Wetland Compensation and Landscape Change in a Rapidly Urbanizing Context.

While there are regulatory requirements that regulators should assess the impact of landscape-scale changes on the success of US Clean Water Act wetland compensatory mitigation sites, these requirements are poorly specified and very little work has been done to characterize how landscape change impacts CWA compensation sites. We created a rapid assessment method with both site-based and landscape-scale components, and used it to assess a population of wetland compensation sites in suburban St. Paul, Minnesota in 1997. We resampled the sites in 2010. The watersheds of these 22 compensation sites are characterized by rapid urbanization, the increase in impervious surfaces, and the loss of agriculture. This has resulted in extreme hydrographs at compensation sites and a fragmenting landscape context of more and smaller undeveloped patches. The ecosystem services provided by these compensation sites in 2010 are not significantly different than in 1997, indicating resilience in the face of landscape change, but not showing a trajectory of improvement. Reference sites were established for each ecosystem service, but two reference sites declined dramatically; results point to the importance of understanding ongoing landscape change even at benchmark sites. Compensation sites are typically located in rapidly changing and fragmenting landscapes, and understanding the relationship between landscape and compensation site will be important to ensuring appropriate compensation for impacts regulated by the Clean Water Act.

Determining vegetation metric robustness to environmental and methodological variables.

Land managers need reliable metrics for assessing the quality of restorations and natural areas and prioritizing management and conservation efforts. However, it can be difficult to select metrics that are robust to sampling methods and natural environmental differences among sites, while still providing relevant information regarding ecosystem changes or stressors. We collected herbaceous-layer vegetation data in wetlands and grasslands in four regions of the USA (the Midwest, subtropical Florida, arid southwest, and coastal New England) to determine if commonly used vegetation metrics (species richness, mean coefficient of conservatism [mean C], Floristic Quality Index [FQI], abundance-weighted mean C, and percent non-native species cover) were robust to environmental and methodological variables (region, site, observer, season, and year), and to determine adequate sample sizes for each metric. We constructed linear mixed effects models to determine the influence of these environmental and methodological variables on vegetation metrics and used metric accumulation curves to determine the effect of sample size on metric values. Species richness and FQI varied among regions, and year and observer effects were also highly supported in our models. Mean C was the metric most robust to sampling variables and stabilized at less sampling effort compared to other metrics. Assessment of mean C requires sampling a small number of quadrats (e.g. 20), but assessment of species richness or FQI requires more intensive sampling, particularly in species-rich sites. Based on our analysis, we recommend caution be used when comparing metric values among sites sampled in different regions, different years, or by different observers.

Wetland compensation and its impacts on ß-diversity.

The anthropogenic degradation of natural ecological communities can cause biodiversity loss in the form of biotic homogenization (i.e., reduced ß-diversity). Biodiversity offsetting practices, such as compensatory wetland mitigation, may inadvertently cause biotic homogenization if they produce locally homogenous or regionally recurring communities. The fact that compensation wetlands often resemble degraded wetlands suggests that potential impacts to ß-diversity are likely. Yet, it is unknown how high-quality, low-quality (degraded), and compensation wetlands compare in terms of ß-diversity. We compared the ß-diversity of high-quality, low-quality, and compensation wetlands at local and regional scales. ß-diversity was quantified as the average distance to group centroids in multivariate space based on pairwise comparisons of community composition. The local spatial structure of ß-diversity was assessed using species turnover across plots. Indicator species analysis was used to describe compositional differences potentially contributing to differences in ß-diversity. Overall, the ß-diversity of compensation sites did not differ from high-quality or low-quality natural wetlands. However, compensation wetlands had a high degree of internal turnover along the hydrological gradient, which culminated in homogenous zones in the wettest areas. Compared to high-quality wetlands, low-quality wetlands had significantly lower ß-diversity at local scales, but significantly greater ß-diversity at regional scales. Indicator species results showed that compensation wetlands were distinguished by low conservation value species typically found in old fields and waste areas. This analysis also indicated that the invasive grass Phalaris arundinacea was indicative of low-quality and compensation wetlands. This species is likely contributing to differing patterns of ß-diversity between high-quality and low-quality wetlands. These results indicate that conclusions regarding ß-diversity depend on scale and scope of analysis. Particularly, the unique architecture of compensation wetlands makes conclusions regarding within-site ß-diversity dependent on the observer's position along the hydrological gradient. Additionally, while we conclude that compensation wetlands are not contributing to biotic homogenization at the regional scale, these wetlands are distinct from both high-quality and low-quality wetlands in their composition and structure. Therefore, assessments of the overall success of wetland mitigation programs should acknowledge the reality of these differences.

An Assessment of Long-Term Compliance with Performance Standards in Compensatory Mitigation Wetlands.

Under the US Clean Water Act, wetland restoration is used to compensate for adverse impacts to wetlands. Following construction, compensation wetlands are monitored for approximately 5 years to determine if they comply with project-specific performance standards. Once a compensation site complies with performance standards, it is assumed that the site will continue to meet standards indefinitely. However, there have been few assessments of long-term compliance. We surveyed, in 2012, 30 compensation sites 8-20 years after restoration to determine whether projects continued to meet performance standards. Additionally, we compared floristic quality of compensation sites to the quality of adjacent natural wetlands to determine whether wetland condition in compensation sites could be predicted based on the condition of nearby wetlands. Compensation sites met, on average, 65% of standards during the final year of monitoring and 53% of standards in 2012, a significant decrease in compliance. Although forested wetlands often failed to meet standards for planted tree survival, the temporal decrease in compliance was driven by increasing dominance by invasive plants in emergent wetlands. The presumption of continued compliance with performance standards after a 5-year monitoring period was not supported. Wetlands restored near better quality natural wetlands achieved and maintained greater floristic quality, suggesting that landscape context was an important determinant of long-term restoration outcomes. Based on our findings, we recommend that compensation wetlands should be monitored for longer time periods, and we suggest that nearby or adjacent natural wetlands provide good examples of reasonably achievable restoration outcomes in a particular landscape.

National-Level Wetland Policy Specificity and Goals Vary According to Political and Economic Indicators.

Growing recognition of the importance of wetlands to human and ecosystem well-being has led countries worldwide to implement wetland protection policies. Different countries have taken different approaches to wetland protection by implementing various policies, including territorial exclusion, market-based offsetting, and incentive programs for land users. Our objective was to describe the relationship between components of national-level wetland protection policies and national characteristics, including natural resource, economic, social, and political factors. We compiled data on the wetland policies of all 193 countries recognized by the U.N. and described the relationships among wetland policy goals and wetland protection mechanisms using non-metric multidimensional scaling. The first non-metric multidimensional scaling axis strongly correlated with whether a country had a wetland-specific environmental policy in place. Adoption of a comprehensive, wetland-specific policy was positively associated with degree of democracy and a commitment to establishing protected areas. The second non-metric multidimensional scaling axis defined a continuum of policy goals and mechanisms by which wetlands are protected, with goals to protect wetland ecosystem services on one end of the spectrum and goals to protect biodiversity on the other. Goals for protecting ecosystem services were frequently cited in policy documents of countries with agriculture-based economies, whereas goals associated with wetland biodiversity tended to be associated with tourism-based economies. We argue that the components of a country's wetland policies reflect national-level resource and economic characteristics. Understanding the relationship between the type of wetland policy countries adopt and national-level characteristics is critical for international efforts to protect wetlands.

Group-based modeling of ecological trajectories in restored wetlands.

Repeated measures taken at the same restoration sites over time are used to describe restoration trajectories and identify sites that are trending toward unexpected outcomes. Analogously, social scientists use repeated measures of individuals to describe developmental trajectories of behaviors or other outcomes. Group-based trajectory modeling (GBTM) is one statistical method used in behavioral and health sciences for this purpose. I introduce the use of GBTM to identify clusters of similar restoration trajectories within a sample of sites. Data collected at 54 restored wetlands in Illinois for up to 15 years post-restoration were used to describe trajectories of six indicators: plant species richness, number of Carex (sedge) species, mean coefficient of conservatism (mean C), native plant cover, perennial plant cover, and planted species cover. For each indicator, I used GBTM to classify wetlands into three to four groups with distinct trajectories. In general, cover by native and planted species declined, while species richness and mean C increased over time or peaked then declined. Site context and management may explain trajectory group membership. Specifically, wetlands restored more recently and those restored within forested contexts were more likely to follow increasing trajectories. I show GBTM to be useful for identifying typical restoration trajectory patterns, developing hypotheses regarding factors driving those patterns and pinpointing critical times for intervention. Furthermore, GBTM might be applied more broadly in ecological research to identify common patterns of community assembly in large numbers of plots or sites.

Habitat specialization along a wetland moisture gradient differs between ammonia-oxidizing and denitrifying microorganisms.

Gradients in abiotic parameters, such as soil moisture,can strongly influence microbial community structure and function. Denitrifying and ammonia-oxidizing microorganisms,in particular, have contrasting physiological responses to abiotic factors such as oxygen concentration and soil moisture. Identifying abiotic factors that govern the composition and activity of denitrifying and ammonia-oxidizing communities is critical for understanding the nitrogen cycle.The objectives of this study were to (i) examine denitrifier andarchaeal ammonia oxidizer community composition and (ii) assess the taxa occurring within each functional group related to soil conditions along an environmental gradient. Soil was sampled across four transects at four locations along a dry to saturated environmental gradient at a restored wetland. Soil pH and soil organic matter content increased from dry to saturated plots. Composition of soil denitrifier and ammonia oxidizer functional groups was assessed by terminal restriction fragment length polymorphism (T-RFLP) community analysis, and local soil factors were also characterized. Microbial community composition of denitrifiers and ammonia oxidizers differed along the moisture gradient (denitrifier:ANOSIM R = 0.739, P < 0.001; ammonia oxidizers: ANOSIMR = 0.760, P < 0.001). Individual denitrifier taxa were observed over a larger range of moisture levels than individual archaeal ammonia oxidizer taxa (Wilcoxon rank sum, W = 2413, P value = 0.0002). Together, our data suggest that variation in environmental tolerance of microbial taxa have potential to influence nitrogen cycling in terrestrial ecosystems.

Accumulation and release of organic phosphorus (P) from legacy P-affected soils to adjacent drainage water.

Legacy effects of P in agricultural soils have been highlighted in recent literature. However, co-accumulation and release of organic P (Po) have often been ignored in current agro-environmental assessments. The mineralizable Po fraction has a potential to increase the activity of phosphate in pore water, increasing fertility or degrading water quality. In this study, the effects of agricultural management practices (fertilizer applied corn-soybean rotation cropland and dairy manure applied pasture) on the Po/phosphate ratio were investigated in P-rich (290-1232 mg kg-1) agricultural soils and adjacent ditchwater using experimental soil-water chemistry. The effect of agricultural management was significant on both Po and the Po/phosphate ratio in soil and adjacent ditchwater. The Po content, dominated by orthophosphate monoesters, in the manure-amended pasture (average ~ 245 mg kg-1) was significantly greater than that in the fertilizer-applied cropland (average 103 mg kg-1). The Po/phosphate ratio was also significantly greater in the manure-amended pasture (0.54) than in the fertilizer-applied cropland (0.42). Similarly, water quality data also showed that ditchwater near the pasture had a significantly greater flux of dissolved non-reactive P and a greater Po/phosphate ratio compared to the water near the fertilizer-applied sites. Furthermore, a greater Po/phosphate ratio in ditchwater was often observed during wet periods, and the ratio was positively correlated to the discharge (r = 0.42, p = 0.003). The study showed the agricultural management-specific Po accumulation and release and - Po/phosphate ratio that might affect the fate of P in agroecosystems.

Microbial community structure and denitrification in a wetland mitigation bank.

Wetland mitigation is implemented to replace ecosystem functions provided by wetlands; however, restoration efforts frequently fail to establish equivalent levels of ecosystem services. Delivery of microbially mediated ecosystem functions, such as denitrification, is influenced by both the structure and activity of the microbial community. The objective of this study was to compare the relationship between soil and vegetation factors and microbial community structure and function in restored and reference wetlands within a mitigation bank. Microbial community composition was assessed using terminal restriction fragment length polymorphism targeting the 16S rRNA gene (total bacteria) and the nosZ gene (denitrifiers). Comparisons of microbial function were based on potential denitrification rates. Bacterial community structures differed significantly between restored and reference wetlands; denitrifier community assemblages were similar among reference sites but highly variable among restored sites throughout the mitigation bank. Potential denitrification was highest in the reference wetland sites. These data demonstrate that wetland restoration efforts in this mitigation bank have not successfully restored denitrification and that differences in potential denitrification rates may be due to distinct microbial assemblages observed in restored and reference (natural) wetlands. Further, we have identified gradients in soil moisture and soil fertility that were associated with differences in microbial community structure. Microbial function was influenced by bacterial community composition and soil fertility. Identifying soil factors that are primary ecological drivers of soil bacterial communities, especially denitrifying populations, can potentially aid the development of predictive models for restoration of biogeochemical transformations and enhance the success of wetland restoration efforts.

Trajectories of vegetation-based indicators used to assess wetland restoration progress.

Temporal trends in attributes of restored ecosystems have been described conceptually as restoration trajectories. Measures describing the maturity or ecological integrity of a restoration site are often assumed to follow monotonically increasing trajectories over time and to eventually reach an asymptote representative of a reference ecosystem. This assumption of simple, predictable restoration trajectories underpins federal and state policies in the United States that mandate wetland restoration as compensation for wetlands damaged during development. We evaluated the validity of this assumption by tracking changes in 11 indicators of floristic integrity, often used to determine legal compliance, in 29 mitigation wetlands. Each indicator was expressed as a percentile relative to the distribution of that indicator among > 100 naturally occurring reference wetlands. Nonlinear regression was used to fit two alternative restoration trajectories to data from each site: an asymptotic (negative exponential) increase in the indicator over time and a peaked (double exponential) relationship. Depending on the particular indicator, between 48% and 76% of sites displayed trends that were at least moderately well described (R2 > 0.5) by one of the two models. Floristic indicators based on species richness, including native richness, number of native genera, and the floristic quality index, rapidly increased to asymptotes exceeding levels in a majority of reference wetlands. In contrast, indicators based on species composition, including mean coefficient of conservatism and relative importance of perennial species, increased very slowly. Thus, some indicators of restoration progress followed increasing trajectories and achieved or surpassed levels equivalent to high-quality reference sites within five years, whereas others appeared destined to either not reach equivalency or to take much longer than mitigation wetlands are typically monitored. Finally, some indicators of restoration progress, such as relative importance of native species, often increased over the first five to 10 years and then declined, which would result in a misleading assessment of progress if based on typical time scales of monitoring. Therefore, the assumption of simple, rapid, and predictable restoration trajectories that underlies wetland mitigation policy is unrealistic.

Relative influence of landscape vs. local factors on plant community assembly in restored wetlands.

Ecological restoration often involves only the manipulation of abiotic factors at the local scale. However, processes external to a restoration site determine the range of local conditions within the site, constraining the level of restoration progress that can be achieved by on-site manipulations. We examined the relationship of landscape and local explanatory variables to plant species composition in 28 restored wetlands in Illinois, USA. Using constrained ordination combined with variation partitioning, we determined the independent and joint effects of three spatially hierarchical sets of variables: (1) macroscale landscape features reflecting site setting within regional landscapes, (2) mesoscale landscape features reflecting nearby propagule sources and buffers from disturbances, and (3) local environmental factors. Because the relative influence of landscape- vs. local-scale factors on restoration success may depend on particular restoration goals, we repeated the analyses using three multivariate plant community responses that represented three frequently stated goals: (1) replicating species composition, (2) restoring a particular wetland community type, and (3) constructing sites with high value for plant conservation. Explanatory variables at landscape and local scales had independent and nearly equally strong relationships to plant species composition. In contrast, when species were aggregated based on plant traits, the independent contribution of local predictors was greater than the independent contributions of macroscale or mesoscale landscape predictors, reflecting convergence of plant trait composition in sites with similar local conditions. Local predictors explained a significant amount of variation in plant conservation value among sites, but much of the variation could be explained by large-scale landscape setting, indicating that landscape constraints on local environmental conditions limited the level of floristic conservation value achievable. The appropriate scale at which to focus restoration efforts will vary depending upon restoration objectives. Restoration of particular wetland community types might be successfully achieved through manipulation of local abiotic factors. In contrast, restoration of a particular species assemblage or reconstruction of wetlands with high value for conservation requires consideration of landscape processes and available species pools.

Floristic conservation value, nested understory floras, and the development of second-growth forest.

Nestedness analysis can reveal patterns of plant composition and diversity among forest patches. For nested floral assemblages, the plants occupying any one patch are a nested subset of the plants present in successively more speciose patches. Elimination of sensitive understory plants with human disturbance is one of several mechanisms hypothesized to generate nonrandom, nested floral distributions. Hypotheses explaining distributions of understory plants remain unsubstantiated across broad landscapes of varying forest types and disturbance histories. We sampled the vegetation of 51 floodplain and 55 upland forests across Illinois (USA) to examine how the diversity, composition, and nestedness of understory floras related to their overstory growth and structure (basal area), and their overall floristic conservation value (mean C). We found that plant assemblages were nested with respect to site species richness, such that rare plants indicated diverse forests. Floras were also nested with respect to site mean C and basal area (BA). However, in an opposite pattern from what we had expected, floras of high-BA stands were nested subsets of those of low-BA stands. A set of early-successional plants restricted to low-BA stands, and more importantly, the absence of a set of true forest plants in high-BA stands, accounted for this pattern. Additionally, we observed a decrease in species richness with increasing BA. These results are consistent with the hypothesis that recovery of true forest plants does not occur concurrently with overstory regeneration following massive anthropogenic disturbance. Nestedness by site mean C indicates that high conservation value (conservative) plants co-occur in highly diverse stands; these forests are assumed to be less disturbed historically. Because site mean C was uncorrelated with BA, BA-neutral disturbances such as livestock usage are suggested as accounting for between-site differences in mean C. When considered individually, conservative plants were actually more likely to be found in low-BA stands (uplands only). This suggests that floras of historically more open-canopied oak-hickory uplands are being degraded by canopy closure from increasing density of "mesophytic, nonpyrogenic" trees. It also indirectly suggests that recent moderate logging is uncorrelated with floristic conservation values.

Performance criteria, compliance success, and vegetation development in compensatory mitigation wetlands.

The US Army Corps of Engineers often requires wetland creation or restoration as compensation for wetlands damaged during development. These wetlands are typically monitored postconstruction to determine the level of compliance with respect to site-specific performance standards. However, defining appropriate goals and measuring success of restorations has proven difficult. We reviewed monitoring information for 76 wetlands constructed between 1992 and 2002 to summarize the performance criteria used to measure progress, assess compliance with those criteria, and, finally, to evaluate the appropriateness of those criteria. Goals were overwhelmingly focused on plant communities. Attributes used to assess the quality of restored plant communities, including percent native species and the Floristic Quality Index, increased over time but were apparently unrelated to the number of species planted. Compliance frequencies varied depending on site goals; sites often failed to comply with criteria related to survival of planted vegetation or requirements that dominant plant species should not be exotic or weedy, whereas criteria related to the establishment of cover by vegetation or by wetland-dependent plants were often met. Judgment of a site's success or failure was largely a function of the goals set for the site. Some performance criteria were too lenient to be of value in distinguishing failed from successful sites, whereas other criteria were unachievable without more intensive site management. More appropriate goals could be devised for restored wetlands by basing performance standards on past performance of similar restorations, identifying consistent temporal trends in attributes of restored sites, and using natural wetlands as references.