Date Awarded

2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Virginia Institute of Marine Science

Advisor

James E. Perry, III

Abstract

This study investigated vegetation changes in a former tidal freshwater marsh (TFM) to determine the role that salinity intrusion plays in vegetation dynamics. Field observations along a narrow salinity gradient in the Pamunkey River revealed that vegetation is shifting to fewer dominants with increasing salinity. Two remaining dominants, Peltandra virginica and Zizania aquatica (hereafter Peltandra and Zizania, respectively), had variable net CO2 assimilation throughout the growing season. Peltandra net CO2 assimilation declined both over the growing season and in marshes with higher salinity; whereas, Zizania generally increased over the growing season peaking in late summer. The same species' tissue nutrients tracked similarly when compared across marshes of different salinity throughout the season, suggesting that the plants have adapted to their environment. Soils, however, contained higher carbon (C) and nitrogen (N) in a TFM relative to higher salinity marshes across years and within a single season. The soil N: phosphorus (P) ratio is relatively stable and well above 16:1 in the TFM, suggesting P-limitation. The other marshes appeared to be in transition with high fluctuations throughout the season and variability within the marsh. Soils responded to changes in salinity faster than vegetation by adsorbing or releasing nutrients. A mesocosm testing plant traits subjected to four low salinity levels found Spartina alterniflora (hereafter Spartina) unresponsive to salinity ranging from 0 to 6, although CO2 assimilation decreased between treatments fresh and 6. Two TFM species, Peltandra and Leersia oryzoides (hereafter Leersia ), responded to salinity over 2 with decreases in aboveground and belowground biomass. The same two species exhibited an improvement in biomass quality (measured by C:N and C:P) over the salinity gradient, and both held greater N and P in the vegetation pool relative to Spartina. The pool of nutrients held in vegetation may shift with salinity intrusion, and the enhanced biomass quality may lead to greater herbivory due to improved palatability. A second mesocosm study paired Spartina, a facultative halophyte, with each of three TFM species: Peltandra, Leersia, and Phragmites australis (hereafter Phragmites). Spartina outperformed Peltandra in all aspects measured -- both aboveground and belowground. In the presence of Leersia, Spartina offered mixed responses, but Phragmites changed Spartina responses considerably. The results of this experiment suggest that Spartina can succeed in less benign environments in the presence of at least some species. A field manipulation excluding insect herbivory within treatments in three marshes along a salinity gradient found that overall biomass did not respond to the removal of insect herbivory, except for Peltandra in Cumberland Marsh (TFM). Peltandra biomass in TFM exclosures was approximately double that of controls, but this result was not significant in other marshes. Zizania N-content was higher in exclosures, suggesting a loss of this nutrient with herbivory, perhaps from rebuilding scarred tissue and/or loss through guttation. Given Peltandra's salt intolerance and Spartina's ability to outperform common species, it is possible that Sweet Hall Marsh, a previous TFM transitioning to an oligohaline marsh, will become a Spartina-Phragmites marsh in the future driven by bottom-up controls.

DOI

https://dx.doi.org/doi:10.25773/v5-db2g-qt58

Rights

© The Author

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