A Trial of fire and ice : assessing the ability of invasive tree pyrus calleryana to resist disturbance during grassland invasion in the American midwest

Date of Award


Degree Name

M.S. in Biology


Department of Biology


Ryan McEwan


Ecological invasion is one of the most important processes of global change influencing ecosystems in practically every biome on Earth. Invasive species shift ecosystem dynamics, community structures, nutrient cycling, and ecosystem function. Invasive species pose a significant challenge to land managers who are charged with maintaining biodiversity and managing long-term ecosystem structure. Pyrus calleryana is an ornamental tree species that escaped cultivation and grows rapidly in highly disturbed soils with high light intensities. Former agricultural fields are highly susceptible to invasion from Pyrus calleryana. Observational evidence suggests that two features of this species' biology may be particularly important to invasion success: (a) an extended leaf phenology and (b) the ability to persist in grasslands following disturbance via epicormic sprouting. While this tree is one of the most problematic invasive species within the Eastern and Central parts of the United States, it is relatively understudied. This thesis is divided into two distinct studies that address the invasion biology of Pyrus calleryana. The first study, detailed in Chapter One, is an analysis of the timing of leaf development (phenology) in relationship to co-occurring native trees. In the second study, detailed in Chapter Two, I conducted a replicated field experiment to assess P. calleryana sprouting as a mechanism of persistence in response to experimental disturbance treatments including prescribed fire and experimental freezing using a treatment of liquid nitrogen. The experiments we conducted in this project are in cooperation with the conservation staff at the Five Rivers Metroparks and all activities took place on their properties. In the first study, I discovered that P. calleryana has a longer leaf duration than native trees through earlier leaf out, and delayed abscission. In addition, a late season frost event allowed for a serendipitous study that revealed much greater frost tolerance in P. calleryana than native trees. Extended phenology and frost tolerance indicate strong potential for growing season carbon acquisition for P. calleryana compared to native trees. In the field experiment (Chapter Two), I discovered that common practices such as mowing and prescribed fire may be facilitating the invasion of Pyrus calleryana due to its aggressive sprout response. Data indicate that experimental freezing was marginally more useful than cutting or herbicide in sprout control; however, herbicide application was the only reliable method for preventing resprouting. The results of these studies illuminate the invasion biology and may help inform more effective management of this problematic invasive species across the region, leading to more sustainable management of biodiverse habitats.


Environmental Science, callery pear, pyrus calleryana, invasion, resprouting, epicormic sprouts, restoration, phenology, fire, ice

Rights Statement

Copyright © 2021, author.