Abstract

This study examined both inter- and intraspecific variation of four dominant salt marsh macrophytes, a high marsh shrub, Baccharis halimifolia, a high marsh rush, Juncus roemerianus, a mid-marsh grass, Spartina patens, and the low marsh grass that is ubiquitous in wetlands along the Atlantic and Gulf coasts of the United States, short-form S. alterniflora.

Chapter One evaluates the seasonal C pool dynamics of the four species including the seasonal allocation of above- and belowground C pools, C pool loss through decomposition, and soil C concentration in a wetland fringing Little Assawoman Bay, one of Delaware's Coastal Bays. To determine whether the rate of vertical accretion and organic matter accumulation differed between the low, S. alterniflora and high, J. roemerianus zones, soil cores from the two zones were used to measure ¹³⁷Cs and ²¹⁰Pb activity. Total plant C pools of the mid- and low marsh grass species, Spartina patens (4360 g C m^-2) and Spartina alterniflora (4197 g C m^-2), were similar and almost two and three times larger than total pools of Juncus roemerianus (2508 g C m^-2) and Baccharis halimifolia (1490 g C m^-2), respectively. Moving from the high to low marsh zones, the C pool shifted from primarily aboveground to belowground. Baccharis had the greatest aboveground C storage (1140 g m^-2) and the slowest rate of C loss.

Chapter Two examines interspecific variation in N pool dynamics in the four species including seasonal allocation of N pools above- and belowground, N loss through decomposition, N resorption efficiency, and soil N concentration. The seasonal fluctuation in the total N pools of the herbaceous species was due to belowground N pool dynamics, particularly fine root and dead large and small-sized macroorganic matter fluxes. Comparisons among the species revealed that the location, magnitude and timing of N storage and dispersal differed, which is important in the context of how species will shift in response to environmental change.

Chapter Three describes the above- and belowground species associations in the fringing wetland and whether or not species shifts have occurred. Accurate productivity measurements in fringing wetlands may be dependent on species-specific organic matter separation, particularly belowground. Vegetation change in salt marshes may also become apparent when comparing above- and belowground species-specific live and dead organic matter. We surveyed species richness, frequency, and percent cover and measured above- and belowground biomass in three vegetation zones. Our study illustrates the importance of species-specific belowground biomass estimates to provide evidence of species shifts in both the low and high marsh zones.

Chapter four examines intraspecific variation in morphological characteristics and carbon, nutrient, and mineral concentration and allocation within B. halimifolia, J. roemerianus, S. patens, and S. alterniflora. Ecotypic variation in morphology and composition and allocation of C, nutrients, and minerals in wetland plants may influence ecosystem functions such as the deposition and trapping of sediments, detritus production, secondary productivity, the cycling and storage of organic and inorganic nutrients, belowground organic matter production, and long-term C storage. We examined the expression of morphological traits and C, nutrient, and mineral composition and allocation among southern ecotypes, a tissue-culture regenerant, and a native mid-Atlantic ecotype for each of four salt marsh species after two growing seasons within natural stands in a mid-Atlantic salt marsh. Overall, we found that the expression of phenotypic variation was greatest in the low marsh, Spartina alterniflora than in the higher marsh species likely due to both the greater spatial variation in elevation and soil conditions in the higher marsh and potentially a lower tolerance of higher marsh species to environmental stress. The differences that we found among ecotypes have important implications for enhancing and developing ecosystem processes in restoration and creation projects.

Chapter Five characterizes carbon pool dynamics of the salt marsh species, J. roemerianus, S. patens, and S. alterniflora using a STELLA model. The model was developed to examine the relationships between C pools and fluxes within species, to simultaneously compare the timing and magnitude of seasonal fluxes of C of each of the three species within a square meter, and to examine how changes to model parameters influence C pool dynamics and the accumulation of C belowground. (Abstract shortened by UMI.)