Assessing long-term outcomes of tidal restoration in New England salt marshes.

Salt marshes are valuable coastal ecosystems, but many have been degraded by roads, railways, and other infrastructure that restrict tidal flow and impound watershed runoff. Restoration of tidal flow to tide-restricted salt marshes generally aims to restore native vegetation and habitat functions. Biological communities may take one or more decades to recover following tidal restoration, but outcomes are seldom assessed on that timescale. We assessed the long-term outcomes of eight tidal restorations in Rhode Island, USA using observed changes in plant and nekton communities from pre-restoration to present, and newly-collected data from a rapid assessment method. The time-series vegetation and nekton data suggest that while restoration actions promoted biological recovery, ambient factors such as inundation stress and eutrophication have worked to offset it. Rapid assessment results indicate that the cover of Phragmites australis is higher and the cover of meadow high marsh is lower at restoration marshes compared with a broad reference sample, suggesting incomplete recovery on average, although outcomes varied across the restoration marshes. Habitat integrity increased with the degree of adaptive management following restoration, as well as the age of restoration, but salt marsh restoration practitioners may need to shift their methods and expectations to accommodate human influences on ambient environmental conditions, particularly prevalent, increasing inundation stress associated with sea-level rise. Our study highlights the value of standardized long-term biological monitoring in assessing salt marsh restoration outcomes, and demonstrates how rapid assessment data can add valuable context to restoration findings.

Evaluation of Plot-Scale Methods for Assessing and Monitoring Salt Marsh Vegetation Composition and Cover.

Vegetation is a key component of salt marsh monitoring programs, but different methods can make comparing datasets difficult. We compared data on vegetation composition and cover collected with 3 methods (point-intercept, Braun-Blanquet visual, and floristic quality assessment [FQA]) in 3 Rhode Island salt marshes. No significant differences in plant community composition were found among the methods, and differences in individual species cover in a marsh never exceeded 6% between methods. All methods were highly repeatable, with no differences in data collected by different people. However, FQA was less effective at identifying temporal changes at the plot scale. If data are collected from many plots in a marsh, any of the methods are appropriate, but if plot-scale patterns are of interest, we recommend point-intercept.

Evaluating how variants of floristic quality assessment indicate wetland condition.

Biological indicators are useful tools for the assessment of ecosystem condition. Multi-metric and multi-taxa indicators may respond to a broader range of disturbances than simpler indicators, but their complexity can make them difficult to interpret, which is critical to indicator utility for ecosystem management. Floristic Quality Assessment (FQA) is an example of a biological assessment approach that has been widely tested for indicating freshwater wetland condition, but less attention has been given to clarifying the factors controlling its response. FQA quantifies the aggregate of vascular plant species tolerance to habitat degradation (conservatism), and model variants have incorporated species richness, abundance, and indigenity (native or non-native). To assess bias, we tested FQA variants in open-canopy freshwater wetlands against three independent reference measures, using practical vegetation sampling methods. FQA variants incorporating species richness did not correlate with our reference measures and were influenced by wetland size and hydrogeomorphic class. In contrast, FQA variants lacking measures of species richness responded linearly to reference measures quantifying individual and aggregate stresses, suggesting a broad response to cumulative degradation. FQA variants incorporating non-native species, and a variant additionally incorporating relative species abundance, improved performance over using only native species. We relate our empirical findings to ecological theory to clarify the functional properties and implications of the FQA variants. Our analysis indicates that (1) aggregate conservatism reliably declines with increased disturbance; (2) species richness has varying relationships with disturbance and increases with site area, confounding FQA response; and (3) non-native species signal human disturbance. We propose that incorporating species abundance can improve FQA site-level relevance with little extra sampling effort. Using our practical sampling methods, an FQA variant ignoring species richness and incorporating non-native species and relative species abundance can be logistically efficient, easily understood, and effective for wetland assessment.

Laying it on thick: Ecosystem effects of sediment placement on a microtidal Rhode Island salt marsh.

Heightened recognition of impacts to coastal salt marshes from sea-level rise has led to expanding interest in using thin-layer sediment placement (TLP) as an adaptation tool to enhance future marsh resilience. Building on successes and lessons learned from the Gulf and southeast U.S. coasts, projects are now underway in other regions, including New England where the effects of TLP on marsh ecosystems and processes are less clear. In this study, we report on early responses of a drowning, microtidal Rhode Island marsh (Ninigret Marsh, Charlestown, RI) to the application of a thick (10-48 cm) application of sandy dredged material and complimentary extensive adaptive management to quickly build elevation capital and enhance declining high marsh plant species. Physical changes occurred quickly. Elevation capital, rates of marsh elevation gain, and soil drainage all increased, while surface inundation, die-off areas, and surface ponding were greatly reduced. Much of the marsh revegetated within a few years, exhibiting aspects of classic successional processes leading to new expansive areas of high marsh species, although low marsh Spartina alterniflora recovered more slowly. Faunal communities, including nekton and birds, were largely unaffected by sediment placement. Overall, sediment placement provided Ninigret Marsh with an estimated 67-320 years of ambient elevation gain, increasing its resilience and likely long-term persistence. Project stakeholders intentionally aimed for the upper end of high marsh plant elevation growth ranges to build elevation capital and minimize maintenance costs, which also resulted in new migration corridors, providing pathways for future marsh expansion.