Defense Date


Document Type


Degree Name

Doctor of Philosophy


Integrative Life Sciences

First Advisor

Donald R. Young


Causes and consequences of woody encroachment into grass dominated systems have been widely studied, however functional mechanisms which promote encroachment are largely unknown. Many expansive woody species are shrubs with rhizobial or actinorhizal N-fixing symbiotic associations. Morella cerifera L. (Myricaceae) is an actinorhizal N-fixing shrub which rapidly expands into grasslands on the barrier islands off the coast of Virginia, USA. The objective of this research was to determine physiological drivers of woody encroachment resulting in increased woody cover of M. cerifera on Southeastern, US barrier islands. Variations in physiology and resource use efficiencies (RUE) of M. cerifera and co-occurring shrubs were determined, and edaphic characteristics beneath shrub thicket canopies and in open areas were quantified as indications of resource availability. Analysis of dune vegetation and soils showed severe freshwater limitation and reduced plant height of dune forbs suggesting dunes represent an upper elevational boundary for M. cerifera distribution. Soil N availability was higher beneath shrubs compared to open areas, and both physiology and isotope effects showed facilitation of the non-fixing shrub, Baccharis halimifolia, by M. cerifera which may lead to increased rates of woody encroachment as B. halimifolia colonizes expanding thicket edges. Morella cerifera and other N-fixers had higher %refixation within stems which resulted in higher carbon use efficiency (CUE) and water use efficiency of N-fixing shrubs compared to non-fixers. Results of an N-fertilization experiment suggest B. halimifolia has higher dependence on and demand for soil nutrients compared to M. cerifera. Morella cerifera showed no signs of resource deficiency or reduced physiological capacity even at 0 ppm total Nsoil. Morella cerifera transitioned from utilizing solely fixation derived N to soil N as N concentrations increased providing another mechanism leading to increased CUE and, indirectly, overall RUE. In summary greater RUE, lower resource demand, and greater resource availability for M. cerifera compared to co-occurring shrubs may result from symbiotic root associations with bacteria and fungi. While expansion of M. cerifera thickets is limited to lower elevational interdunal depressions, expansion may continue and result in increased rates of woody encroachment through facilitation of co-occurring shrubs.


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Is Part Of

VCU University Archives

Is Part Of

VCU Theses and Dissertations

Date of Submission

December 2011

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Life Sciences Commons