Biophysical and morphological leaf adaptations to drought and salinity in salt marsh grasses

Leaf energy budgets were constructed for 13 species of estuarine C₄ grasses (Poaceae) to elucidate the biophysical effects of drought and salinity on the interception and dissipation of solar energy. Spartina alterniflora, S. anglica, S. argentinensis, S. bakeri, S. cynosuroides, S. densiflora, S. foliosa, S. foliosa × S. alterniflora hybrids, S. gracilis, S. patens, S. pectinata, S. spartinae, and Distichlis spicata plants were grown under controlled soil water potential gradients in a greenhouse. Species were grouped into four major ecological functional types, based on elevational zonation ranges: low marsh species, middle marsh species, high marsh species, and freshwater species. Different functional types are adapted to different environmental conditions, and responded differently to reduced water potentials. Latent heat flux decreased similarly across species in response to decreasing water potential. Latent heat loss was found to decrease by as much as 65% under decreasing water potential, leading to an increase in leaf temperature of up to 4 °C. Consequently, radiative and sensible heat losses increased under decreasing water potential. Sensible heat flux increased as much as 336% under decreasing water potential. Latent heat loss appeared to be an important mode of temperature regulation in all species, and sensible heat loss appeared to be more important in high marsh species compared to low marsh species. High marsh species are characterized by narrower leaves than middle and low marsh species, leading to a smaller boundary layer, and providing higher conductance to sensible heat loss. This may be an adaptation for high marsh species to regulate leaf temperature without access to large amounts of water for transpirational cooling. Stomatal conductance decreased with decreasing water potential across species: leaf conductances to water vapor and CO₂ decreased as much as 69% under decreasing water potential. Additionally, oxidative stress appeared to increase in these plants during times of drought or salinity stress. Ascorbate peroxidase activities increased with decreasing soil water potential, indicating increased cellular reactive oxygen species. High marsh species had higher ascorbate peroxidase activities compared to low marsh species, indicating higher tolerance to drought- or salinity-induced stresses. It was concluded that different species of marsh grasses are adapted for growth in different zones of salt marshes. Adaptations include biophysical, biochemical, and morphological traits that optimize heat exchange with the environment.     

Interactive effects of elevation and burial with wrack on plant community structure in some Rhode Island salt marshes

1 The effects of disturbances and elevation on marsh plant communities were examined using experimental disturbances along an elevation gradient in marshes with different disturbance histories. In addition, differences in species composition among five marshes were determined at elevations at which the greatest concentration of burial by wrack occurred.
2 Experimental wrack burial generally caused significant mortality of the high-marsh competitive dominants, Juncus gerardi and Spartina patens , and strongly increased the abundance of the fugitive perennial, Distichlis spicata .
3 The effects of experimental wrack burial interacted strongly with abiotic factors associated with elevation to influence the distributions of both competitive dominants and annual fugitive plants.
4 Frequent wrack burial in a marsh appears to lead to a persistent assemblage of plants dominated by competitively subordinate fugitives. This assemblage of fugitives tends to occur at intermediate elevations within the marsh, where wrack gets stranded for long periods of time and where the resistance of Juncus gerardi to wrack burial is lowest.
5 We suggest that wrack-burial disturbances interact strongly with marsh elevation to influence the zonation of plants in New England salt marshes, and discuss some implications of our results.     

Nitrogen addition does not reduce belowground competition in a salt marsh clonal plant community in Mississippi (USA)

Previous studies have suggested that belowground competition for nutrients influences plant zonation in salt marshes. In this study, I tested the hypothesis that competition for nitrogen structured a clonal plant community in a nitrogen-limited salt marsh in coastal Mississippi, USA. In contrast to most previous field studies that have investigated mechanisms of competition, I examined clonal growth responses of established genets of a nitrogen-demanding low-intertidal species (Spartina alterniflora) to nitrogen addition and the removal of a nitrogen-conserving high-intertidal species (Juncus roemerianus). Nitrogen addition stimulated clonal invasion of the Juncus zone by Spartina but did not reduce the significant competitive effects of Juncus on Spartina. Simulated Juncus shade did not reduce invasion of the Juncus zone by Spartina, indicating that belowground competition reduced clonal invasion. In the last year of the study, the border shifted unexpectedly towards the Spartina zone, resulting in competitive displacement of Spartina by Juncus. Nitrogen addition did not prevent or slow this displacement, further contradicting the nitrogen competition hypothesis. Although growth rates were much more strongly limited by nitrogen in Spartina than in Juncus, nitrogen addition did not cause the displacement of Juncus by Spartina after three growing seasons. I conclude that zonation of Spartina and Juncus is maintained by preemption of space and greater tolerance of low nitrogen supplies by Juncus in the high marsh. These results contrast sharply with findings of reduced belowground competition with nutrient addition in previous studies and highlight the important role of nutrient-mediated competition for space between clonal plants.     

Nitrogen further promotes a dominant salt marsh plant in an increasingly saline environment

Aims Human alterations of the environment are combining in unprecedented ways, making predictions of alterations to natural communities a difficult and pressing challenge. Estuarine systems have been subject to a high degree of modification, including increased nitrogen (N) inputs and altered salinity, factors important in shaping estuarine plant communities. As human populations increase and the climate changes, both N and salinity levels are likely to increase in these coastal marshes. Our objective was to evaluate the interactive effects of N and salinity on US West Coast salt marsh species; in particular, the performance of the dominant species Sarcocornia pacifica (pickleweed) alone and in mixed species assemblages. We expected increased salinity to favor S. pacifica but that N enrichment could help maintain greater species richness through use of N in salinity tolerance mechanisms.Methods We crossed treatments of N (added or not) and salinity (salt added or not) in a field experiment at a salt marsh in the San Francisco Estuary, California, USA, in each of three habitats: (i) monotypic pickleweed on the marsh plain, (ii) monotypic pickleweed along channels and (iii) mixed assemblages along channels. In a greenhouse experiment, we crossed treatments of N (added or not) and salinity (at three levels to simulate brackish to saline conditions) in (i) pots of pickleweed only and (ii) the same species mix as in the field.Important findings N addition doubled S. pacifica biomass and branching in both channel and marsh plain habitats regardless of salinity and greatly increased its dominance over Distichlis spicata and Jaumea carnosa in mixed assemblages along channels. In the greenhouse, S. pacifica biomass increased 6- to 10-fold with N addition over the range of salinities, while D. spicata and J. carnosa biomass increased with N addition only at lower salinity levels. Thus, while localized management could influence outcomes, expected overall increases in both N and salinity with human population growth and climate change are likely to enhance the production of S. pacifica in US West Coast marshes while reducing the diversity of mixed species assemblages. This decline in diversity may have implications for the resilience of marshes already subject to multiple stressors as the climate changes.     

Sedimentation and resuspension in a New England salt marsh

Particulate matter in a salt marsh can undergo repeated sedimentation and resuspension. Sedimentation measured with sediment traps, increases with tidal amplitude in areas with fast tidal currents, but is unaffected by tidal amplitude in areas with slow currents. The total sedimentation of particulate nitrogen in areas with slow tidal currents is three times as large as the gross tidal exchanges of particulate nitrogen between the marsh and coastal waters. Net tidal export of particles by the marsh suggests that sedimentation is more than offset by resuspension. Resuspension of fine (4–40 µm) particles occurs early in the flood tide in tidal creeks with slow currents. This resuspension does not increase with tidal amplitude, suggesting that it is not caused by tidal currents.     

Recovery of a northern New England salt marsh plant community from winter icing

High latitude salt marsh plant communities are frequently exposed to conspicuous winter ice disturbances, which trigger secondary succession. In this paper, we document the recovery of a northern New England salt marsh from a severe winter icing event in 1998. Ice disturbances that killed plants but that left the underlying peat intact recovered rapidly. However, ice damage that killed plants and removed the underlying peat, led to areas of physiologically harsh edaphic conditions, specifically waterlogged and anoxic soils that limited plant recolonization. A transplant experiment revealed that only the most stress-tolerant plants were capable of invading the most stressful portions of ice disturbances. A second experiment that artificially dried disturbance patches accelerated patch recovery. These data suggest that recovery from intense ice disturbance is dependent on stress-tolerant plants invading edaphically harsh disturbances, eventually facilitating the recolonization of the community. This process likely takes longer than a decade for full recovery to occur in the areas where both plants and the peat base are removed.     

Variation in nutrient availability and plant species diversity across forb and graminoid zones of a Northern New England high salt marsh

Plant zonation patterns across New England salt marshes have been investigated for years, but how nutrient availability differs between zones has received little attention. We investigated how N availability, P availability, and plant N status varied across Juncus gerardii, Spartina patens, and mixed forb zones of a Northern New England high salt marsh. We also investigated relationships between several edaphic factors and community production and diversity across the high marsh. P availability, soil salinity, and soil moisture were higher in the mixed forb zone than in the two graminoid zones. NH⁺ ₄-N availability was highest in the J. gerardii zone, but NO^– ₃-N availability and mid season net N mineralization rates did not vary among zones. Plant tissue N concentrations were highest in the mixed forb zone and lowest in the S. patens zone, reflecting plant physiologies more so than soil N availability. Community production was highest in the J. gerardii zone and was positively correlated with N availability and negatively correlated with soil moisture. Plant species diversity was highest in the mixed forb zone and was positively correlated with P availability and soil salinity. Thus, nutrient availability, plant N status, and plant species diversity varied across zones of this high marsh. Further investigation is needed to ascertain if soil nutrient availability influences or is a result of the production and diversity differences that exist between vegetation zones of New England high salt marshes.     

Impact of a parasitic plant on the zonation of two salt marsh perennials

Animal, fungal, and bacterial consumers can have dramatic effects on the structure of plant communities, often by consuming dominant competitors and indirectly increasing the abundance of inferior competitors. We investigated the role of a consumer plant, the parasite Cuscuta salina, on plant zonation in a western salt marsh. Cuscuta had a strong host species preference in experiments, disproportionally infecting Salicornia virginica, the dominant competitor in most of the marsh. In plots with Cuscuta, which infected 18% of our study area over a 3-year period, Salicornia cover decreased and the cover of Arthrocnemum increased substantially in comparison to plots without Cuscuta. Deep in the Salicornia zone, the cover of Arthrocnemum in Cuscuta-infected plots increased by 558% in 1 year relative to uninfected plots. At the ecotone, the cover of Arthrocnemum in Cuscuta-infected plots increased by only 41% during the same time interval. These data suggest that the relative benefit of a consumer to a less-preferred, subordinate competitor may be strongest where competition is the most asymmetrical as predicted by recent theoretical models. By weakening the competitive dominant, which in the absence of the parasite can create virtual monocultures, Cuscuta enhanced community diversity and altered the ecotone between Salicornia and Arthrocnemum. Cuscuta patches were highly dynamic at the ecotone between Salicornia and Arthrocnemum, and thus the changes we measured in our sample plots were likely to be representative of large portions of the marsh. Our findings emphasize the importance of trophic interactions in salt marsh structure and zonation. Received: 23 April 1997 / Accepted: 10 October 1997     

Experimental warming causes rapid loss of plant diversity in New England salt marshes

Anthropogenic climate change is predicted to cause widespread biodiversity loss due to shifts in species' distributions, but these predictions rarely incorporate ecological associations such as zonation. Here, we predict the decline of a diverse assemblage of mid-latitude salt marsh plants, based on an ecosystem warming experiment. In New England salt marshes, a guild of halophytic forbs occupies stressful, waterlogged pannes. At three sites, experimental warming of < 4 °C led to diversity declines in pannes and rapid takeover by a competitive dominant, Spartina patens . In Rhode Island, near their southern range limit, pannes were more sensitive to warming than farther north, and panne area also declined in control plots over the three-season experiment. These results suggest that warming will rapidly reduce plant diversity in New England salt marshes by eliminating a high diversity zone. Biodiversity in zoned ecosystems may be more affected by climate-driven shifts in zonation than by individual species' distribution shifts.     

Molecular characterization of sulfate-reducing bacteria in a New England salt marsh

Sulfate reduction, mediated by sulfate-reducing bacteria (SRB), is the dominant remineralization pathway in sediments of New England salt marshes. High sulfate reduction rates are associated with the rhizosphere of Spartina alterniflora when plants elongate aboveground. The growth process concurrently produces significant amounts of new rhizome material belowground and the plants leak dissolved organic compounds. This study investigated the diversity of SRB in a salt marsh over an annual growth cycle of S. alterniflora by exploring the diversity of a functional gene, dissimilatory sulfite reductase (dsrAB). Because the dsrAB gene is a key gene in the anaerobic sulfate-respiration pathway, it allows the identification of microorganisms responsible for sulfate reduction. Conserved dsrAB primers in polymerase chain reaction (PCR) generated full-length dsrAB amplicons for cloning and DNA sequence analysis. Nearly 80% of 380 clone sequences were similar to genes from Desulfosarcina and Desulfobacterium species within Desulfobacteraceae. This reinforces the hypothesis that complete oxidizers with high substrate versatility dominate the marsh. However, the phylotypes formed several clades that were distinct from cultured representatives, indicating a greater diversity of SRB than previously appreciated. Several dsrAB sequences were related to homologues from gram-positive, thermophilic and non-thermophilic Desulfotomaculum species. One dsrAB lineage formed a sister group to cultured members of the delta-proteobacterial group Syntrophobacteraceae. A deeply branching dsrAB lineage was not affiliated with genes from any cultured SRB. The sequence data from this study will allow for the design of probes or primers that can quantitatively assess the diverse range of sulfate reducers present in the environment.     

Strong associations between plant genotypes and bacterial communities in a natural salt marsh

Although microbial communities have been shown to vary among plant genotypes in a number of experiments in terrestrial ecosystems, relatively little is known about this relationship under natural conditions and outside of select model systems. We reasoned that a salt marsh ecosystem, which is characterized by twice‐daily flooding by tides, would serve as a particularly conservative test of the strength of plant–microbial associations, given the high degree of abiotic regulation of microbial community assembly resulting from alternating periods of inundation and exposure. Within a salt marsh in the northeastern United States, we characterized genotypes of the foundational plant Spartina alterniflora using microsatellite markers, and bacterial metagenomes within marsh soil based on pyrosequencing. We found significant differences in bacterial community composition and diversity between bulk and rhizosphere soil, and that the structure of rhizosphere communities varied depending on the growth form of, and genetic variation within, the foundational plant S. alterniflora. Our results indicate that there are strong plant–microbial associations within a natural salt marsh, thereby contributing to a growing body of evidence for a relationship between plant genotypes and microbial communities from terrestrial ecosystems and suggest that principles of community genetics apply to this wetland type.     

Biological consequences of ice rafting in a New England salt marsh community

Ice rafting of salt marsh peat is a recurrent phenomenon in north temperate regions. This process was simulated in a northern New England salt marsh to test several hypotheses concerning the effects of peat transport from high to low intertidal heights on the growth and mortality of key sessile organisms: the ribbed mussel Geukensia demissa (Dillwyn), the fucoid alga Fucus vesiculosus L. var. spiralis (Farlow) and the cordgrass Spartina alterniflora (Loisel.). Growth rates increased when Geukensia and Fucus were transported to the lower intertidal; however, Spartina died when similarly transported. Predation pressure (primarily from Carcinus maenus L.) on Geukensia was greater when it was rafted to the lower intertidal zone than in the upper intertidal habitat and was size specific; mussels >3.5cm reached a size-escape from crab predation.A winter survey of dislodged mussels revealed that 72% of the mussels collected were dead and 86% had been overgrown by large Fucus plants, >2.5 × the natural frequency of Fucus overgrowth (32%). In marsh habitats where hard substratum is rare, 91% of the Fucus were growing on Geukensia. A dislodgement experiment showed that a significantly greater percentage of Geukensia was dislodged after ice-out when Fucus was attached to the shell than those mussels without Fucus overgrowth. In the spring, a population survey conducted in the salt marsh examined densities, biomass and population structure of Geukensia, as well as densities, percent cover and biomass of Fucus. Values obtained in the foremarsh were compared to those from the peat islands recently rafted to the tidal flats. Both biomass and densities of Geukensia were similar in the two areas; however, the size-frequency distributions of the mussels were different. Since fewer large mussels, Fucus and Fucus-overgrown mussels were found on the newly transported peat islands, this pattern appears to reflect dislodgement of larger Geukensia by attached algae during ice transport. Two ice-related sources of mortality were identified for Geukensia: (1) Fucus overgrowth acted as a vector for mussel dislodgement and was an indirect source of mortality; and (2) ice crushing was a direct source of mortality for non-overgrown mussels.     

ENSO episodes modify plant/terrestrial-herbivore interactions in a southwestern Atlantic salt marsh

Hemisphere scale events such as El Niño-Southern Oscillation (ENSO) can alter rainfall regimes worldwide, with important effects on species abundance and distribution. The evidence of ENSO effects on terrestrial communities is, however, restricted to a few ecosystem types. We explored the effects of ENSO episodes on plant/terrestrial-herbivore interactions through changes in the rainfall regime in a southwestern Atlantic salt marsh (Mar Chiquita coastal lagoon, Argentina. 37° 40′S, 57° 23′W). Surveys showed a positive relationship between winter rainfall and the abundance of the wild guinea pig Cavia aperea. The highest salt marsh abundances of C. aperea were associated with rainy periods during El Niño episodes, and the lowest ones were associated with the driest La Niña episodes. Rainfall was negatively associated with marsh sediment salinity, and experiments revealed that increased salinity reduces growth and increases mortality of cordgrass (Spartina densiflora). Salt increase also causes the highest percentage of dry area in S. densiflora leaves and reduced carbon content, and more salt content and secretion in S. densiflora stems. A factorial experiment in which we manipulated C. aperea presence and salinity along the edges of S. densiflora patches showed that plants can asexually invade unvegetated areas when salinity is reduced and C. aperea is excluded. Conversely, S. densiflora edges retracted when salinity was increased or there was C. aperea herbivory. Changes in nutritional quality of S. densiflora could explain the low herbivory of (and lack of impacts from) C. aperea in plots with high salinity. Thus, plant distribution responds directly to climate oscillations through changes in salt stress, and indirectly, through changes in plant-herbivore interactions. Herbivores respond indirectly to climate oscillations through changes in plant food quality, which suggests that top-down effects increase when bottom-up stressors are relaxed. ENSO events have direct and indirect effects on marsh communities that modulate the relative importance of top-down and bottom-up effects and have a considerable effect on the primary productivity of S. densiflora marshes.     

Spatial and temporal changes in salt marsh distribution in the Dee estuary,NW England,determined from aerial photographs

Vegetation changes in salt marsh communities of the Dee estuary, northwest England, were analysed with a combination of remote sensing techniques using data dating back to the 1950s. The distribution of communities in 1997 was classified using Airborne Thematic Mapper data and used to develop a methodology for the analysis of black and white photographs of the marsh. These methods were then applied retrogressively to a time sequence of monochrome photographs running from 1955 to 1975. At the apex of the salt marshes on the English shore of the Dee estuary, the marsh expanded dramatically to 1975, and consisted predominantly of pioneer and low marsh vegetation types. Between 1975 and 1997, however, there was only a slight increase in salt marsh area, but with an increase in mid and high marsh vegetation, replacing pioneer marsh. In a second area of the salt marsh on the English shore, a different pattern of salt marsh expansion was observed. The area occupied by marsh continued to increase right up to 1997, with extensive pioneer vegetation suggesting a process of continuing expansion. However, the pattern of marsh colonisation appeared to be different in 1997 compared to 1975. The significance of the changes in salt marsh distribution within the Dee estuary are discussed in relation to the historical pattern of salt marsh colonisation, the importance of Spartina anglica in the process and the implications for strategic management of the estuarine resources.     

The effect of competition and salinity on the growth of a salt marsh plant species

Summary Young rhizome sprouts of the herbaceous perennial Jaumea carnosa were propagated from material collected in a salt marsh along the central California coast. The sprouts were transplanted to flats of sand sown with different densities of seeds of a representative glycophyte, Lolium perenne L. Derby, turf type. Controls flats contained only Jaumea or Lolium. Three series of replicated flats were watered from above with dilutions of seawater in 1/10 strength Hoagland solution, such that dissolved salts were 400, 4000 or 11,600 ppm. Two other series were continuously subirrigated with 400 or 11,600 ppm salt water. After 61 days of treatment in a greenhouse with a 30/11°C thermoperiod (mean daily max/min), all plants were harvested and weighed. In the monospecific control flats, the growth of both species declined with increasing salinity, but the relative decline of Lolium was three times that of Jaumea. Jaumea's root: shoot ratio was also less affected by salinity. Both species grew well when subirrigated by 400 ppm salt water, but grew poorly when subirrigated by 11,600 ppm salt water, indicating that aeration alone is not the most significant factor in the marsh. The effect of interspecific competition on Jaumea was marked at low salinity, depressing growth by 52% compared to controls, but at high salinity the competitive effect was insignificant, whether the plants were watered from above or subirrigated. This supports the hypothesis that intolerant halophytes such as Jaumea are restricted in nature to salt marshes because they are poor competitors with glycophytes on non-saline soils.This research was supported by the John Simon Guggenheim Memorial Foundation and by the University of California Bodega Marine Laboratory