Date of Completion

8-17-2016

Embargo Period

2-13-2017

Keywords

Invasive species, phenology, community ecology

Major Advisor

John C. Volin

Associate Advisor

Eric L. Kruger

Associate Advisor

Michael R. Willig

Associate Advisor

Robin L. Chazdon

Associate Advisor

Thomas E. Worthley

Field of Study

Natural Resources: Land, Water, and Air

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

Mechanisms underlying the ability of invasive exotic plant species (IES) to establish outside of their native ranges and outcompete native species (NS) are not yet fully understood, especially in low-resource environments. In deciduous forest understories, where light availability is low, but seasonally variable, one potential contributor to success of IES is extended leaf phenology, which may result from earlier leaf flush or later leaf abscission compared to that of NS. If either is the case, IES would tend to exploit a broader temporal niche for assimilating resources, which is particularly beneficial if it results in photosynthetic activity when the canopy is leafless (i.e., when light availability is high and competition low). Therefore, the role of phenology with respect to the hypothetical growth advantage of IES is twofold; extended leaf phenology may allow IES to take advantage of a vacant niche and canopy phenology may mediate the amount of resources available during that niche. Despite growing efforts to understand the mechanistic basis of IES success in low-light environments, relatively little is known regarding influences of overstory phenology on understory IES and NS carbon gain and physiological responses. The goal of my research was to explore the role of community-level phenological interactions in the success of IES, focusing on mechanistic causes for their hypothesized growth advantage over NS in deciduous forest understories in Connecticut.

Strong relationships characterized seasonal growth and traits associated with overstory and understory phenology, including a positive linkage with the extension of seedling leaf phenology beyond canopy leaf lifespan. Compared to NS, IES generally exhibited greater phenological extension and achieved greater seasonal growth. This advantage was most pronounced under canopies that allowed greater cumulative understory light across the growing season. After accounting for phenology, growth differences between IES and NS were explained by a number of intrinsic growth determinants, including greater specific leaf area and more efficient whole-plant photosynthesis. However, photosynthetic capacities and their temperature responses were also temporally dependent and greater in IES than in NS during spring and summer. This contributed to a greater importance of pre-canopy periods with respect to IES growth. These findings demonstrate the significance of a community-level understanding of phenological interactions and environmental conditions in elucidating IES growth advantages over NS in deciduous forest understories.

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