| Summary |
Sea-level rise would increase the inundation frequency of high salt marshes. Furthermore, a rise in sea level may cause an increase in the amount of wrack deposited in a high marsh community. Since changes in sea level occur slowly, communities may also change gradually, in response to increased inundation. Therefore, abrupt disturbances, such as storm induced deposition of Spartina alterniflora wrack, may be necessary to induce rapid changes in saltmarsh plant communities. Previous ecological studies in this area have generally not focused on more than one type of disturbance. Therefore, there is a lack of knowledge on how disturbances interact with each other in eliciting community responses. Furthermore, few studies in salt marshes have investigated the impact of abrupt disturbances which result in change of community structure. I experimentally manipulated inundation and wrack deposition to discern the individual and combined effects of these factors on high marsh plant species (i.e., Juncus roemerianus, Spartina patens, and Distichlis spicata) at a mainland marsh on the Eastern Shore of Virginia. This two year study involved (1) the experimental pumping of salt water onto the high marsh, (2) experimental placement of S. alterniflora wrack on the plant community, and (3) removing half of the wrack after six months of deposition. The experimental design was a randomized block that involved three flooding treatments (flooded, border control, and control), two wrack treatments (wrack and nonwrack), and three blocks (X, Y, and Z). The data collected from the experimental plots included (1) the tracking of the growth and senescence of J. roemerianus leaves, (2) estimations of growing and senescing J. roemerianus leaf densities, (3) quantification of aboveground biomass (g/m2), (4) vegetative analysis of the change in ground cover, and (5) estimation of the annual net primary productivity (ANPP, g/(m2 x y)) of J. roemerianus. Furthermore, I monitored two additional J. roemerianus populations that were inundated naturally on a daily basis and compared them to the experimental control plots to study the effects of greater, natural inundation on J. roemerianus. I also conducted an experiment to study the effects of shading on the regrowth of J. roemerianus. The effect of increased inundation on the plant community was species specific. Increased inundation did not significantly affect the aboveground biomass and ANPP of J. roemerianus, but it did result in slower growing and faster senescing J. roemerianus leaves. Furthermore, the reference J. roemerianus populations, which have undergone prolonged flooding (> 2 years), were significantly different from the control plots. Therefore, the growth and senescence rates ofJ. roemerianus are more sensitive to changes in inundation than aboveground biomass and ANPP; although aboveground biomass and ANPP probably decline with long-term inundation. Increased frequency of inundation decreased S. patens aboveground biomass, but ponding of water increased S. patens aboveground biomass. Increased inundation alone did not cause a significant effect on the aboveground biomass of D. spicata and did not promote the growth of minor species such as short S. alterniflora. The effects of wrack deposition in reducing the aboveground biomass of the plant community were non-species specific, while regrowth and colonization after wrack deposition were species specific. Six months of wrack deposition significantly reduced the aboveground biomass of the three dominant species. One year after the removal of wrack, the aboveground biomass and leaf densities of J. roemerianus were still significantly lower in the post-wrack areas as compared to the nonwrack areas. In contrast, the aboveground biomass values for S. patens and D. spicata were not significantly lower in the post-wrack areas as compared to the nonwrack areas. Furthermore, living D. spicata biomass was significantly higher in the post-wrack areas. D. spicata also colonized the bare areas where J. roemerianus previously grew. The interaction between the responses to increased inundation and wrack deposition occurs through time. During periods of deposition, the extreme effect ofthe wrack on the plants masked any interaction between flooding and deposition. However, after the removal of wrack, increased inundation inhibited D. spicata, a primary colonizing species, from colonizing some ofthe bare areas. When increased inundation inhibits the colonization of post-wrack areas, peat accumulation may decrease and erosion may increase in the bare areas. This would result in microtopographic changes that may eventually lead to the formation of marsh potholes. Lastly, shading did not have a significant effect on the regrowth ofJ. roemerianus after clipping. However, shading inhibited the regrowth of species other than J. roemerianus. This sensitivity of other species to shading may be one reason why J. roemerianus stands are highly monospecific. The possibility of J. roemerianus being able to outcompete other high marsh species via shading deserves further study. |
