Defense Date

2009

Document Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Biology

First Advisor

Donald Young

Abstract

Shrub expansion in herbaceous ecosystems is emerging as an important ecological response to global change, especially in mesic systems where increases in canopy biomass are greatest. Two consequences of woody encroachment are increases in belowground resources, such as carbon and nitrogen, and reductions in above-ground resources such as light, which affect diversity, community trajectory, and ecosystem function. My objective was to determine how expansion of the nitrogen-fixing shrub Morella cerifera affected the resource environment across a chronosequence of shrub expansion on a Virginia barrier island. I quantified changes in carbon (C) and nitrogen (N) cycling, canopy structure and understory light associated with M. cerifera expansion. Litterfall in shrub thickets exceeded litterfall for other woody communities in the same region, and due to high N concentration, resulted in a return of as much as 169 kg N ha-1 yr-1 to the soil, 70% of which was from symbiotic N fixation. Litter and soil C and N pools were 3-10 times higher in shrub thickets than in adjacent grasslands. Understory light in shrub thickets decreased to as low as 0.5% of above-canopy light. Sunflecks in shrub thickets were shorter, smaller and less intense than sunflecks in forest understories. However, relative to other shrub species such as Elaeagnus umbellata, M. cerifera was less efficient at intercepting light. Although M. cerifera had the highest leaf area index (LAI) of five shrub species studied, M. cerifera was relatively inefficient at light attenuation due to low levels of branching, steep leaf angles and a relatively shallow canopy. The shift from grassland to shrub thicket on barrier islands, and in other mesic systems, results in a significant change in canopy structure that alters understory resource availability and greatly alters ecosystem function and trajectory.

Rights

© The Author

Is Part Of

VCU University Archives

Is Part Of

VCU Theses and Dissertations

Date of Submission

April 2009

Included in

Biology Commons

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