Date of Graduation

2016

Document Type

Dissertation

Degree Type

PhD

College

Eberly College of Arts and Sciences

Department

Biology

Committee Chair

James B McGraw

Committee Co-Chair

Mary B Adams

Committee Member

Frank Gilliam

Committee Member

William Peterjohn

Committee Member

Richard Thomas

Abstract

Historical accounts suggest that American ginseng was once far more abundant than it is today, and extensive changes in forest canopy structure and ginseng habitat caused by clearcut timber harvest occurred coincidentally to the precipitous decline in abundance. The presence of natural American ginseng populations after widespread clearcuts suggests that ginseng can survive under sparse canopies, and the presence of ginseng in the second growth forests common today suggest that ginseng can survive under dense canopies. However, performance may not be optimal at either end of the disturbance spectrum, and the net effect of direct and indirect anthropogenic canopy disturbances on ginseng has been unknown. The present suite of studies addresses the question "how does American ginseng respond to changes in forest dynamics that result from changes in canopy disturbance regimes caused by humans?" I first determined how different types of disturbances affected canopy openness on the scale of a small herbaceous plant. In Chapter 2, I found that canopy openness after timber harvest was greatest at the most intensely harvested site, and that subsequent understory canopy closure differed from that occurring in higher strata. Relative canopy closure was greatest in areas with high densities of shade-intolerant weedy and invasive species. The purpose of Chapter 3 was to determine if canopy disturbances via patch cut timber harvest were physiological stressors to American ginseng, or whether increases in light following timber harvest benefited carbon assimilation and growth. I found that ginseng is a 'slow opportunist;' photosynthesis and growth increased following canopy disturbance, but stimulation lagged behind changes in canopy structure. Although ginseng benefited from a patch cut timber harvest, it was unclear whether varying intensities of timber harvest deferentially affected the survival, growth, and reproduction of ginseng. In Chapter 4, I found that survival generally decreased following timber harvest, and was lowest at the most intensely harvested site. However, growth and seed production increased in individuals that survived. In order to provide an integrated assessment of survival, growth, and fertility following canopy disturbance, I quantified the demographic response of five populations of American ginseng to natural disturbances whose regimes are being altered by climate change. In Chapter 5, I found that population growth rate increased after natural canopy disturbances, and did so even at the current frequency of disturbance within American ginseng populations. Additionally, canopy disturbances produced sustained increases in population size via influxes of seeds to the seed bank. These studies provide clear evidence that American ginseng is surprisingly resilient in the face of rapid environmental shifts. Further, American ginseng, and perhaps similar herbs, benefit from moderate intensity, anthropogenic canopy disturbances whose frequencies and spatial extents are predicted to increase.

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