Backfilling canals to restore wetlands: empirical results in coastal Louisiana

Wetland restoration is largely a developing science and engineering enterprise. Analyses of results are too few and constrained to observations over a few years. We report here on the effectiveness of one restoration technique used sparsely in coastal Louisiana for several decades. Canals have been dredged in coastal Louisiana wetlands since 1938 for oil and gas exploration and extraction. These canals are typically dredged to 2.5 m depth and are 20 to 40 m wide. Canal lengths vary from 100 m to several 1000s m in the case of outer continental shelf pipeline canals that cross the wetlands.Today, thousands of miles of canals crisscross these wetlands. Studies have linked dredged canals to a number of undesirable effects on the wetland environment including alterations in salinity, flooding and drainage patterns, direct loss of marsh by convention to open water, and increases in marsh erosion rates. These effects have led state and federal agencies charged with managing the wetland resource to look for methods of mitigating canal impacts. One possible method of managing spoil banks after the abandonment of a drilling site is to return spoil material from the spoil banks to the canal with the hope that marsh vegetation will be reestablished on the old spoil banks and in the canal. The movement of former spoil bank material back into the canal is referred to as backfilling.The purpose of this study was to (1) examine how backfilled canals changed over 10 years, (2) examine factors influencing success with multiple regression statistical models, and, (3) compare costs of backfilling with other Louisiana marsh restoartion projects. We examined the sites to document and interpret changes occurring since 1983/4 and to statistically model the combined data derived from these new and previous analyses. Specifically, we wanted to determine the recovery rates of vegetation, water depth, and soils in backfilled canals, restored spoil banks, and in nearby marshes, and to quantify the influence of plugging canals on these rates.The major factors determining backfilling restoration success are the depth of the canal, soil type, canal dimensions, locale, dredge operator skill, and permitting conditions. Plugging the canal has no apparent effect on water depth or vegetation cover, with the exception that submerged aquatic vegetation may be more frequently observed behind backfilled canals with plugs than in backfilled canals without plugs. Canal age, soil organic matter content, and whether restoration was done as mitigation on-site or off-site were the most important predictors of final canal depth. Canal length and percentage of spoil returned (+) had the greatest effect on the restoration of vegetation cover. Backfilled canals were shallower if they were older, in soils lower in organic matter, and backfilled off-site. Backfilling the canal restores wetlands at a cost of $1,200 to $3,400/ha, which compares very favorably with planned restoration projects in south Louisiana.Corresponding Editor: Anonumous     

Restoration of dredged canals in wetlands: a comparison of methods

A comparison of two methods for restoring dredged canals to wetlands was examined at the Jean Lafitte National Historical Park and Preserve’s Barataria Preserve Unit near New Orleans, LA. Both northern and southern canals had the remnant dredged spoil material returned to the canal, but the southern canal had additional sediment pumped in from a nearby lake. The water depth in the southern canal shallowed significantly from 1.2 to 0.4 m following backfilling and sediment addition, while the depth of the northern canal (which received no additional sediment) remained unchanged following backfilling. Neither site had complete soil restoration, but the former spoil areas of the northern canal showed greater restoration than the southern canal. The vegetation on the former spoil areas of the northern canal closely resembled that of the reference marsh, while the former spoil areas of the southern canal had species indicative of spoil banks and other elevated areas. After 3 years wetland vegetation was established on approximately 65% of the former spoil areas at both sites and 20–25% of the open water areas. Sediment addition to the southern canal raised costs by a factor of eight times compared to that of the northern canal. The results of this study document the restoration potential of both methods, but also show that backfilling without supplemental sediment additions can restore abandoned canals at a fraction of the cost of other methods.     

Ecological Design is More Important Than Compensatory Mitigation for Successful Restoration of Alpine Spoil Heaps

Spoil heaps of surplus rock from hydropower tunnel construction negatively impact alpine landscapes unless restored. Such spoil heaps have been created for more than 100 years, but we still lack knowledge about the relative importance of compensatory mitigation (seeding and fertilization), spoil‐heap construction method, local environmental factors, and regional climatic factors for restoration success. We studied the species composition of 19 alpine spoil heaps in Western Norway and their undisturbed surroundings using ordination and statistical modeling. Substrate grain size was the principal factor explaining differences in species composition between spoil heaps and their surroundings. Soil characteristics, that is, organic matter content and pH, and reutilization of topsoil were also important. Seeding and fertilization had negligible effects on restoration success. Slow recovery was observed for total vegetation cover and species richness of vascular plants and lichens while bryophyte cover recovered rapidly. Lower bryophyte cover and bryophyte and vascular plant species richness on older than on younger spoil heaps indicated recent changes in spoil‐heap construction practices that favor plant colonization. Our results indicate that spoil‐heap design is more important for restoration success than compensatory mitigation. We therefore suggest spoil heaps designed with a fine‐grained top substrate preferably from stockpiled local topsoil, with uneven surface topography that mimics natural topographic variation, and recommend discontinuation of seeding and fertilization.     

Unrealized Expectations for Restoration of a Floodplain Plant Community

The ongoing restoration of the channelized Kissimmee River is expected to promote reestablishment of the prolonged, deep inundation regimes that sustained broadleaf marsh as the dominant wetland plant community on the historical floodplain. The success of the restoration was evaluated at locations on the remnant floodplain where broadleaf marsh had been replaced by a mesophytic shrub community, and on the lower portion of the reconstructed floodplain, which was recreated by backfilling of a flood control canal and degradation of associated spoil mounds. During the 8‐year post‐restoration period (2001–2008) mean annual hydroperiods and depths on the restored floodplain were not significantly different from pre‐channelization hydrologic conditions at historical reference sites. Increased hydroperiods and depths eliminated the mesophytic shrub (primarily Myrica cerifera) and associated fern cover, and led to colonization of floating and mat‐forming species, but did not result in the reestablishment of a broadleaf marsh community. Signature broadleaf marsh species, Sagittaria lancifolia and Pontederia cordata, were found in all remnant floodplain plots and colonized 8 of the 10 reconstructed floodplain plots, but had mean cover ranging from only 0.9 to 6.1%. Several factors may have contributed to unsuccessful reestablishment of broadleaf marsh, including unfavorable edaphic conditions, brief drawdown (low stage) periods for establishment of seedlings, flood induced mortality, and an invasion of the exotic shrub, Ludwigia peruviana, which had post‐restoration mean cover of 17–19%. Study results indicate hydrologic restoration of floodplain plant communities can be influenced by more discrete aspects of the river flood pulse than average hydroperiods and depths.     

Long-term response of fishes and other fauna to restoration of former salt hay farms: multiple measures of restoration success

This synthesis brings together published and unpublished data in an evaluation of restoration of former salt hay farms to functioning salt marshes. We compared nine years of field measurements between three restored marshes (Dennis, Commercial, and Maurice River Townships) and a reference marsh (Moores Beach) in the mesohaline portion of Delaware Bay. In the process, we compared channel morphology, geomorphology, vegetation, sediment organic matter, fish assemblages, blue crabs, horseshoe crabs, benthic infauna, and diamondback terrapins. For fishes we compared structural (distribution, abundance) and functional (feeding, growth, survival, reproduction, production) aspects to evaluate the restored marshes in an Essential Fish Habitat context. Marsh vegetation and drainage density responded gradually and positively with restored marshes approximating the state of the reference marsh within the nine-year study period. The fauna responded more quickly and dramatically with most measures equal or greater in the restored marshes within the first one or two years after restoration. Differences in response time between the vegetation and the fauna imply that the faunal response was more dependent on access to the shallow intertidal marsh surface and intertidal and subtidal creeks than on characteristics of the vegetated marsh. The fishes in created subtidal creeks in restored marshes responded immediately and maintained fish assemblages similar to the reference marsh over the study period. The intertidal creek fish assemblages tended to become more like the reference marsh in the last years of the comparison. Overall, these results document the success of the restoration and how marshes function for both resident and transient fauna, especially fishes.     

Tidal Marsh Restoration: Trends in Vegetation Change Using a Geographical Information System (GIS)

Adequately evaluating the success of coastal tidal marsh restoration has lagged behind the actual practice of restoring tidally restricted salt marshes. A Spartina-dominated valley marsh at Barn Island Wildlife Management Area, Stonington, Connecticut, was tidally restricted in 1946 and consequently converted mostly to Typha angustifolia. With the re-introduction of tidal flooding in 1978, much of the marsh has reverted to Spartina alterniflora. Using a geographical information system (GIS), this study measures restoration success by the extent of geographical similarity between the vegetation of the restored marsh and the pre-impounded marsh. Based on geographical comparisons among different hydrologic states, pre-impounded (1946), impounded (1976), and restored (1988) tidal marsh restoration is a convergent process. Although salt marsh species currently dominate the restored system, the magnitude of actual agreement between the pre-impounded vegetation and that of the restored marsh is only moderate. Further restoration of the salt marsh vegetation may be limited by continued tidal restriction, marsh surface subsidence, and reduced accretion rates. General trends of recovery are identified using a gradient approach and the geographic pattern’ of vegetation change. In the strictest sense, if restoration refers only to vegetation types that geographically replicate preexisting types, then only 28% of the marsh has been restored. Restoration in a broader sense, however, representing the original salt marsh vegetation regardless of spatial position, amounts to 63% restored. Unrestored marsh, dominated by Typha angustifolia and Phragmites australis, remains at 37%. By emphasizing trends during vegetation recovery, this evaluation technique aims to understand the restoration process, direct future research goals, and ultimately aid in future restoration projects.     

Removing the sands (sins?) of our past: dredge spoil removal and saltmarsh restoration along the Indian River Lagoon, Florida (USA)

Estuarine saltmarshes are widely recognized as highly productive and biologically diverse systems. The Indian River Lagoon (IRL) is a 250?km long ??Estuary of National Significance?? along Florida??s east coast and is known as one of North America??s most diverse estuaries. Like most North American estuaries, the IRL is facing a number of problems, among them loss of emergent wetlands. Between 75 and 90% of the original mangrove and saltmarsh acreage historically bordering the IRL has been lost or impacted, either through direct filling for development or impoundment for mosquito control. This loss has affected IRL water quality and fisheries, since these habitats are now removed from the estuarine system. Active programs are now underway to restore mosquito impoundments by reconnection with culverts or removal of dikes, but restoration of dredge spoil is more problematic, as many of these sites have been developed. However, where undeveloped spoil is found on public lands, restoration is a possibility. One such site, Pine Island Conservation Area (PICA), jointly owned by the Brevard County Environmentally Endangered Lands Program and the St. Johns River Water Management District, contained 25.4?ha of dredge spoil, originating from 1969, when the property was owned by a development company. Following public acquisition in 1996, plans were developed to remove the spoil and restore the site to historic saltmarsh elevation. Between 2003 and 2006 all of the material was removed and the restoration of normal hydrology has resulted in ??volunteer?? recruitment by appropriate marsh vegetation, without a need for any plantings.     

Comparison of Designed Channel Restoration and Riparian Buffer Restoration Effects on Riparian Soils

Stream restoration is often employed in efforts to stabilize eroding channel banks. Banks are stabilized through a designed channel approach, which involves grading and armoring of stream banks using heavy machinery, or alternatively through planting of seedlings and saplings to establish forested riparian buffers. We hypothesized that designed channel restoration would have detrimental impacts on riparian soils but that soils would recover over time, and we hypothesized that riparian buffer restoration would not impact riparian soils. We tested these hypotheses by comparing soil attributes (bulk density, soil organic matter, and root biomass) at reaches that had undergone designed channel and riparian buffer restoration in different years (project ages ranged from 2 to 16 years) to paired urban (unrestored) control reaches. Soil properties in sub‐surface soil layers (10–20 and 20–30 cm depth) at both recent (<10 years old) and older (>10 years old) designed channel reaches differed significantly from paired urban control soils; bulk density was higher and root biomass lower in manipulated reaches compared to urban control reaches. At many designed channel reaches, bulk density exceeded values known to restrict root growth. These results indicate that compaction and disturbance of riparian soils may be a significant unintended consequence of designed channel restoration and can persist for at least a decade. In contrast, we found no significant differences in soil properties between riparian buffer restoration reaches and urban control reaches. Thus, the results indicate that riparian buffer restoration is a more ecologically favorable method than designed channel restoration for bank stabilization.     

Fifteen Years of Vegetation and Soil Development after Brackish-Water Marsh Creation

Aboveground biomass, macro‐organic matter (MOM), and wetland soil characteristics were measured periodically between 1983 and 1998 in a created brackish‐water marsh and a nearby natural marsh along the Pamlico River estuary, North Carolina to evaluate the development of wetland vegetation and soil dependent functions after marsh creation. Development of aboveground biomass and MOM was dependent on elevation and frequency of tidal inundation. Aboveground biomass of Spartina alterniflora, which occupied low elevations along tidal creeks and was inundated frequently, developed to levels similar to the natural marsh (750 to 1,300 g/m²) within three years after creation. Spartina cynosuroides, which dominated interior areas of the marsh and was flooded less frequently, required 9 years to consistently achieve aboveground biomass equivalent to the natural marsh (600 to 1,560 g/m²). Aboveground biomass of Spartina patens, which was planted at the highest elevations along the terrestrial margin and seldom flooded, never consistently developed aboveground biomass comparable with the natural marsh during the 15 years after marsh creation. MOM (0 to 10 cm) generally developed at the same rate as aboveground biomass. Between 1988 and 1998, soil bulk density decreased and porosity and organic C and N pools increased in the created marsh. Like vegetation, wetland soil development proceeded faster in response to increased inundation, especially in the streamside zone dominated by S. alterniflora. We estimated that in the streamside and interior zones, an additional 30 years (nitrogen) to 90 years (organic C, porosity) are needed for the upper 30 cm of created marsh soil to become equivalent to the natural marsh. Wetland soil characteristics of the S. patens community along upland fringe will take longer to develop, more than 200 years. Development of the benthic invertebrate‐based food web, which depends on organic matter enrichment of the upper 5 to 10 cm of soil, is expected to take less time. Wetland soil characteristics and functions of created irregularly flooded brackish marshes require longer to develop compared with regularly flooded salt marshes because reduced tidal inundation slows wetland vegetation and soil development. The hydrologic regime (regularly vs. irregularly flooded) of the “target” wetland should be considered when setting realistic expectations for success criteria of created and restored wetlands.     

Salt Marsh Restoration in Connecticut: 20 Years of Science and Management

In 1980 the State of Connecticut began a tidal marsh restoration program targeting systems degraded by tidal restrictions and impoundments. Such marshes become dominated by common reed grass (Phragmites australis) and cattail (Typha angustifolia and T. latifolia), with little ecological connection to Long Island Sound. The management and scientific hypothesis was that returning tidal action, reconnecting marshes to Long Island Sound, would set these systems on a recovery trajectory. Specific restoration targets (i.e., pre‐disturbance conditions or particular reference marshes) were considered unrealistic. However, it was expected that with time restored tides would return ecological functions and attributes characteristic of fully functioning tidal salt marshes. Here we report results of this program at nine separate sites within six marsh systems along 110 km of Long Island Sound shoreline, with restoration times of 5 to 21 years. Biotic parameters assessed include vegetation, macroinvertebrates, and use by fish and birds. Abiotic factors studied were soil salinity, elevation and tidal flooding, and soil water table depth. Sites fell into two categories of vegetation recovery: slow, ca. 0.5%, or fast, more than 5% of total area per year. Although total cover and frequency of salt marsh angiosperms was positively related to soil salinity, and reed grass stand parameters negatively so, fast versus slow recovery rates could not be attributed to salinity. Instead, rates appear to reflect differences in tidal flooding. Rapid recovery was characterized by lower elevations, greater hydroperiods, and higher soil water tables. Recovery of other biotic attributes and functions does not necessarily parallel those for vegetation. At the longest studied system (rapid vegetation recovery) the high marsh snail Melampus bidentatus took two decades to reach densities comparable with a nearby reference marsh, whereas the amphipod Orchestia grillus was well established on a slow‐recovery marsh, reed grass dominated after 9 years. Typical fish species assemblages were found in restoration site creeks and ditches within 5 years. Gut contents of fish in ditches and on the high marsh suggest that use of restored marsh as foraging areas may require up to 15 years to reach equivalence with reference sites. Bird species that specialize in salt marshes require appropriate vegetation; on the oldest restoration site, breeding populations comparable with reference marshland had become established after 15 years. Use of restoration sites by birds considered marsh generalists was initially high and was still nearly twice that of reference areas even after 20 years. Herons, egrets, and migratory shorebirds used restoration areas extensively. These results support our prediction that returning tides will set degraded marshes on trajectories that can bring essentially full restoration of ecological functions. This can occur within two decades, although reduced tidal action can delay restoration of some functions. With this success, Connecticut's Department of Environmental Protection established a dedicated Wetland Restoration Unit. As of 1999 tides have been restored at 57 separate sites along the Connecticut coast.     

Carbon accumulation and vertical accretion in a restored versus historic salt marsh in southern Puget Sound,Washington, United States

Few comparisons exist between vertical accretion (VA) and carbon accumulation rates (CARs) in restored versus historic (i.e. reference) marshes. Here, we compare these processes in a formerly diked, sparsely vegetated, restored salt marsh (Six Gill Slough, SG), whose surface is subsided relative to the tidal frame, to an adjacent, relatively pristine, historic salt marsh (Animal Slough, AS). Six sediment cores were collected at both AS and SG approximately 6 years after restoration. Cores were analyzed for bulk density (BD), % loss of ignition, % organic carbon, and ²¹⁰Pb. We found that sharp changes in BD in surface layers of SG cores were highly reliable markers for the onset of restoration. The mean VA since restoration at SG (0.79 [SD = 0.29] cm/year) was approximately twice that of AS (0.41 [SD = 0.16] cm/year). In comparison, the VA at AS over 50 years was 0.30 (SD = 0.09) cm/year. VA consisted almost entirely of inorganic sediment at SG whereas at AS it was approximately 55%. Mean CARs at SG were somewhat greater than at AS, but the difference was not significant due to high variability (SG: 81–210 g C m^?2 year^?1; AS: 115–168 g C m^?2 year^?1). The mean CAR at AS over the past 50 years was 118 (SD = 23) g C m^?2 year^?1. This study demonstrates that a sparsely vegetated, restored salt marsh can quickly begin to accumulate carbon and that historic and restored marshes can have similar CARs despite highly divergent formation processes.     

Interim Responses of Floodplain Wetland Vegetation to Phase I of the Kissimmee River Restoration Project: Comparisons of Vegetation Maps from Five Periods in the River's History

Phase I of the Kissimmee River Restoration Project (KRRP) reestablished intermittent inundation of the river's floodplain by backfilling 12 km of the C‐38 flood control canal in 2001. We compared floodplain vegetation maps based on 2003 and 2008 aerial imagery (2 and 7 years following completion of Phase I, respectively) to vegetation maps from 1954 (pre‐channelization), 1974 (3 years after channelization), and 1996 (25 years after channelization) to evaluate broad‐scale vegetation responses to Phase I restoration. Results indicate that the extent of wetland plant communities expanded rapidly, more than doubling in area within 2 years after completion of Phase I, and that by 2008 wetlands had nearly recovered to pre‐channelization levels. However, full reestablishment of the pre‐channelization wetland mosaic has not yet occurred. Prior to channelization, much of the floodplain was dominated by a broadleaf marsh (BLM) community associated with extended, deep annual flooding, while shorter‐hydroperiod communities dominated the floodplain in 2003 and 2008. Prior to restoration construction, the reestablishment of BLM was predicted to be slow because suitable hydrology is dependent on project components that will not be in place until all restoration components are completed (projected for 2019). Hydrologic data indicate that the duration and variability of floodplain inundation have not yet achieved restoration targets over the entire Phase I study area. Other factors affecting vegetation responses are likely involved, including the age and viability of soil seed banks, the rarity of relict propagule sources following the channelized period, and competition from an invasive wetland shrub species.     

Impacts of hydrological restoration on lowland river floodplain plant communities

Many lowland floodplain habitats have been disconnected from their rivers by flood defence banks. Removing or lowering these banks can reinstate regular flooding and thus restore these important wetland plant communities. In this study we analyse changes in wetland hydrology and plant community composition following the lowering of flood defence banks at a floodplain of the River Don in the United Kingdom (UK). The aim of the restoration project was to improve the quality of “floodplain grazing marsh” habitat, which is a group of wetland communities that are of conservation interest in the UK. We analyse changes in species richness and community composition over a period of 6 years, and compare the presence of indicator species from the target floodplain grazing marsh plant communities. The lowering of the flood banks increased the frequency of flood events, from an estimated average of 1.7 floods per year to 571 floods per year. The increased flooding significantly increased the proportion of time that the wetland was submerged, and the heterogeneity in hydrological conditions within the floodplain. There were significant differences in composition between the pre-restoration and restored plant communities. Plants with traits for moisture tolerance became more abundant, although the communities did not contain significantly more ‘target’ floodplain grazing marsh species at the end of the study period than prior to restoration. Colonisation by floodplain grazing marsh species may have been limited because environmental conditions were not yet suitable, or because of a shortage of colonising propagules. While the desired target plant community has not been achieved after 5 years, it is encouraging that the community has changed dynamically as a result of hydrological changes, and that moisture-tolerant species have increased in occurrence. Over the next few decades, the restored flood regime may cause further environmental change or colonisation events, thus helping increase the occurrence of desired floodplain grazing marsh indicator species.

     

Rapid Seed Bank Development in Restored Tidal Freshwater Wetlands

Studies of seed bank development have rarely been included in evaluations of wetland restoration. We compared the seed bank of a recently restored tidal freshwater marsh in Washington, D.C., Kingman Marsh, with seed banks of another restored site (Kenilworth Marsh) and two reference marshes (Dueling Creek and Patuxent Marsh). The density and richness of emerging seedlings from Kingman Marsh seed bank samples increased from less than 4 seedlings and 2 taxa/90-cm² sample in 2000 (the year of restoration) to more than 130 seedlings and 10 taxa/90-cm² sample in 2003. The most important seed bank taxa at Kingman Marsh included Cyperus spp., Juncus spp., Lindernia dubia , Ludwigia palustris , and the non-native Lythrum salicaria . These taxa are not abundant in most mid-Atlantic tidal freshwater marshes but are almost identical to those described for a created tidal freshwater wetland in New Jersey. Seed banks of both the restored sites contained few seeds of several important species found at the reference sites. Flooding had a significant negative effect on emerging seedling density and taxa density, suggesting that slight decreases in soil elevation in restored wetlands will dramatically decrease recruitment from the seed bank. Because seed banks integrate processes affecting growth and reproduction of standing vegetation, we suggest that seed banks are a useful metric of wetland restoration success and urge that seed bank studies be incorporated into monitoring programs for restored wetlands.     

The Significance of Meander Restoration for the Hydrogeomorphology and Recovery of Wetland Organisms in the Kushiro River,a Lowland River in Japan

The meanders and floodplains of the Kushiro River were restored in March 2011. A 1.6‐km stretch of the straightened main channel was remeandered by reconnecting the cutoff former channel and backfilling the straightened reach, and a 2.4‐km meander channel was restored. Additionally, flood levees were removed to promote river–floodplain interactions. There were four objectives of this restoration project: to restore the in‐stream habitat for native fish and invertebrates; to restore floodplain vegetation by increasing flooding frequency and raising the groundwater table; to reduce sediment and nutrient loads in the core wetland areas; to restore a river–floodplain landscape typical to naturally meandering rivers. In this project, not only the natural landscape of a meandering river but also its function was successfully restored. The monitoring results indicated that these goals were likely achieved in the short term after the restoration. The abundance and species richness of fish and invertebrate species increased, most likely because the lentic species that formerly inhabited the cutoff channel remained in the backwater and deep pools created in the restored reach. In addition, lotic species immigrated from neighboring reaches. The removal of flood levees and backfilling of the formerly straightened reach were very effective in increasing the frequency of flooding over the floodplains and raising the water table. The wetland vegetation recovered rapidly 1 year after the completion of the meander restoration. Sediment‐laden floodwater spread over the floodplain, and approximately 80–90% of the fine sediment carried by the water was filtered out by the wetland vegetation.     

Evaluating the ecological performance of wetland restoration in the Yellow River Delta,China

Long-term monitoring is essential to evaluate the effects of wetland restoration projects. A monitoring program before and after restoration has been carried out in the study area located in the Yellow River Delta since 2001. Water quality, soil salinity, soil organic matter, plant community, and bird species were chosen as indicators in this program. During the past seven years, the restored wetland showed increasing efficiency in reducing water pollution levels. Soil quality was constantly improved through salinity reduction and soil organic matter accumulation. The vegetation community quickly re-established after the restoration was initiated in 2002. The restored vegetation communities provide favorable habitat conditions for birds and thirty-seven bird species were observed in October 2007. Based on Canonical Correspondence Analysis (CCA), plant species and vegetation community are mainly influenced by soil salinity and water depth. These indicate that conducting freshwater to the project area is an efficient measure for vegetation restoration. While monitoring results show that the restoration project had positive effects on the wetland ecosystem over the past seven years, two issues remain for future study: (1) the contribution of harvesting vegetation to stabilizing nutrient removal rate and the accumulation of soil organic matter in the soil; and (2) the effects of excessive propagation of Phragmites australis on spatial heterogeneity and plant diversity.     

Evaluating tidal marsh sustainability in the face of sea-level rise: a hybrid modeling approach applied to San Francisco Bay

Background

Tidal marshes will be threatened by increasing rates of sea-level rise (SLR) over the next century. Managers seek guidance on whether existing and restored marshes will be resilient under a range of potential future conditions, and on prioritizing marsh restoration and conservation activities.

Methodology

Building upon established models, we developed a hybrid approach that involves a mechanistic treatment of marsh accretion dynamics and incorporates spatial variation at a scale relevant for conservation and restoration decision-making. We applied this model to San Francisco Bay, using best-available elevation data and estimates of sediment supply and organic matter accumulation developed for 15 Bay subregions. Accretion models were run over 100 years for 70 combinations of starting elevation, mineral sediment, organic matter, and SLR assumptions. Results were applied spatially to evaluate eight Bay-wide climate change scenarios.

Principal Findings

Model results indicated that under a high rate of SLR (1.65 m/century), short-term restoration of diked subtidal baylands to mid marsh elevations (−0.2 m MHHW) could be achieved over the next century with sediment concentrations greater than 200 mg/L. However, suspended sediment concentrations greater than 300 mg/L would be required for 100-year mid marsh sustainability (i.e., no elevation loss). Organic matter accumulation had minimal impacts on this threshold. Bay-wide projections of marsh habitat area varied substantially, depending primarily on SLR and sediment assumptions. Across all scenarios, however, the model projected a shift in the mix of intertidal habitats, with a loss of high marsh and gains in low marsh and mudflats.

Conclusions/Significance

Results suggest a bleak prognosis for long-term natural tidal marsh sustainability under a high-SLR scenario. To minimize marsh loss, we recommend conserving adjacent uplands for marsh migration, redistributing dredged sediment to raise elevations, and concentrating restoration efforts in sediment-rich areas. To assist land managers, we developed a web-based decision support tool (www.prbo.org/sfbayslr).     

Using Functional Trajectories to Track Constructed Salt Marsh Development in the Great Bay Estuary, Maine/New Hampshire, U.S.A.

A growing number of studies have assessed the functional equivalency of restored and natural salt marshes. Several of these have explored the use of functional trajectories to track the increase in restored marsh function over time; however, these studies have disagreed as to the usefulness of such models in long‐term predictions of restored marsh development. We compared indicators of four marsh functions (primary production, soil organic matter accumulation, sediment trapping, and maintenance of plant communities) in 6 restored and 11 reference (matched to restored marshes using principal components analysis) salt marshes in the Great Bay Estuary. The restored marshes were all constructed and planted on imported substrate and ranged in age from 1 to 14 years. We used marsh age in a space‐for‐time substitution to track constructed salt marsh development and explore the use of trajectories. A high degree of variability was observed among natural salt marsh sites, displaying the importance of carefully chosen reference sites. As expected, mean values for constructed site (n = 6) and reference site (n = 11) functions were significantly different. Using constructed marsh age as the independent variable and functional indicator values as dependent variables, nonlinear regression analyses produced several ecologically meaningful trajectories (r ²> 0.9), demonstrating that the use of different‐aged marshes can be a viable approach to developing functional trajectories. The trajectories illustrated that although indicators of some functions (primary production, sediment deposition, and plant species richness) may reach natural site values relatively quickly (<10 years), others (soil organic matter content) will take longer.     

Microbial and Phosphate Dynamics in a Restored Shrub Steppe in Southwestern Wyoming

Soil microbial activity and soil nutrients were monitored on a revegetated coal surface mine in southwestern Wyoming from the initial planting in 1982 through 1987. Total soil nitrogen (N) and organic matter did not change during this period. However, despite no changes in available phosphorus (P) concentrations, the total P declined over 50% during the five-year period, with no apparent reduction in the loss rates. The greatest loss was in the bound inorganic P pool. Moisture appeared not to limit microbial mass-C. Microbial mass-C was higher under shrubs than in interspaces and increased with time. Total organic matter did not increase. Thus, the ratio of microbial mass-C to organic matter-C increased during the study period. This suggests that the input of readily decomposable substrate may limit microbial activity. During the study period, all above-ground litter was removed by wind. Root production in the surface soils was low and highly variable and, in this habitat, probably did not contribute largely to the organic matter status. These data suggest that despite an apparent recovery of many parameters used to indicate reclamation or restoration success, the soil-bound P pools could be undergoing a loss. Microbial-C and organic matter changes indicate a system that is not approaching equilibrium within the required monitoring period of most restoration efforts. These parameters could eventually reduce the recovery potential of restored sites.     

Effect of time on the natural regeneration of salt marsh

Abstract. The effect of time on natural regeneration of two salt marshes was studied in relation to plant and edaphic factors. The study was carried out in two naturally restoring salt marshes, differing in restoration time, in Txingudi (Bay of Biscay). After 20 yr, the younger salt marsh had the same plant species richness and high species similarity as a 35 yr old salt marsh (17 and 16, respectively, similarity index = 0.9), but both sites had lower species richness and similarity than a nearby natural salt marsh (36 plant species and similarity indices of 0.45 with the 35 yr old marsh and 0.46 with the 20 yr old marsh). Plant species present in the two recovering salt marshes followed a similar distribution pattern in relation to organic matter, conductivity and moisture content although this zonation differed from the natural salt marsh. The range of edaphic factors measured was also similar, but differed from those in the natural salt marsh. The process of plant species recolonization and spatial distribution might be delayed by a low probability of species arrival and by the time need for the restoration of hydrologic and edaphic factors. This study supports the necessity of long‐term monitoring in measuring coastal salt marsh restoration.