Juncus Roemerianus Research Articles

Acute salt marsh dieback in the Mississippi River deltaic plain: a drought‐induced phenomenon?

ABSTRACTAims Extensive dieback of salt marsh dominated by the perennial grass Spartina alterniflora occurred throughout the Mississippi River deltaic plain during 2000. More than 100,000 ha were affected, with 43,000 ha severely damaged. The aim of this work was to determine if sudden dieback could have been caused by a coincident drought and to assess the significance of this event with respect to long‐term changes in coastal vegetation.Location Multiple dieback sites and reference sites were established along 150 km of shoreline in coastal Louisiana, USA.Methods Aerial and ground surveys were conducted from June 2000 to September 2001 to assess soil conditions and plant mortality and recovery.Results Dieback areas ranged in size from ∼300 m2−5 km2 in area with 50–100% mortality of plant shoots and rhizomes in affected zones. Co‐occurring species such as Avicennia germinans (black mangrove) and Juncus roemerianus (needlegrass rush) were unaffected. Historical records indicate that precipitation, river discharge, and mean sea level were unusually low during the previous year. Although the cause of dieback is currently unknown, plant and soil characteristics were consistent with temporary soil desiccation that may have reduced water availability, increased soil salinity, and/or caused soil acidification (via pyrite oxidation) and increased uptake of toxic metals such as Fe or Al. Plant recovery 15 months after dieback was variable (0–58% live cover), but recovering plants were vigorous and indicated no long‐lasting effects of the dieback agent.Main conclusions These findings have relevance for global change models of coastal ecosystems that predict vegetation responses based primarily on long‐term increases in sea level and submergence of marshes. Our results suggest that large‐scale changes in coastal vegetation may occur over a relatively short time span through climatic extremes acting in concert with sea‐level fluctuations and pre‐existing soil conditions.

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.

Species‐specific patterns of litter processing by terrestrial isopods (Isopoda: Oniscidea) in high intertidal salt marshes and coastal forests

Summary The species‐specificity of litter processing by three species of isopods at the interface between salt marsh and coastal forest habitats in the south‐eastern United States was examined. To quantify isopod performance, measurements were taken of feeding, digestion and growth of isopods fed on three litter types (Juncus roemerianus, Quercus virginiana and Pinus palustris) and on artificial diets containing one of three classes of model phenolic compounds (simple phenolics and hydrolysable and condensed tannins). To quantify the ecosystem impact of isopods, promotion of microbial respiration, changes in detritus chemistry, and the quantity of litter processed by isopod populations were measured. The results support three hypotheses concerning isopod–litter interactions. (i) Isopod performance on different litter types can be predicted based on chemical litter traits, e.g. phenolic concentrations and C : N ratios. (ii) Fully terrestrial isopods are better adapted to the range of phenolics found in angiosperm litter than are semiterrestrial species inhabiting the supralittoral. (iii) Isopod species differ with respect to their impact on decomposition processes due to species‐specific digestive capabilities, different effects on microbial decomposition and different rates of net litter processing. Because isopods are transitional between semiterrestrial and terrestrial habitats, unlike most other salt marsh detritivores, they are likely to play a unique role in decomposition processes and in the flux of materials between salt marsh and terrestrial habitats.

Fungi on Juncus and Spartina: New marine species of Anthostomella, with a list of marine fungi known on Spartina

All 39 marine species described so far from Spartina spp. are listed. Anthostomella torosa sp. nov. is described from culms of Juncus roemerianus in North Carolina, and A. spissitecta sp. nov. is described from leaf sheaths of saltmarsh Spartina alterniflora in the USA (Connecticut, Florida, North Carolina, Rhode Island) and from S. densiflora in Argentina (Buenos Aires). Both species occur close to the rhizome and are classified as obligate marine fungi. The new species are compared with similar taxa, and a table comprises characteristics of the nine known aquatic Anthostomella species.

Monitoring the recovery of Juncus roemerianus marsh burns with the normalized difference vegetation index and Landsat Thematic Mapper data

Nine atmospherically corrected Landsat Thematic Mapper images were usedto generate mean normalized difference vegetation indices (NDVI) at 11burn sites throughout a coastal Juncus roemerianus marsh in St. MarksNational Wildlife Refuge, Florida. Time-since-burn, the time lapse from thedate of burn to the date of image collection, was related to variation inmean NDVI over time. Regression analysis showed that NDVI increasedfor about 300 to 400 days immediately after the burn, overshooting thetypical mean NDVI of a nonburned marsh. For about another 500 to 600days NDVI decreased until reaching a nearly constant NDVI of about0.40. During the phase of increasing NDVI the ability to predicttime-since-burn was within about ±60 days. Within the decreasingphase this dropped to about ±88 days.Examination of each burn site revealed some nonburn related influences onNDVI (e.g., seasonality). Normalization of burn NDVI by site-specificnonburn control NDVI eliminated most influences. However, differentialresponses at the site-specific level remained related to either storm impactsor secondary burning. At these sites, collateral data helped clarify theabnormal changes in NDVI. Accounting for these abnormalities,site-specific burn recovery trends could be broadly standardized into fourgeneral phases: Phase 1 – preburn, Phase 2 – initial recovery (increasingNDVI), Phase 3 – late recovery (decreasing NDVI), and Phase 4 – finalcoalescence (unchanging NDVI). Phase 2 tended to last about 300 to 500days, Phase 3 an additional 500 to 600 days, and finally reaching Phase 4,900 to 1,000 days after burn.

Diversity of fungi on six species of Gramineae and one species of Cyperaceae in Hong Kong

Samples of standing senescent culms of Panicum maximum, Pennisetum purpureum, Phragmites australis, Miscanthus floridulus, Saccharum arundinaceum and Thysanolaena maxima (Gramineae), and Schoenoplectus litoralis (Cyperaceae) were collected in Hong Kong between 1997 to 1999. A total of 205 fungal taxa were identified on these samples, including 61 ascomycetes, and 144 mitosporic taxa. Common fungal genera included Diaporthe, Leptosphaeria, Massarina, Ophiobolus and Ophioceras (ascomycetes), and Monodictys, Phaeoisaria, Periconia, Phoma, Phomopsis, Rhinocladiella, Septoria and Sporidesmium (mitosporic taxa). Different grass species were host to different fungal communities and diversities of taxa. Diversity indices for fungi on the hosts varied from 3.3 to 8.7, the highest index being from Pennisetum purpureum, and were overall higher from species offering more durable, strongly sclerenchymatic substrates. No single saprobic fungus collected in this study is thought to be specific to any one grass, however, certain fungi tended to reoccur on single grass species, but not on adjacent grasses. A ‘core fungal group’ was commonly associated with the decaying grasses and this ‘core’ was thought to be important in nutrient cycling in the grasses. A comparison of the fungi occurring on grasses with those on other monocotyledonous hosts is made. The numbers of fungi known to occur on Juncus roemerianus and Phragmites australis are briefly summarised.

Leaching requirement and salinity threshold for the yield and agronomic characteristics of halophytes under salt stress

An experiment was carried out at Nahshala Farm, north west of Al-Ain City, U.A.E. during the 1998–2000 growing seasons, using six halophytes (Batis maritima, Distichlis spicata, Juncus roemerianus, Paspalum vaginatum, Salicornia bigelovii and Spartina alterniflora) and two levels of leaching fraction (0·25 and 0·50) under three irrigation salinity levels (10, 20, and 40 gL−1) in a randomized complete block design arranged in split plots. The purpose of the research was to apply the theory of crop salt tolerance on aboveground yields and agronomic characteristics of halophytes to determine their thresholds for salinity and to determine leaching requirements. The results indicated that the halophyte species tested can grow with minimum reduction in the growth potential at <20 gL−1of mean salinity of soil solution. Leaching fraction (LF) of 0·25 at the highest salinity of irrigation water (40 gL−1) was inadequate to attain the steady-state salt balance during the growth period, although the drainage salinity reached more than 90 g L−1. Furthermore, if the same level of LF is used for longer period, soil salinity under this high salt treatment will continue to rise, and plant growth may deteriorate. Leaching fraction of 0·50 is preferable if salinity of irrigation water was more than 20 gL−1and dry matter production is considered, although the amount of water use will be excessive

Rapid shoreward encroachment of salt marsh cordgrass in response to accelerated sea-level rise.

The distribution of New England salt marsh communities is intrinsically linked to the magnitude, frequency, and duration of tidal inundation. Cordgrass (Spartina alterniflora) exclusively inhabits the frequently flooded lower elevations, whereas a mosaic of marsh hay (Spartina patens), spike grass (Distichlis spicata), and black rush (Juncus gerardi) typically dominate higher elevations. Monitoring plant zonal boundaries in two New England salt marshes revealed that low-marsh cordgrass rapidly moved landward at the expense of higher-marsh species between 1995 and 1998. Plant macrofossils from sediment cores across modern plant community boundaries provided a 2,500-year record of marsh community composition and documented the migration of cordgrass into the high marsh. Isotopic dating revealed that the initiation of cordgrass migration occurred in the late 19th century and continued through the 20th century. The timing of the initiation of cordgrass migration is coincident with an acceleration in the rate of sea-level rise recorded by the New York tide gauge. These results suggest that increased flooding associated with accelerating rates of sea-level rise has stressed high-marsh communities and promoted landward migration of cordgrass. If current rates of sea-level rise continue or increase slightly over the next century, New England salt marshes will be dominated by cordgrass. If climate warming causes sea-level rise rates to increase significantly over the next century, these cordgrass-dominated marshes will likely drown, resulting in extensive losses of coastal wetlands.

Vegetation succession and carbon sequestration in a coastal wetland in northwest Florida: Evidence from carbon isotopes

Measurements of stable carbon isotopic ratios as well as carbon (C), nitrogen (N), and phosphorus (P) contents in soils and plants were made along a chronovegetation sequence stretching from high marsh to low marsh in a coastal wetland in northwest Florida. The wetland is dominated by Juncus roemerianus , which is a C3 plant and has an average δ13C of −27‰. Lesser amounts of other species, including C4 plants, are also present in the area. The δ13C values of soil organic matter from low and middle marshes range from −24 to −27 ‰, which are consistent with the current plant community. However, the δ13C values of soil organic matter from high marsh show significant variations, from −23‰ in the surface soil to −17‰ at depth. This large C isotopic variation within soil profiles indicates a shift in local vegetation, from a C4‐dominated community to the current C3 plant‐dominated marsh, as a result of landward expansion of the wetland due to sea level rise. Radiocarbon dates on soil organic matter indicate that this ecological change occurred in the past hundred years or so as a result of sea level rise presumably due to global warming. Soil organic carbon inventory was ∼29 ± 3.6 kg m−2 in low marsh (the oldest part of the wetland), 15 ± 3.6 kg m−2 in middle marsh, and 13 ± 6.0 kg m−2 in high marsh (the youngest and most inland part of the wetland). N and P inventories are also higher in low marsh than in high marsh and seem to correlate directly with aboveground productivity in the marshes. The much higher C storage in low marsh than in high marsh indicates that carbon sequestration increased significantly as coastal wetland evolves from high marsh (initial stage) to low marsh (steady state). This has important implications to the global C cycle. As sea level rises owing to global warming, coastal wetlands are expected to expand landward in many areas where topography is gentle, which would provide a significant sink for atmospheric carbon dioxide.

Fungi on Juncus roemerianus : new coelomycetes with notes on Dwayaangam junci

Pycnodallia dupla gen. sp. nov. and Hymenopsis chlorothrix sp. nov. are described from saltmarsh Juncus roemerianus in North Carolina. Conidiomata of P. dupla develop on senescent inflorescences and involucral leaves, between 40 and 60 cm above the rhizome, therefore, the species can be considered as facultative marine. Conidiomata of H. chlorothrix are found on dead culms, 16–43 cm above the rhizome. This species is classified as obligately marine. Dwayaangam junci , the anamorph of Orbilia junci , has been found for the first time in nature. Conidia develop on the medulla of leaves, in lesions caused by grasshoppers.

Fungal biomass and productivity in standing-decaying leaves of black needlerush (Juncus roemerianus)

Ascomycetous fungal decomposers generate the major part of the microbial biomass on and in standing naturally decaying shoots for one (smooth cordgrass, Spartina alterniflora ) of the two major ramet-forming marsh plants of the south-eastern USA. Evidence from frequency of occurrence of sexual structures (ascomata) in the second major ramet-forming marsh plant (black needlerush, Juncus roemerianus ) suggests that fungi are major secondary producers in the needlerush decomposition system also. To test this, samples of needlerush blades at three stages of decay were collected seasonally for three years. Average living-fungal standing crop (as ergosterol) of needlerush blades (254 µ g g – 1 organic mass of decay system) was ~65%of a comparable average for smooth cordgrass, but fungal mass was equally active between the two species (near 30 pmol acetate incorporated into ergosterol µ g – 1 ergosterol h –1 ). Although maximum living-fungal crop for needlerush was found in winter, and in winter and spring for smooth cordgrass, in many other respects, patterns of change in needlerush fungal variables were quite different from those for smooth cordgrass. For example, living-fungal standing crop rose 1.5-fold with decay stage for needlerush, but was constant for cordgrass, perhaps because of lesser mycophagous activity and lesser competition from bacterioplankton associated with tidal flooding in the high-marsh, needlerush zones.

Host-specific ecotype diversity of rhizoplane diazotrophs of the perennial glasswort Salicornia virginica and selected salt marsh grasses

The degree of host specificity of most plant root associated bacteria is poorly under- stood. In this study we examined the physiological diversity of oxygen utilizing, culturable dia- zotrophs from the rhizoplane of the high marsh perennial glasswort Salicornia virginica and com- pared them to diazotrophs from other salt marsh plants (tall and short Spartina alterniflora, Juncus roemerianus, and Spartina patens) from the same ecosystem. Forty-six pure culture strains were recovered from the rhizoplane of S. virginica by stab inoculating freshly collected roots into com- bined nitrogen-free semi-solid media, followed by streak plating of clonal outgrowths. The majority of these strains were Gram-negative obligately aerobic or microaerophilic rods, but 3 Gram-positive strains were also isolated and characterized. API 20NE test strips were used for preliminary charac- terization of all strains, yielding 22 physiologically different API strain groups. One representative strain was selected from each API group and tested for the presence of nifH, denoting strains capa- ble of N2-fixation. Seventeen strains (14 Gram-negative, 3 Gram-positive) were nifH-positive and were characterized further using BIOLOG test plates. Four well-supported strain clusters were iden- tified by bootstrapped cluster analysis of the BIOLOG substrate utilization profiles. These clusters differed in utilization of carbohydrates, carboxylic acids, and amino acids. S. virginica diazotrophs were physiologically quite different from rhizoplane diazotrophs from the low marsh plants S. alterni- flora and J. roemerianus, but much more similar to diazotrophs from another high marsh plant, S. patens. We hypothesize that the observed physiological differentiation between high marsh and low marsh diazotrophs reflects differences in selection pressures in the rhizoplane microenvironment produced by plants with differing abilities to ventilate the rhizosphere. In addition, high and low marsh branches were further resolved into host-specific strain clusters, which also implies a strong impact of other host features, such as the suite of carbon exudate compounds produced, on the dis- tributions of specific diazotroph strains. These findings imply endemic, host-specific distributions of salt marsh diazotrophs and are consistent with the great diversity of diazotrophs that have been observed in this ecosystem to date.

Effects of irrigation water salinity and leaching fraction on the growth of six halophyte species

A pot experiment was carried out at Nahshala Farm, about 50 km from Al-Ain, UAE, during the 1998/99 growing seasons, using six halophytes: Spartina sp., Distichlis palmeri, Paspalum vaginatum, Juncus roemerianus, Salicornia bigelovii and Batis maritima, under two levels of leaching fraction, 0·25 and 0·50 and three levels of irrigation salinity, 10, 20 and 40 g/l. The objectives of the experiment were twofold: (1) to find out the optimum and threshold of saline water irrigation to keep salinity level down as much as possible in the soil using the leaching fraction technique; and (2) to study the response (growth and biomass production) of some halophytes to different levels of salinity. The experiment was conducted in triplicate with a split-plot design arranged in a randomized complete block. Results indicate that these halophyte species can be grown productively at a leaching fraction between 0·25 and 0·50 when salinity of the irrigation water is less than 20 g/l. At higher salinities, Salicornia bigelovii can grow and yield satisfactorily under these conditions, while the other species may require more frequent irrigation at higher leaching fractions. Some of these tested halophytes may be able to revegetate the salt-affected lands and be a potential source of forage in these harsh habitats. This study supports the idea of seawater agriculture by demonstrating the possibility of using some high salt-tolerant halophytes at relatively higher leaching fraction in order to maintain satisfactory yield production of such halophytes.

Microdiversity of Culturable Diazotrophs from the Rhizoplanes of the Salt Marsh Grasses Spartina alterniflora and Juncus roemerianus

Salt marshes dominated by Spartina alterniflora (smooth cordgrass) are among the most productive ecosystems known, despite nitrogen limitation. Rhizoplane/rhizosphere diazotrophy (nitrogen fixation) serves as a significant source of combined nitrogen in these systems. Several recent studies have demonstrated remarkable physiological and phylogenetic macro- and microdiversity within this important functional group of organisms. However, the ecological significance of this diversity is presently unknown. The physiological characteristics of the culturable, oxygen-utilizing fraction of the rhizoplane diazotroph assemblages from Spartina alterniflora and from another salt marsh grass, the black needle rush Juncus roemerianus, were examined in combination with an assessment of the phylogenetic relatedness by whole genome DNA-DNA hybridization. Analysis of substrate utilization data permitted quantitative evaluation of fully cross-hybridizing strain groups and physiological clusters. Phylogenetically related strains, defined by DNA homology >/=90% relative to the positive control, displayed extensive physiological diversity. Seven bootstrap-supported physiological clusters, composed largely of phylogenetically dissimilar strains, showed similar utilization patterns for at least one class of ecologically relevant substrates (carbohydrates, carboxylic acids, or amino acids). These diazotrophs appear to be physiologically adapted for utilization of specific substrates or classes of substrates, lending support to diazotrophic functional redundancy. Microenvironmental heterogeneity is credited for promoting this diversity by selecting for physiologically specialized diazotroph populations to occupy defined niches in situ. One outcome of this physiological diversity is maintenance of a crucial environmental function (nitrogen fixation) over a broad range of environmental conditions.

Fungi on Juncus roemerianus 14. Three New Coelomycetes, including Floricola, anam.-gen. nov.

Floricola striata anam.-gen. et sp. nov. is described from senescent inflorescences, culms and leaves, Septoriella unigalerita anam.-sp. nov. and Stagonospora abundata anam.-sp. nov. from senescent standing leaves of Juncus roemerianus at the U. S. east coast. Floricola striata and Stagonospora abundata are facultative marine species from the base and middle of the plant, whereas Septoriella unigalerita can be classified as a terrestrial, but halotolerant species, growing in the middle and tip of the leaves.

Effects of Increased Inundation and Wrack Deposition on a High Salt Marsh Plant Community

Salt marshes respond to both slowly increasing tidal inundation with sea level rise and abrupt disturbances, such as storm-induced wrack deposition. The effects of inundation pattern and wrack deposition have been studied independently but not in combination. We manipulated inundation of tidal creek water and wrack presence individually and in combination, in two neighboring communities within a Virginia high salt marsh during 1994 and 1995. The effects of these manipulations were assessed by measurements of aboveground plant biomass. Altered inundation by itself produced little response in the various categories of plant biomass measured. Wrack deposition affected all species (i.e., Juncus roemerianus, Spartina patens, and Distichlis spicata) showing a significant reduction in aboveground biomass, as expected. Recovery after wrack deposition was dependent on the species. S. patens and D. spicata recovered from wrack deposition within one growing season, while J. roemerianus did not. Because the effects of wrack deposition greatly exceeded those of experimentally increased inundation, the possible interactions between the two were masked. Increased inundation may have inhibited the colonization of bare areas by some species after the removal of wrack from an area, although statistical significance at α=0.01 was not reached. Our results confirm that wrack deposition can cause the redistribution of species within the high marsh community. Altered inundation may have a greater effect on the re-establishment of the plant community after wrack deposition than it does without wrack deposition.

Silica Content of a Mesohaline Tidal Marsh in North Carolina

Studies of silicon (Si) cycling in estuarine systems have primarily focused on terrestrial inputs, benthic fluxes and uptake and recycling of Si by diatoms. However, intertidal marsh vegetation, which often covers a large area and contributes a high biomass to estuarine systems, also contains Si. Silica contents of two marsh plants, Spartina alterniflora and Juncus roemerianus, were measured in a mesohaline tidal marsh in the Cape Fear River estuary, North Carolina. Plant silica contents in the two species were significantly different in three age categories with new culms having the lowest and dead culms having the highest silica content. Mean silica contents in S. alterniflora were 4·1, 5·2 and 7·8mgg−1dry biomass in new, mature and dead culms, respectively. Mean silica contents in J. roemerianus were 0·67, 4·2 and 9·2mg−1dry biomass in new, mature and dead culms, respectively. Porewater dissolved silicic acid (DSi) was measured at three depths (0–10, 11–20 and 21–30cm) and was lowest in the upper 10cm. Mean porewater DSi concentrations were 198μM from January to May and 295μM from June to September. Sediment biogenic silica content (BSi) was 0·76 and 1·0% in J. roemerianus and S. alterniflora vegetative zones, respectively. Biomass estimates were used to obtain an estimate of the amorphous silica inventory of the marsh. Vascular plants and sediments contained approximately 7·1gSiO2m−2and 82·0gSiO2m−2, respectively. These data indicate that intertidal marshes may be an important reservoir of Si in estuarine systems.

Fungi on Juncus roemerianus 12. Two New Species of Mycosphaerella and Paraphaeosphaeria (Ascomycotina)

Mycosphaerella eurypotami sp. nov. and Paraphaeosphaeria apicicola sp. nov. are described from senescent leaves of the endemic needlerush, Juncus roemerianus, an estuarine plant of the U. S. east coast and the Gulf of Mexico. Whereas M. eurypotami fruits in the middle and tip of the leaves, P. apicicola appears only in the apices. Both species are exposed to extreme conditions, such as salt spray and rain, freezing in the winter and subtropical summer temperatures, and alternations between wetting and drying. So far, M. eurypotami has been collected in North Carolina and Virginia, and P. apicicola in North Carolina. Mycosphaerella eurypotami is compared with Monascostroma innumerosum and Mycosphaerella juncellina, and P. apicicola with other Paraphaeosphaeria species.

Nitrogen effects on an interaction chain in a salt marsh community.

Nutrients can structure communities by influencing both plant interactions and plant herbivore interactions, though rarely do studies integrate these processes. In this study we examined how nitrogen fertilization influenced (1) the positive interaction between the marsh elder, Iva frutescens, and the black rush, Juncusgerardi, and (2) the quality of Iva as a host plant for the aphid, Uroleuconambrosiae. Previous studies have shown that by mitigating soil salt accumulation and hypoxia, Juncus is essential to the survival of Iva and its aphid herbivore at mid-marsh elevations. To address the effects of nitrogen on this interaction, we compared fertilized and unfertilized Iva plants subject to Juncus removal and control treatments in the field. Additionally, we measured the monthly population growth rates of aphids transplanted onto these Iva plants. Iva leaf biomass and flower number results indicated that fertilizing Iva eliminated its dependence upon Juncus, such that fertilized plants grown without Juncus were not different from unmanipulated plants. Aphid monthly population growth rates through mid-summer revealed that fertilization also eliminated the indirect dependency of aphids on Juncus, so that aphid growth rates on fertilized Iva without Juncus neighbors were similar to rates on unmanipulated Iva. Results also indicated that fertilizing Iva grown with Juncus increased Iva size, potentially enabling these plants to support larger aphid populations. Our results suggest that only under conditions of nitrogen limitation are the positive effects of Juncus essential to the mid-marsh persistence of Iva and its aphid herbivore. Furthermore, we found that nitrogen effects on aphid populations may arise not only from a direct effect of nutrients on Iva size but also through the indirect effects of nitrogen on the interaction between Juncus and Iva. We argue that studies integrating processes occurring both within and between trophic levels, are important to fully understanding the community-wide effects of nutrients.

Physiological diversity of the rhizosphere diazotroph assemblages of selected salt marsh grasses.

Rhizosphere diazotroph assemblages of salt marsh grasses are thought to be influenced by host plant species and by a number of porewater geochemical parameters. Several geochemical variables can adversely affect plant productivity and spatial distributions, resulting in strong zonation of plant species and growth forms. This geochemically induced stress may also influence the species compositions and distributions of rhizosphere diazotroph assemblages, but little is currently known about these organisms. The diversity and key physiological features of culturable, O2-tolerant rhizosphere diazotrophs associated with the tall and short growth forms of Spartina alterniflora and with Juncus roemerianus were examined. A total of 339 gram-negative strains were isolated by a root stab culture approach and morphologically and physiologically characterized by using API and BIOLOG tests. Eighty-six distinct groups composed of physiologically similar strains were identified. Of these groups, 72% were shown to be capable of N2 fixation through molecular analyses, and a representative strain was chosen from each diazotroph group for further characterization. Cluster and principal-components analysis of BIOLOG data allowed the designation of physiologically distinct strain groupings. Most of these groups were dominated by strains that were not identifiable to species on the basis of API or BIOLOG testing. Representatives of several families including the Enterobacteriaceae, Vibrionaceae, Azotobacteraceae, Spirillaceae, Pseudomonadaceae, and Rhizobiaceae were recovered, as well as strains with no clear taxonomic affiliations. This study identifies numerous potentially important physiological groups of the salt marsh diazotroph assemblage.