Graduation Semester and Year

2008

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Quantitative Biology

Department

Biology

First Advisor

Laura Gough

Abstract

Increasing atmospheric carbon dioxide (CO2) concentrations have caused higher air temperatures in the Arctic. Because arctic organisms are temperature limited, warmer conditions lead to higher decomposition and nutrient cycling rates by soil microorganisms resulting in positive feedbacks; more CO2 is released from warmer soils thus increasing global atmospheric CO2 concentrations. Higher decomposition and nutrient cycling rates increase soil nutrient availability for plants resulting in increased net primary productivity (NPP) and shifts in community structure. If these "warmer" communities contain more woody shrubs, they may store more carbon in plant tissue, potentially offsetting increases in CO2 from decomposition. However, little is known about how higher trophic levels affect plants under these conditions. I studied how mammalian herbivores affect individual plant growth, community structure and decomposition in two common arctic plant communities in northern Alaska that experienced ten years of experimental fertilization and herbivore exclosures in a factorial design. While fertilization increased growth of individual plants, increased community NPP and shifted species composition, the effects of herbivores were different in each community. In moist acidic tussock (MAT) tundra, long-term fertilization and herbivory decreased the growth of individual graminoids and increased deciduous shrub growth. Indeed, the graminoid Eriophorum vaginatum in MAT was not tolerant of herbivory regardless of fertilization. In contrast, graminoid growth in dry heath tundra (DH) increased with fertilization and was not affected by herbivores, while herbivores suppressed increases from fertilization in deciduous shrub growth. The DH graminoid Hierochloe alpina growing with fertilization was tolerant of and not strongly affected by herbivores. Additionally, I found evidence that the effects of fertilization and herbivores on individual growth scaled up to affect community structure in both communities. With higher soil nutrients and herbivore exposure in MAT, deciduous shrub abundance increased and graminoids decreased, while at DH the opposite was true. Lastly, I found no difference in E. vaginatum and H. alpina leaf decomposition due to herbivore processing with only minor changes attributed to fertilization. These results show that herbivores may play an important role in shaping plant responses to warmer temperatures in arctic ecosystems ultimately effecting feedbacks to the global carbon cycle.

Disciplines

Biology | Life Sciences

Comments

Degree granted by The University of Texas at Arlington

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