Date of Award

2016

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy in Forest Science (PhD)

Administrative Home Department

School of Forest Resources and Environmental Science

Advisor 1

Thomas G. Pypker

Committee Member 1

Randall K. Kolka

Committee Member 2

Andrew J. Burton

Committee Member 3

Rodney A. Chimner

Committee Member 4

Bradley H. Baltensperger

Abstract

Throughout many North American forests, the invasive emerald ash borer (EAB, Agrilus planipennis Fairmaire (Coleoptera: Buprestidae)) is a significant threat to biodiversity and ecosystem processes. The fate of Fraxinus nigra Marshall (black ash) is of particular concern, as it is frequently a dominant canopy species across much of its range. A multiyear manipulative study was conducted to investigate the potential impacts of EAB on vegetation dynamics and nitrogen (N) cycling in nine F. nigra-dominated wetlands of Upper Michigan, USA. The effects of early- and late-stage infestations were simulated by girdling (‘Girdle’) or felling (‘Ash-Cut’) all ash stems greater than 2.5 cm in diameter.

Vegetation responses were limited to lower canopy layers and herbaceous species. No change in growth rate of residual overstory species was observed, while sapling growth rate of non-F. nigra species increased in the ‘Girdle’ treatment. Species composition of the seedling layer is shifting to co-dominant species, as densities of Acer rubrum L. (red maple) and Betula alleghaniensis Britton (yellow birch) increased following treatment. The strongest response was exhibited in the herbaceous community, the areal cover of which nearly doubled by the third post-treatment growing season. Though this expansion did not appear to as yet inhibit regeneration of woody species, the potential to impact future forest recovery remains a concern.

Despite reduced demand for nitrogen by the dominant overstory species, soil N availability did not increase following treatment, though an increasing trend in soil NO3- was observed in the ‘Girdle’ treatment. These responses are potentially due to increased microbial activity and biomass, leading to increased N immobilization, which may influenced growth rate responses in retained species. Biochemical and morphological changes observed in overstory foliage were thus driven predominantly by aboveground conditions, as leaves acclimated to shade were replaced with sun-acclimated leaves. Succession to A. rubrum or B. alleghaniensis, the litter from which has lower N content than F. nigra, may lead to slower but more complete litter decomposition, potentially affecting future soil carbon storage and nutrient cycling rates.

All examined forest canopies were net sinks for atmospheric N deposition. However, this sink strength was reduced by the ‘Girdle’ and ‘Ash-Cut’ treatments and led to increased deposition of NO3- to the forest floor in the ‘Ash-Cut’ treatment. Although these effects were relatively minor in these forests, the observed response has important implications in regions with greater N deposition. In forests of these regions, the reduction in retention following the loss of Fraxinus species may exacerbate the impacts of N pollution.

These results highlight the interconnected nature of ecosystem processes and suggest potential consequences of EAB invasion in this forest type. In the near term, F. nigra mortality may result in increased export of NO3- from the ecosystem, which may be amplified by reducing the canopy sink strength for atmospherically deposited N. In the long term, the recovery of woody vegetation is uncertain, due to competition with herbaceous species. Should recovery of a closed forest canopy occur, the foliar and litter characteristics of the replacement species may reduce carbon storage and site fertility, through feedback mechanisms between the canopy and forest floor. Understanding these potential responses is critical to developing mitigation strategies to reduce the impacts of EAB on ecosystem processes and the provisioning of ecosystem services.

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