forest management on understory dynamics in a post-agriculture forest comprised of mixed mesophilic hardwoods. Specifically, we treated forested plots with one of three management treatments: 1) overstory thinning (girdling of approximately 20% of trees to open the canopy) 2) overstory thinning coupled with invasive shrub removal, or 3) unmanaged control. Three 1-hectare plots of each treatment were established. We monitored herbivory, growth, leaf number, and recruitment of all native and invasive tree/shrub species across 116 randomly placed meter-squared plots while accounting for changes in light availability, soil conditions, and understory composition. Additionally, we quantified fruit production and dispersal potential of two understory shrubs in individuals occurring within treatment plots.
Results/Conclusions
Two-years post-management we did not find evidence that treatment impacted growth in terms of seedling height, but did influence total leaf number (F26,569=9.79, p<.001). Canopy openness decreased insect herbivory (F1,573=4.30, p=.04). Preliminary comparisons between a native and invasive shrub show inverse trends in fruit production in response to canopy thinning, although more time and replicates are needed to confirm. Specifically, the native Lindera benzoin increases fruit production when in close proximity to tree thinning, however, the invasive Frangula alnus has less fruit when present in more heavily thinned areas. Management also impacted fruit predation (F2,91=10.0, p<.001), with predation rates declining in forest areas managed by overstory thinning in combination with removal of invasive shrubs (p=.02). However, fruit predation increased with overstory thinning alone (p<.001). We suspect forest management will be an important driver of dynamics within the understory, differentially impacting species across demographic stages and setting up forest dynamics that can drive trajectories for plant communities as they mature.