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.     

How will warming affect the salt marsh foundation species Spartina patens and its ecological role?

Foundation species structure environments and create refuge from environmental stress. In New England high salt marsh, the grass Spartina patens is a foundation species that reduces salinity, anoxia, desiccation, and thermal stresses through canopy shading and root proliferation. In a factorial S. patens-removal and warming field experiment, foundation species removal strongly impacted every aspect of the community, reiterating the important role of the foundation species S. patens in the high marsh. Given this central role, we hypothesized that facilitation by the foundation species would be even more important under warmer conditions by ameliorating more severe thermal stress. However, the ecological role of S. patens was unaffected by experimental warming, and, independent of the presence of the foundation species, warming had only weak effects on the salt marsh ecological community. Only the foundation species itself responded strongly to warming, by significantly increasing aboveground production in warmed plots. Apparently, amelioration of thermal stress is not as important for salt marsh ecosystem function as S. patens’ moderation of salinity and desiccation stresses. From these experimental results, we anticipate that climate change-associated thermal stress will not greatly affect S. patens-dominated high marsh communities. In contrast, foundation species loss, an emergent conservation issue in Atlantic salt marshes, represents a critical threat to salt marsh ecosystem function.     

Warming increases plant biomass and reduces diversity across continents,latitudes, and species migration scenarios in experimental wetland communities

Atmospheric warming may influence plant productivity and diversity and induce poleward migration of species, altering communities across latitudes. Complicating the picture is that communities from different continents deviate in evolutionary histories, which may modify responses to warming and migration. We used experimental wetland plant communities grown from seed banks as model systems to determine whether effects of warming on biomass production and species richness are consistent across continents, latitudes, and migration scenarios. We collected soil samples from each of three tidal freshwater marshes in estuaries at three latitudes (north, middle, south) on the Atlantic coasts of Europe and North America. In one experiment, we exposed soil seed bank communities from each latitude and continent to ambient and elevated (+2.8 °C) temperatures in the greenhouse. In a second experiment, soil samples were mixed either within each estuary (limited migration) or among estuaries from different latitudes in each continent (complete migration). Seed bank communities of these migration scenarios were also exposed to ambient and elevated temperatures and contrasted with a no‐migration treatment. In the first experiment, warming overall increased biomass (+16%) and decreased species richness (?14%) across latitudes in Europe and North America. Species richness and evenness of south‐latitude communities were less affected by warming than those of middle and north latitudes. In the second experiment, warming also stimulated biomass and lowered species richness. In addition, complete migration led to increased species richness (+60% in North America, + 100% in Europe), but this higher diversity did not translate into increased biomass. Species responded idiosyncratically to warming, but Lythrum salicaria and Bidens sp. increased significantly in response to warming in both continents. These results reveal for the first time consistent impacts of warming on biomass and species richness for temperate wetland plant communities across continents, latitudes, and migration scenarios.     

Outline of A New Approach to Evaluate Ecological Integrity of Salt Marshes

The integrity of coastal salt marshes can be determined from the extent to which they provide key ecosystem services: food and habitat for fish and wildlife, good water quality, erosion and flood control, and recreation and cultural use. An outline of a new approach for linking ecosystem services with metrics of structure and function to evaluate the ecological integrity of salt marshes is described. One main objective of the approach is to determine whether differences in structure and function can be detected among salt marshes with similar geomorphology and hydrology but different degrees of anthropogenic stress. The approach is currently being applied to salt marshes of Narragansett Bay, RI, USA. Stable nitrogen isotopic ratios of the marsh biota reflected the nitrogen sources from the adjacent watersheds and were significantly correlated with percent residential land use. Results show that plant zonation significantly ( r = —0.82; p < 0.05) relates with percent residential land use and is potentially a sensitive indicator of anthropogenic disturbance of New England salt marshes. We are currently examining species diversity, denitrification rates, and susceptibility to erosion among the sites for additional indicators of salt marsh condition. Our results to date suggest that this approach will provide the methods needed for managers to systematically monitor and evaluate the integrity of salt marshes     

Outline of A New Approach to Evaluate Ecological Integrity of Salt Marshes

The integrity of coastal salt marshes can be determined from the extent to which they provide key ecosystem services: food and habitat for fish and wildlife, good water quality, erosion and flood control, and recreation and cultural use. An outline of a new approach for linking ecosystem services with metrics of structure and function to evaluate the ecological integrity of salt marshes is described. One main objective of the approach is to determine whether differences in structure and function can be detected among salt marshes with similar geomorphology and hydrology but different degrees of anthropogenic stress. The approach is currently being applied to salt marshes of Narragansett Bay, RI, USA. Stable nitrogen isotopic ratios of the marsh biota reflected the nitrogen sources from the adjacent watersheds and were significantly correlated with percent residential land use. Results show that plant zonation significantly ( r = —0.82; p < 0.05) relates with percent residential land use and is potentially a sensitive indicator of anthropogenic disturbance of New England salt marshes. We are currently examining species diversity, denitrification rates, and susceptibility to erosion among the sites for additional indicators of salt marsh condition. Our results to date suggest that this approach will provide the methods needed for managers to systematically monitor and evaluate the integrity of salt marshes     

The dynamics of bottom–up and top–down control in a New England salt marsh

Traditionally, salt marsh ecosystems were thought to be controlled exclusively by bottom–up processes. Recently, this paradigm has shifted to include top–down control as an additional primary factor regulating salt‐marsh community structure. The most recent research on consumer impacts in southern US marshes has shown that top–down forces often interact with biotic and abiotic factors, such as secondary fungal infection in grazer‐induced wounds, soil nutrients and climatic variation, to influence ecosystem structure. In a more northern salt marsh, located in New England, we examined the separate and interactive effects of nutrient availability, insect herbivory and secondary fungal infection, on growth of the foundation species, Spartina alterniflora. We used a factorial design with two levels of nutrients (control and addition) insects (control and removal) and fungi (control and removal). Nutrient addition increased plant biomass by 131% in the absence of herbivores. When insect consumers were allowed access to fertilized plots, biomass was reduced by nearly 45% when compared with treatments with nutrients and insecticide. In contrast, insect herbivores did not affect plant biomass in unfertilized control treatments. These differences suggest that consumer effects are triggered under high nutrient levels only. We also found that secondary fungal infections in grazer‐induced wounds, in contrast to lower latitude marshes, did not significantly impact primary production. Our results suggest that while New England salt marshes may typically be under bottom–up control, eutrophication can trigger dual control with inclusion of top–down regulation. However, unlike lower latitude marshes, consumer control of plant growth in northern US salt marshes is not dependent on herbivores facilitating fungal infections that then control grass growth, suggesting that the intensity of disease mediated top–down control by small grazers may be regulated by climate and/or grazer identity that co‐vary with latitude.     

Migration, invasion and decline: changes in recruitment and forest structure in a warming-linked shift of European beech forest in Catalonia (NE Spain)

Altitudinal upward shifts of species' ranges have occurred across a wide range of taxonomic groups and geographical locations during the twentieth century in response to current climate warming. However, actual data of plant species' altitudinal shifts are still scarce and not always clear. Here we provide a more detailed investigation of a previously reported European beech Fagus sylvatica forest altitudinal shift in the Montseny Mountains (Catalonia, NE Spain) now based on field photographic survey and on the population age structure and the recruitment patterns in the high Fagus limit (HFL), the central forest area (CFA) and the low Fagus limit (LFL). Monitoring of the lowest altitudinal range shows that beech forest is being progressively replaced by Mediterranean holm oak forest. Holm oaks are characterized by recruitment rates more than three times higher than those of beech in the LFL in the last decades. The percentage of young individuals in the LFL is only half that in the HFL and CFA. In the highest altitudinal range, present day and early 20th century photographs show that the HFL has gained density and has shifted altitudinally upwards, advancing with establishment of new, vigorous outpost trees (13 individuals per each 100 m of tree-line). They are mostly (89%) younger than 35 yr old and mostly (97%) located up to 70  m (with a few up to 105 m) ground surface distance above the current tree line (36–51 m altitude) at the highest altitudes (1600–1700 m). The beech forest upward shift is a likely consequence of warming, but land-use practice changes (cessation of burning by shepherds) have made it possible. These changes in vegetation distribution and population structure constitute a new indication of the complex global change effects on life in mountain ecosystems.     

Seventy years of continuous encroachment substantially increases ‘blue carbon’ capacity as mangroves replace intertidal salt marshes

Shifts in ecosystem structure have been observed over recent decades as woody plants encroach upon grasslands and wetlands globally. The migration of mangrove forests into salt marsh ecosystems is one such shift which could have important implications for global ‘blue carbon’ stocks. To date, attempts to quantify changes in ecosystem function are essentially constrained to climate‐mediated pulses (30 years or less) of encroachment occurring at the thermal limits of mangroves. In this study, we track the continuous, lateral encroachment of mangroves into two south‐eastern Australian salt marshes over a period of 70 years and quantify corresponding changes in biomass and belowground C stores. Substantial increases in biomass and belowground C stores have resulted as mangroves replaced salt marsh at both marine and estuarine sites. After 30 years, aboveground biomass was significantly higher than salt marsh, with biomass continuing to increase with mangrove age. Biomass increased at the mesohaline river site by 130 ± 18 Mg biomass km^?2 yr^?1 (mean ± SE), a 2.5 times higher rate than the marine embayment site (52 ± 10 Mg biomass km^?2 yr^?1), suggesting local constraints on biomass production. At both sites, and across all vegetation categories, belowground C considerably outweighed aboveground biomass stocks, with belowground C stocks increasing at up to 230 ± 62 Mg C km^?2 yr^?1 (± SE) as mangrove forests developed. Over the past 70 years, we estimate mangrove encroachment may have already enhanced intertidal biomass by up to 283 097 Mg and belowground C stocks by over 500 000 Mg in the state of New South Wales alone. Under changing climatic conditions and rising sea levels, global blue carbon storage may be enhanced as mangrove encroachment becomes more widespread, thereby countering global warming.     

Salt marshes along the coast of The Netherlands

The area of salt marshes does no longer increase. The recent erosion coincides with a rise in MHT-level in the last 25 years. Despite the decrease in area, sedimentation continues, especially in the lower salt marsh, which acts as a sink of nitrogen. Assimilation and mineralization of nitrogen are in balance in most plant communities along the gradient from lower to higher salt marshes. Mineralization of nitrogen increases towards the higher salt marsh, whereas the above-ground production and the mean nitrogen content of plants decrease. There is a positive correlation between quality of food plants in salt marshes and breeding success of Brent geese in the arctic tundra. Sedimentation on mainland salt marshes can compensate for the expected sea level rise. This is not the case for island salt marshes, if the relative sea level rise is more than 0.5–1.0 cm yr⁻¹. The natural succession on salt marshes results in an accumulation of organic material, which is related to the dominance of single plant species. It is not clear to which extent this process is enhanced by eutrophication from acid deposition and seawater. Human exploitation of unprotected salt marshes is old and heavy in the system of mound settlements. Reclamation rates by dikes in the last centuries were higher than the rate of area increase. Grazing by cattle as a management practice results in both a higher plant species-richness and community diversity than abandoning; hay-making is intermediate, but shows less structural diversity than grazing with low stocking density. The invertebrate fauna is favoured by a short period of abandoning, but eventually characteristic salt marsh invertebrates are replaced by inland species. Many bird species prefer grazed salt marshes. The final section gives some perspectives. Provided that no further embankments take place the optimal nature management option for plants and animals is a vegetation pattern, which includes areas with a low canopy (grazed) and areas with a tall canopy.     

Comparison of adult mosquito community structure on various habitats

The community structure of adult mosquitoes was compared from New Jersey light trap collections in six different types of habitats in Citrus County, Florida, USA. From October 1998 to December 2000, mosquitoes were collected three times a week from the following habitats (swamps, swamps and freshwater marshes, pine fiat-woods, pine fiat-woods and scrub, salt marshes, and salt marshes and mangroves). Mosquito density was highest in the swamps and freshwater marshes habitat, with an average of 95.65 specimens per trap.Density was lowest in the flatwoods and scrub habitat, with an average of 14.38 specimens per trap. Species dominance differed among habitats. Salt marshes produced the greatest aggregation index, while pine flatwoods produced the lowest. Conversely, diversity analysis showed that pine flatwoods had the greatest diversity, while salt marshes the lowest diversity. Similarity indices indicated that the adult mosquito communities from pine flatwoods and pine flatwoods and scrub were very similar (0.8583). The adult mosquito community of salt marshes was different from that of swamps and freshwater marshes (the similar index was 0.0217).     

Bird diversity and environmental gradients in Britain: a test of the species–energy hypothesis

1.  We tested the species diversity–energy hypothesis using the British bird fauna. This predicts that temperature patterns should match diversity patterns. We also tested the hypothesis that the mechanism operates directly through effects of temperature on thermoregulatory loads; this further predicts that seasonal changes in temperature cause matching changes in patterns of diversity, and that species' body mass is influential.
2.  We defined four assemblages using migration status (residents or visitors) and season (summer or winter distribution). Records of species' presence/absence in a total of 2362, 10 × 10-km, quadrats covering most of Britain were used, together with a wide selection of habitat, topographic and seasonal climatic data.
3.  We fitted a logistic regression model to each species' distribution using the environmental data. We then combined these individual species models mathematically to form a diversity model. Analysis of this composite model revealed that summer temperature was the factor most strongly associated with diversity.
4.  Although the species–energy hypothesis was supported, the direct mechanism, predicting an important role for body mass and matching seasonal patterns of change between diversity and temperature, was not supported.
5.  However, summer temperature is the best overall explanation for bird diversity patterns in Britain. It is a better predictor of winter diversity than winter temperature. Winter diversity is predicted more precisely from environmental factors than summer diversity.
6.  Climate change is likely to influence the diversity of different areas to different extents; for resident species, low diversity areas may respond more strongly as climate change progresses. For winter visitors, higher diversity areas may respond more strongly, while summer visitors are approximately neutral.     

Mean conditions predict salt marsh plant community diversity and stability better than environmental variability

Environmental variability and the frequency of extreme events are predicted to increase in future climate scenarios; however, the role of fluctuations in shaping community composition, diversity and stability is not well understood. Identifying current patterns of association between measures of community stability and climatic means and variability will help elucidate the ways in which altered variability and mean conditions may change communities in the future. Salt marshes provide essential ecosystem services and are increasingly threatened by sea‐level rise, land‐use change, eutrophication and predator loss, yet the effects of temporal environmental variation on salt marshes remain unknown. We synthesized long‐term plant community monitoring data from 11 sites on both coasts of the United States. We used an information‐theoretic approach and linear models to determine the associations among long‐term mean conditions, interannual environmental variability, and plant community stability and diversity. We found that salt marsh community stability and diversity were more strongly related to long‐term means of temperature and precipitation than to interannual variation. Warm and wet environments had fewer species and less turnover among years. Our results suggest that communities in cool, dry environments may be more resilient to climate warming due to greater species richness and turnover. Mean conditions are sufficient to predict contemporary patterns of salt marsh plant community dynamics, but environmental variability may have stronger impacts as it increases with climate change.     

Effects of regular salt marsh haying on marsh plants, algae, invertebrates and birds at Plum Island Sound, Massachusetts

The haying of salt marshes, a traditional activity since colonial times in New England, still occurs in about 400 ha of marsh in the Plum Island Sound estuary in northeastern Massachusetts. We took advantage of this haying activity to investigate how the periodic large-scale removal of aboveground biomass affects a number of marsh processes. Hayed marshes were no different from adjacent reference marshes in plant species density (species per area) and end-of-year aboveground biomass, but did differ in vegetation composition. Spartina patens was more abundant in hayed marshes than S. alterniflora, and the reverse was true in reference marshes. The differences in relative covers of these plant species were not associated with any differences between hayed and reference marshes in the elevations of the marsh platform. Instead it suggested that S. patens was more tolerant of haying than S. alterniflora. Spartina patens had higher stem densities in hayed marshes than it did in reference marshes, suggesting that periodic cutting stimulated tillering of this species. Although we predicted that haying would stimulate benthic chlorophyll production by opening up the canopy, we found differences to be inconsistent, possibly due to the relatively rapid regrowth of S. patens and to grazing by invertebrates on the algae. The pulmonate snail, Melampus bidendatus was depleted in its δ¹³C content in the hayed marsh compared to the reference, suggesting a diet shift to benthic algae in hayed marshes. The stable isotope ratios of a number of other consumer species were not affected by haying activity. Migratory shorebirds cue in to recently hayed marshes and may contribute to short term declines in some invertebrate species, however, the number of taxa per unit area of marsh surface invertebrates and their overall abundances were unaffected by haying over the long term. Haying had no impact on nutrient concentrations in creeks just downstream from hayed plots, but the sediments of hayed marshes were lower in total N and P compared to references. In sum, haying appeared to affect plant species composition but had only short-term affects on consumer organisms. This contrasts with many grassland ecosystems, where an intermediate level of disturbance, such as by grazing, increases species diversity and may stimulate productivity. From a management perspective, periodic mowing could be a way to maintain S. patens habitats and the suite of species with which they are associated.     

Changes to spotted turtle (Clemmys guttata) habitat selection in response to a salt marsh restoration

Wetlands Ecology and Management - Development of coastal New England has led to the replacement of up to 37% of salt marshes with degraded freshwater wetlands, primarily through tidal restrictions....     

New England Salt Marsh Recovery: Opportunistic Colonization of an Invasive Species and Its Non-Consumptive Effects

Predator depletion on Cape Cod (USA) has released the herbivorous crab Sesarma reticulatum from predator control leading to the loss of cordgrass from salt marsh creek banks. After more than three decades of die-off, cordgrass is recovering at heavily damaged sites coincident with the invasion of green crabs ( Carcinusmaenas ) into intertidal Sesarma burrows. We hypothesized that Carcinus is dependent on Sesarma burrows for refuge from physical and biotic stress in the salt marsh intertidal and reduces Sesarma functional density and herbivory through consumptive and non-consumptive effects, mediated by both visual and olfactory cues. Our results reveal that in the intertidal zone of New England salt marshes, Carcinus are burrow dependent, Carcinus reduce Sesarma functional density and herbivory in die-off areas and Sesarma exhibit a generic avoidance response to large, predatory crustaceans. These results support recent suggestions that invasive Carcinus are playing a role in the recovery of New England salt marshes and assertions that invasive species can play positive roles outside of their native ranges.     

An ecological 'footprint' of climate change

Recently, there has been increasing evidence of species' range shifts due to changes in climate. Whereas most of these shifts relate ground truth biogeographic data to a general warming trend in regional or global climate data, we here present a reanalysis of both biogeographic and bioclimatic data of equal spatio-temporal resolution, covering a time span of more than 50 years. Our results reveal a coherent and synchronous shift in both species' distribution and climate. They show not only a shift in the northern margin of a species, which is in concert with gradually increasing winter temperatures in the area, they also confirm the simulated species' distribution changes expected from a bioclimatic model under the recent, relatively moderate climate change.     

The natural regeneration of salt marsh on formerly reclaimed land

Question: Does the vegetation of restored salt marshes increasingly resemble natural reference communities over time? Location: The Essex estuaries, southeast England. Methods: Abandoned reclamations, where coastal defences had been breached in storm events, and current salt marsh recreation schemes were surveyed giving a chronosequence of salt marsh regeneration from 2 to 107 years. The presence, abundance and height of plant species were recorded and comparisons were made with adjacent reference salt marsh communities at equivalent elevations. Results: Of the 18 paired sites surveyed, 13 regenerated marshes had fewer species than their adjacent reference marsh, three had an equal number and two had more. The plant communities of only two de‐embankment sites matched that of the reference community. 0–50 year old sites and 51–100 year old sites had fewer species per quadrat than the 101+ year sites and the reference salt marshes. There was a weak relationship between differences in species richness for regenerated and reference marshes and the time since sites were first re‐exposed to tidal inundation. Cover values for the invasive and recently evolved Spartina anglica were greater within regenerated than reference marshes. Conclusions: Salt marsh plants will colonise formerly reclaimed land relatively quickly on resumption of tidal flooding. However, even after 100 years regenerated salt marshes differ in species richness, composition and structure from reference communities.     

Experimental predator removal causes rapid salt marsh die‐off

Salt marsh habitat loss to vegetation die‐offs has accelerated throughout the western Atlantic in the last four decades. Recent studies have suggested that eutrophication, pollution and/or disease may contribute to the loss of marsh habitat. In light of recent evidence that predators are important determinants of marsh health in New England, we performed a total predator exclusion experiment. Here, we provide the first experimental evidence that predator depletion can cause salt marsh die‐off by releasing the herbivorous crab Sesarma reticulatum from predator control. Excluding predators from a marsh ecosystem for a single growing season resulted in a >100% increase in herbivory and a >150% increase in unvegetated bare space compared to plots with predators. Our results confirm that marshes in this region face multiple, potentially synergistic threats.     

中国滨海盐沼湿地碳收支与碳循环过程研究进展

滨海盐沼湿地由于其较高的初级生产力和较缓慢的有机质降解速率而成为缓解全球变暖的有效蓝色碳汇,近年来引起全球范围内的热切关注.我国滨海盐沼湿地分布较广,国内学者对滨海盐沼湿地碳循环及碳收支研究取得了一定进展,深入研究滨海盐沼湿地碳循环有助于对全球碳循环及全球变化的理解,并为利用滨海湿地进行碳的增汇减排提供科学依据.主要从我国滨海盐沼湿地碳循环主要观测方法、碳收支与碳循环过程及特点、碳库的组成与影响因素、气态碳的输入输出、潮汐作用对其碳收支的影响这5个方面出发,对国内的滨海盐沼湿地碳循环与碳收支的研究进展进行了归纳总结,并对今后的研究方向给出如下建议:(1)加强滨海盐沼湿地土壤碳库在深度上和广度上的研究;(2)标准化滨海盐沼湿地碳储量、碳通量的量化方法和观测技术;(3)在研究尺度上要宏观、微观并重,同时加强长期原位监测湿地碳通量的变化与室内模拟研究;(4)量化在潮汐影响下滨海盐沼湿地碳与邻近生态系统之间的横向交换通量.只有对我国滨海盐沼湿地碳库收支进行更准确的评估和长期的碳库动态变化监测,方可进一步认识我国盐沼湿地对全球气候变化的影响及其反馈作用,这对于预测全球变化及制定湿地碳储备功能的提升策略具有重要的意义.     

Long‐term nutrient addition increases respiration and nitrous oxide emissions in a New England salt marsh

Salt marshes may act either as greenhouse gas (GHG) sources or sinks depending on hydrological conditions, vegetation communities, and nutrient availability. In recent decades, eutrophication has emerged as a major driver of change in salt marsh ecosystems. An ongoing fertilization experiment at the Great Sippewissett Marsh (Cape Cod, USA) allows for observation of the results of over four decades of nutrient addition. Here, nutrient enrichment stimulated changes to vegetation communities that, over time, have resulted in increased elevation of the marsh platform. In this study, we measured fluxes of carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) in dominant vegetation zones along elevation gradients of chronically fertilized (1,572 kg N ha^?1 year^?1) and unfertilized (12 kg N ha^?1 year^?1) experimental plots at Great Sippewissett Marsh. Flux measurements were performed using darkened chambers to focus on community respiration and excluded photosynthetic CO₂ uptake. We hypothesized that N‐replete conditions in fertilized plots would result in larger N₂O emissions relative to control plots and that higher elevations caused by nutrient enrichment would support increased CO₂ and N₂O and decreased CH₄ emissions due to the potential for more oxygen diffusion into sediment. Patterns of GHG emission supported our hypotheses. Fertilized plots were substantially larger sources of N₂O and had higher community respiration rates relative to control plots, due to large emissions of these GHGs at higher elevations. While CH₄ emissions displayed a negative relationship with elevation, they were generally small across elevation gradients and nutrient enrichment treatments. Our results demonstrate that at decadal scales, vegetation community shifts and associated elevation changes driven by chronic eutrophication affect GHG emission from salt marshes. Results demonstrate the necessity of long‐term fertilization experiments to understand impacts of eutrophication on ecosystem function and have implications for how chronic eutrophication may impact the role that salt marshes play in sequestering C and N.