Plant Growth In Salt Marshes Research Articles

Modelling the effects of global temperature increase on the growth of salt marsh plants

Gradual increases in temperature and atmospheric CO2 concentrations have resulted from the increased human use of fossil fuels since the beginning of industrial activity. In coastal wetland ecosystems, salt marshes constitute important habitats because they play important ecological roles, acting as carbon sinks by capturing atmospheric CO2 and storing it in living plant tissue. Ecological models are important tools for understanding the results of anthropogenic impacts on a global scale. Global warming poses threats to salt marshes through different effects, e.g., increases in sea level. The objectives of this study were i) to assess how temperature increases will influence the growth of salt marsh plants, ii) to infer the carbon budget of salt marshes under temperature increase scenarios and iii) to predict how salt marsh plants will keep pace with increases in sea level. These goals were achieved by developing growth models of three different plants (Spartina maritima, Scirpus maritimus and Zostera noltei) found in the Mondego estuary. Models were developed for C3 and C4 plant species. The results suggest that a temperature increase enhances the aboveground biomass of salt marsh plants. According to the predictions of the models, the sedimentation rate of S. maritima and Z. noltei can keep pace with increases in sea level, but this is apparently not the case for S. maritimus. If S. maritimus disappears from the Mondego estuary, the carbon sequestration ability of the system should decrease due to the loss of active plant tissue. This conclusion is based on the fact that S. maritimus accumulated more than 80% of the total carbon sequestered in the tissues by the three studied species.

Relationships betweenSpartina alterniflora andLittoraria irrorata in a South Carolina salt marsh

Bottom-up factors such as nutrient availability have long been thought to be the primary regulators of plant growth in salt marshes. However, that paradigm has been challenged by investigations showing that grazing by the periwinkle snailLittoraria irrorata regulatesSpartina alterniflora growth through top-down forces. Our investigation was conducted between November 2003 and March 2007 within the North Inlet Estuary (NIE) salt marsh to examine relationships betweenS. alterniflora andL. irrorata. The primary goal was to determine if observed in situ densities ofL. irrorata significantly decreasedS. alterniflora productivity at the study sites. The results indicate thatS. alterniflora productivity positively correlated withL. irrorata snail density during winter. However, no correlation was observed during summer. Hence, there was no inverse relationship between snail density and primary productivity and no support for the hypothesis of ‘top-down’ control of marsh plant production. A significant relationship betweenS. alterniflora stem density andL. irrorata density was observed during summer and winter, suggesting that stem density may play a key role in determining the distribution of periwinkle snails in NIE. These results challenge the applicability of theL. irrorata-S. alterniflora top-down control model as a generalized phenomenon in southeastern salt marshes.

The effect of rates of sedimentation and tidal submersion regimes on the growth of salt marsh plants

A system of tidal mesocosms has been constructed in order to determine interactions between rates of accretion and duration of inundation. Plants are grown in three tanks in which different tidal regimes are simulated by a system of electronically controlled pumps. Sediment is added to give four different rates of accretion and the mesocosms are run under controlled greenhouse conditions. To date experiments have been conducted with seedlings of Salicornia europaea and Aster tripolium. The longest duration of immersion reduces the growth rate of root and shoot of Salicornia seedlings but Aster was unaffected by 5 h submersion daily. Both species, however, responded positively to all rates of sediment addition up to the equivalent of 60 mm per year. The implication of these findings are discussed in relation to salt marsh development.

TOP-DOWN CONTROL OFSPARTINA ALTERNIFLORAPRODUCTION BY PERIWINKLE GRAZING IN A VIRGINIA SALT MARSH

Top-down forces, such as grazing and predation, have long been thought to be unimportant in controlling plant growth in salt marshes. Instead, bottom-up forces, such as porewater ammonium and oxygen availability, are thought to be the primary regulating factors. In the field, we observed the periwinkle, Littoraria irrorata, grazing on live saltmarsh cordgrass, Spartina alterniflora. To examine the relative importance of periwinkle grazing and nutrient availability in controlling marsh grass growth, we manipulated snail and nitrogen levels in a season-long field experiment in a Virginia salt marsh. Snails differentially affected plant growth at varying nitrogen levels. In unfertilized plots, snail removal increased S. alterniflora aboveground production by 38%, whereas in nitrogen addition plots, snail removal increased cordgrass growth by 78%. Snail addition decreased aboveground production by 51% in unfertilized stands, while in fertilized stands, snail addition led to even greater reductions (66%). By comparison, nitrogen addition increased S. alterniflora production by 443% in snail removal treatments, while in controls and snail addition treatments, fertilization effects were dampened significantly, as nitrogen enrichment increased marsh grass growth by 322% and 189%, respectively. Feeding assays examining the rate at which snails ingested live S. alterniflora revealed that snail consumption alone could not be responsible for these large biomass reductions. Experimental results suggest that grazer-induced defoliation and grazer control of plant demand for nitrogen fertilizer were the likely mechanisms involved. A survey of salt marshes from Maryland to Georgia showed that periwinkle grazing on live S. alterniflora is widespread. Our results show that L. irrorata can exert strong top-down control of S. alterniflora production, and that this effect increases with increasing nitrogen avaliability. Together, these findings question the widely accepted notion that grazers play a relatively unimportant role in the salt marsh community.