Virginie Bouchard

EFFECTS OF MACROPHYTE FUNCTIONAL GROUP RICHNESS ON EMERGENT FRESHWATER WETLAND FUNCTIONS

Most plant diversity-function studies have been conducted in terrestrial ecosystems and have focused on plant productivity and nutrient uptake/retention, with a notable lack of attention paid to belowground processes (e.g., root dynamics, decomposition, trace gas fluxes). Here we present results from a mesocosm experiment in which we assessed how the richness of emergent macrophyte functional groups influences aboveground and belowground plant growth and microbial-mediated functions related to carbon and nitrogen cycling, with an emphasis on methane (CH4) efflux and potential denitrification rates. We found that an increase in the richness of wetland plant functional groups enhanced belowground plant biomass, altered rooting patterns, and decreased methane efflux, while having no effect on aboveground plant production or denitrification potential. We hypothesize that the greater root production and increased rooting depth in the highest diversity treatments enhanced CH4 oxidation to a relatively greater degree than methane production, leading to an overall decrease in CH4 efflux across our plant functional group richness gradient.

IMPACT OF THE INVASIVE NATIVE SPECIES ELYMUS ATHERICUS ON CARBON POOLS IN A SALT MARSH

The spread of the invasive native clonal grass Elymus athericus is one of the most significant changes that have affected the plant communities of European salt marshes in the last decade. The goal of this study was to investigate the rate of this invasion and its impact on C cycling in a non-grazed salt marsh of the Mont Saint-Michel Bay (France). Aboveground net primary production (ANPP), litter quantity and quality, and decomposition rates were quantified in Elymus athericus and in the original plant communities (“control”). Since 1991, Elymus athericus has been replacing the characteristic plant communities at a rate of 4 ha yr−1. ANPP was significantly higher in Elymus athericus (3011±347 gDW m−2 yr−1 and 1181±123 gC m−2 yr−1) than in the control (2028±239 gDW m−2 yr−1 and 771±76 gC m−2 yr−1) stands. Lignin content of Elymus athericus’ litter was 3.5 times greater than in the litter of the control communities, explaining its accumulation under the vegetation (i.e., litter quantity was from 2 to 10 times greater under Elymus athericus than under the control vegetation). C mineralization in the sediment (expressed as rate of CO2 per mass of sediment) was significantly lower under Elymus athericus than under the control vegetation. Our results suggest that Elymus athericus increased C trapping within the salt marsh and thus limits the potential for C exportation toward coastal waters.

No‐net‐loss not met for nutrient function in freshwater marshes: recommendations for wetland mitigation policies

Wetlands provide many important services throughout the world, with an estimated economic value that, in comparison to other ecosystems, far exceeds their relatively small global extent. In recognition of their importance, both national and international regulations exist to protect the worlds remaining wetlands. Of growing interest is the “no-net-loss” policy which permits unavoidable destruction of wetlands if compensated by restoration of degraded wetlands or creation of new wetlands. The fundamental assumption of no-net-loss is that wetlands can be created which function equivalently to natural wetlands. One integral function that wetlands perform is cycling of carbon, nitrogen and phosphorus. Here we demonstrate that loss of this nutrient-related function is not being mitigated by creation or restoration of wetlands. We compare indicators of plant- and microbial-mediated functions, as well as abiotic (e.g., soil character, hydrology) and biotic (e.g., plant community composition) structure, between 10 created or restored and 5 natural freshwater depressional wetlands in central Ohio, USA. Nutrient stocks were generally smaller and transformations slower in created wetlands than in natural wetlands, with little development over time. Of particular concern were differences in C- and N-related function. Created wetlands stored 90% less C within litter and 80% less C within soil and processed 60% less N through denitrification, on average compared to natural wetlands. Our study suggests that subversion of natural wetlands into restored or created wetlands could have large-scale environmental consequences such as reduced capacity for nitrate removal and C sequestration.

AN ASSESSMENT OF WETLAND IMPACTS AND COMPENSATORY MITIGATION IN THE CUYAHOGA RIVER WATERSHED, OHIO, USA

A watershed-based assessment of wetland impacts and compensatory mitigation was conducted for the Cuyahoga River Watershed (CRW) in northeastern Ohio, USA, to explore the effectiveness of wetland mitigation regulations and any resulting cumulative changes to wetland and landscape structure. Mitigation projects from 23 Section 401 certifications and Ohio Isolated Wetland permits were evaluated for permit compliance, wetland structure, and landscape context. Although there was a net gain in wetland area as a result of the 23 permits, the CRW experienced a net loss of wetland acreage due to the exportation to mitigation banks located outside the watershed. The majority of projects (67%) that restored or created wetlands independently (not at a mitigation bank) were not successful at meeting permit requirements in terms of wetland area. The comparison of impacted and mitigation wetland vegetation types revealed an increase in open-water/emergent wetland area and a decrease in area of scrub/shrub and forested wetlands, along with a decrease in the number of wetlands from 134 impacted wetlands to 65 mitigation wetlands. Impacted wetlands were significantly smaller than replacement wetlands. Landscape composition surrounding the wetlands was highly variable, varying from 17%–75% natural land uses and from 18%–82% human land uses. We suggest that an improvement in compliance with permit requirements is necessary. Current wetland policy allows for the exportation of wetlands for mitigation purposes, which can result in the loss of wetlands from some hydrologic units. The consideration of wetland structure needs to be incorporated into the regulatory process to avoid a shift in wetland types that are present. Finally, instead of reviewing projects on a site-by-site basis, a landscape approach should be taken in order to avoid the loss of upland-wetland heterogeneity and the placement of mitigation wetlands in degraded landscapes.

Sheep grazing as management tool in Western European saltmarshes

The effects of sheep grazing on plant community structure and diversity were studied in saltmarshes of the Mont-Saint-Michel bay. This study took place at two scales: (1) at the scale of the entire bay to explore the changes in plant community over a ten year period; and (2) locally with the use of experimental exclosure set up to mimic the abandonment of grazing. Moderate grazing generally enhanced plant richness and diversity, while the absence of grazing and overgrazing lead to a decrease in diversity and richness. The development of management strategies is becoming critical to preserve the diversity of saltmarshes functions.

Regulation of carbon processes by macrophyte species in a Great Lakes coastal wetland.

The objective of this research was to compare and contrast C dynamics within plots occupied by Phragmites australis, Typha spp., and Sagittaria latifolia in a Lake Erie coastal wetland (Ohio, USA). The effect of each species on above- and belowground biomass, soil C pools, soil labile C, litter decomposition rates, and microbial catabolic response profiles were analyzed. Phragmites australis and Typha spp. produced significantly more aboveground biomass (1,522 ± 464 and 1,177 ± 164 g DM m−2, respectively) than S. latifolia (500 ± 80 g DM m−2), although no difference was observed in terms of belowground biomass. After 208 days in the field, litter of S. latifolia had lost 72% of its initial mass while only 47% of the litter of Typha spp. and P. australis had decomposed. This coupled process of high primary production and slow litter decomposition within P. australis and Typha spp. communities did not translate into greater accumulation of C in the soil. In fact, we observed lower rates of C mineralization and greater biomass of methanogens in the S. latifolia plots. Despite similar water level, soil conditions in the S. latifolia community was more saturated, which might have limited availability of C for microbial consumption in these plots. Microbial catabolic responses to 24 substrates demonstrated distinct differences in the respiration responses of the soil microbial communities of the three macrophyte species. The microbial community found in the rhizosphere of P. australis was particularly responsive to phenolic acids. Few differences in C fluxes and pools were observed between plots occupied by P. australis and Typha spp., but the replacement of S. latifolia by one of the two other species could have a significant effect on the C cycle in the Great Lakes coastal wetlands.

The role of spatio‐temporal heterogeneity in the establishment and maintenance of Suaeda maritima in salt marshes

Abstract The effects of disturbance and microtopography on the organization and dynamics of plant communities were studied in a European salt marsh located in the Bay of Mont St. Michel, France. The existence of seed trapping mechanisms was also tested. The study took place in the lower and middle marsh plant communities dominated by the perennials Puccinellia maritima and Halimione portulacoides, respectively and associated with the annual Suaeda maritima. Three treatments were used in series of plots placed in each community: (1) vegetation removal and root destruction to a depth of 10 cm and refilling, (2) non‐remnant herbicide treatment without vegetation removal and (3) creation of depressions (20 cm deep). These treatments were compared with adjacent control plots. The first year of the experiment showed that the perennials facilitated the establishment of Suaeda by trapping its seeds. Estimation of cover, density and biomass over 5 yr following the disturbances showed that in the first 2 yr Suaeda dominated the disturbed plots. Thereafter Suaeda was gradually eliminated by competitive exclusion after ca. 3 yr in the zone originally dominated by Puccinellia maritima and after 4 yr in the zone occupied by Halimione portulacoides. Depressions constituted refuge habitats for Suaeda by limiting competition with the perennials but also led to a high risk of mortality with temporal fluctuations in density. Despite a period of investigation limited to 5 yr, our study demonstrated that natural disturbances of various types occurred and influenced the dynamics of Suaeda, Halimione and Puccinellia. We deduced that natural disturbances and microtopography are responsible for the maintenance of the habitat in a state of non‐equilibrium by favouring the establishment of both spatial and temporal environmental heterogeneity. These conditions appear to be particularly favourable for the maintenance of annual species such as Suaeda maritima. Nomenclature:Tutin et al. (1964–1980).

EFFECT OF A WOODY (SALIX NIGRA) AND AN HERBACEOUS (JUNCUS EFFUSUS) MACROPHYTE SPECIES ON METHANE DYNAMICS AND DENITRIFICATION

Wetlands improve water quality through denitrification, but these ecosystems are also an important source of the greenhouse gas, methane. The objective of this research was to determine the effect of two common macrophyte species (Juncus effusus and Salix nigra) on denitrification and on the methane cycle. The research was conducted in a newly constructed wetland on the Columbus campus of The Ohio State University during two growing seasons. In the wetland, some plots were left unplanted, while others were planted with Salix or Juncus species (i.e., 3 treatments; n = 15 per treatment). For each treatment, we quantified concentrations of methane at two depths (15 and 25 cm) in the sediment, emissions of methane from the sediment and through the plants, and denitrification rates. During most of the second growing season, both species had a limited effect on denitrification and methanogenesis. The effects of the plants became evident by the end of the second growing season and during the third growing season. During the third growing season, Salix species enhanced the release of the greenhouse gas methane to the atmosphere, while Juncus limited the emission of methane. In comparison to the unplanted plots, the long-term removal of nitrate by denitrification was favored in the plots planted with Juncus and was not affected by Salix. Our study provides evidence that certain plants (such as Juncus) can be planted in constructed wetlands to favor denitrification, while buffering methane emission.

Actual litter decomposition rates in salt marshes measured using near-infrared reflectance spectroscopy

Abstract Near-infrared reflectance spectroscopy (NIRS) has been widely applied as a holistic tool to investigate decomposition processes in terrestrial ecosystems. The objectives of this research were to determine the potential of NIRS to predict (1) the halophytic litter chemistry (i.e., carbon and nitrogen content) during decomposition, and (2) the stage of decomposition of halophytic litter. Decomposition experiments were conducted in the laboratory with microcosms placed under a wide range of physical characteristics and in the field with litterbags located along the elevation gradient (i.e., low to upper marsh). Microcosm experiments were used to calibrate the predictive equations. These calibration equations were then applied to the field data to test their capacity to predict %C, %N, and litter mass loss (LML). NIRS can be successfully applied to predict chemical composition of halophyte litter during decomposition processes. We hypothesized that the use of litterbags in the field might lead to a 20–40% overestimation of the decay rate as fine organic debris are lost through the meshes of the litterbags. NIRS can be used as a fast and nondestructive method to more accurately predict decay rates, and thus microbial consumption in aquatic environments.

Plant Community Composition More Predictive than Diversity of Carbon Cycling in Freshwater Wetlands

Changes in the world’s species composition and the loss of biodiversity have prompted a closer investigation of the importance of biodiversity and community composition to ecosystem functioning. However, few studies have explored this relationship outside of controlled experiments. Here, we examined the relationship between plant diversity, primary production, and methane efflux in freshwater wetlands in an across-site field study and assessed the applicability of experimental findings to natural wetlands. Four wetland sites in central Ohio (USA) were divided into two plant communities, one dominated by clonal species and one dominated by non-clonal species. We found that plant diversity was negatively correlated with aboveground biomass in both the clonal and non-clonal communities. Overall, plant community composition was a stronger predictor than diversity of the response variables and in certain instances a stronger predictor than environmental factors such as soil organic matter content, moisture content, and pH. Thus, plant community composition is an important driver of ecosystem functioning in depressional wetlands beyond the well-known environmental factors. Additionally, our work indicates that results from experimental wetland studies of the relationship among diversity, biomass and methane emission are not applicable to the wetland ecosystems included in our study.