Date of Award

2020

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy in Forest Science (PhD)

Administrative Home Department

College of Forest Resources and Environmental Science

Advisor 1

Andrew Burton

Committee Member 1

Yvette Dickinson

Committee Member 2

Curtis Edson

Committee Member 3

Rupali Datta

Abstract

The regeneration and reorganization of organisms and their habitat post-disturbance is a primary mechanism by which ecosystems maintain functionality in a changing environment. This adaptation to disturbance lends dynamism and resiliency to landscapes. Yet, the 20th century was marked by an alarming loss of global biodiversity, pointing to the likelihood that the rate of ecological disturbance generated by human systems today is either too frequent or too intense to be beneficial to ecosystems, and rather poses a risk to ecosystem functionality and the carrying capacity of Earth systems. Compounding these risks is high uncertainty regarding the potential for stronger, more frequent occurrences of natural disturbances as the planet's atmosphere warms. Observing and documenting forest responses to natural disturbances provides opportunity to adapt conventional natural resources management to ecosystem-based management.

The objective of this study was to document forest response to a severe thunderstorm disturbance in mixed northern hardwoods, and to assess the effects of single tree selection harvest on gap opening and community succession in maple (Acer saccharum) dominated stands. For this research, I measured forest canopy response to intermediate storm disturbance using aerial imagery and compared gap morphology of northern hardwood forests across a spectrum of prior human disturbance (primary old growth, unmanaged second growth, and working second growth timber stands). This research provides evidence that single tree selection harvest in second growth forests creates more open canopy than in old growth forest, but gaps are smaller, more frequently distributed, and close rapidly. Severe thunderstorm disturbance did not increase mean gap size in stands with recent single tree selection harvest, indicating this harvest method effectively replaces storm damage as the primary disturbance mechanism the forest depends on for gap regeneration. I surveyed storm damage and seedling regeneration in 14 maple-basswood storm gaps and found basswood (Tilia americana) to be especially vulnerable to tree fall, robust sugar maple regeneration at all sites, and the potential origination for a yellow birch (Betula alleghaniensis) grove where single tree selection and natural wind disturbance coincided. I monitored microsite climate and coarse wood mass and volume in a subset of gaps and forest interior locations and found evidence that Acer seedlings outcompeted other species across a wide range of understory light availability. Thunderstorm disturbance resulted in the deposition of an immense volume of coarse woody debris in gap habitats. Finally, I used stable isotopes to quantify the retention of carbon (C) in coarse woody debris and the proportion transferred to soil after a hypothetical treefall disturbance. My research indicates up to 10% of log C transferred to soil after six years decomposition, but C pathways were highly variable across sites.

Findings detailed in the following chapters support the conclusion that single tree selection management of second growth maple-basswood stands is effective in promoting robust Acer seedling regeneration in the western Upper Peninsula. Single tree selection was also found to moderately improve species richness among seedlings compared to unmanaged second growth, likely due to greater canopy richness. However, seedling species evenness was poor across second growth stands regardless of management or storm disturbance. These results emphasize the importance of adapting ecosystem-based management practices within single tree selection methods. Maximizing the retention of diverse canopy tree species as seed trees can facilitate more diverse regeneration in mixed northern hardwood timber stands. Cutting larger, less frequent gaps would better mimic canopy-gap mosaics of old growth forest, and extend the time period of elevated light availability to foster in-gap regrowth and recruitment. In storm gaps, as the fallen trees decompose new substrates will be available for seed germination and may improve community recruitment over time. In unmanaged second-growth hardwood stands with low canopy diversity, planting may be necessary to improve community diversity, and storm gaps provide light-rich environments where such attempts may be most successful.

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