The determination of rates and stocks of carbon storage in salt marshes, as well as their protection, require that we know where they and their boundaries are. Marsh boundaries are conventionally ...mapped through recognition of plant communities using aerial photography or satellite imagery. We examined the possibility of substituting the use of 1 m resolution LiDAR-derived digital elevation models (DEMs) and tidal elevations to establish salt marsh upper boundaries on the New Brunswick coasts of the Gulf of St. Lawrence and the Bay of Fundy, testing this method at tidal ranges from ≤2 to ≥4 m. LiDAR-mapped marsh boundaries were verified with high spatial resolution satellite imagery and a subset through field mapping of the upland marsh edge based upon vegetation and soil characteristics, recording the edge location and elevation with a Differential Geographic Positioning System. The results show that the use of high-resolution LiDAR and tidal elevation data can successfully map the upper boundary of salt marshes without the need to first map plant species. The marsh map area resulting from our mapping was ~30% lower than that in the province’s aerial-photograph-based maps. However, the difference was not primarily due to the location of the upper marsh boundaries but more so because of the exclusion of mudflats and large creeks (features that are not valued as carbon sinks) using the LiDAR method that are often mapped as marsh areas in the provincial maps. Despite some minor limitations, the development of DEMs derived from LiDAR can be applied to update and correct existing salt marsh maps along extensive sections of coastlines in less time than required to manually trace from imagery. This is vital information for governments and NGOs seeking to conserve these environments, as accurate mapping of the location and area of these ecosystems is a necessary basis for conservation prioritization indices.
We measured total carbon stocks of three marshes: Two formed in association with a developing spit along the Gulf of St. Lawrence coast of New Brunswick, Canada, and another with a lagoon on the ...coast of Maine, USA. Overall, 46 cores and 157 depth recordings were collected to determine depth of the marsh deposits. Total marsh soil volume was estimated by interpolation. In all marshes soil depth varied in a predictable pattern based upon marsh developmental history. In spit marshes deposit age and thickness increased towards the oldest portion of the spit. In the lagoonal marsh, soil depth was greatest in the center and declined towards both the upland and seaward margins. This same pattern held on axes perpendicular to the primary, age axis of the spit marshes. In each marsh C density did not significantly vary with depth so that marsh depth was an acceptable estimator of C stock, and therefore driven by the geomorphic context of the marshes we studied. There were major differences in C stock estimates produced using GIS interpolation, average C contained in all marsh cores, or cores along a single transect. Our study demonstrates that assuming a soil depth of just 0.5 or 1 m can substantially under- or overestimate marsh carbon stocks and the value of that stock on a carbon market.
•Soil C storage is estimated for two spit and one lagoonal salt marsh.•Soil depth varied in a pattern predictable by environmental history.•Significant variation in carbon storage found between estimation techniques.•Consistent C density enables C stock estimates using soil volume within marshes.•Using standard soil depths can substantially over or under- estimate C stocks.
Our study of a St. Lawrence Estuary marsh reveals that, compared to native Spartina patens‐dominated vegetation, invasive Phragmites australis makes a greater contribution to soil volume and carbon ...stock (referred to as blue carbon). Phragmites' contributions to soil volume enhance marsh sustainability in face of sea level rise, and its greater contribution to soil carbon helps to reduce the atmospheric concentration of CO2. Phragmites australis (common reed) is a cosmopolitan species growing in fresh to brackish wetlands. An invasive genetic strain, introduced from Europe or Asia, has expanded extensively along the St. Lawrence River in the last few decades but has been little studied on the estuarine portion. We collected soil cores from three sites within an invasive Phragmites stand and one site within S. patens‐dominated stand in a St. Lawrence Estuary salt marsh near la Pocatiere, Quebec. We measured the bulk density, carbon content, volume, and mass of belowground organic matter in 2‐cm‐thick soil layers of three cores at each Phragmites site. Bulk density and carbon content were measured in 5‐cm‐thick soil layers of three cores at S. patens site. Results showed that soil in the Phragmites stands held 37–77% more blue carbon than in the S. patens‐dominated marsh. Based upon their diameter size, Phragmites rhizomes could be contributing 7.4–10.2 cm to the thickness in the upper 20 cm of soil. We suggest that any management of invasive Phragmites include consideration of its role in increasing blue carbon stocks and marsh resilience along with other ecosystem services.
Plain Language Summary
An aggressive invasive strain of the common reed (Phragmites australis) is widespread on the eastern coast of North America. It is rapidly spreading through salt marshes, the grassy meadows, which straddle mean sea level on the coast of Quebec's St. Lawrence Estuary. This reed is considered to degrade the value of the native salt marsh and its biodiversity by displacing native vegetation and the habitat it provides for birds and wildlife such as the Nelson's Sparrow, designated as a Species of Special Concern in nearby Maine. Our study of a St. Lawrence salt marsh, however, shows that the invasive reed can have positive effects on soil properties. We compared soils below the reed and the most common native grass, salt meadow hay (Spartina patens) and found 37–77% more carbon stored in the reed soil. Thus, growth of the reed increases the ability of salt marshes to reduce concentrations of atmospheric carbon dioxide and mitigate climate change. We measured how much the roots and rhizomes (underground stems) of the two species contributed to the soil volume and found that the reed contributed considerably more. Greater contributions to soil volume means that the elevation of the soil surface will increase faster in the reed marsh and help this tidal ecosystem to keep up with increased rates of sea level rise that will accompany climate warming. Thus, any management of the invasive reed should include consideration of its role in mitigation of climate change and marsh adaptation to sea level rise, along with other ecosystem values provided by the native vegetation.
Key Points
Phragmites australis has invaded Spartina patens‐dominated marshes of the St. Lawrence Estuary
Soils of invasive Phragmites had 37–77% greater C stocks than S. patens soils
Phragmites contributed 7.4–10.2 cm to the thickness of the top 20 cm of soil
The data presented here includes a table of soils measurements taken at high resolution depth intervals (5 cm) for three salt marshes, two along the New Brunswick coast of Canada and one on the ...southern coast of Maine, USA. The data includes a table which includes the bulk density, percent organic matter, percent organic carbon, carbon stock, and rhizome dominance (if identifiable) at 5 cm depth intervals for each soil core. Shapefiles are also included which indicates the GPS position of acquired cores and sites where marsh depth was measured but no material was recovered. These shapefiles also include marsh peat depth and estimates of carbon stock for each point. For further information and interpretation of the included data please see the companion research article titled “The Importance of Geomorphic Context for Estimating the Carbon Stock of Salt Marshes” 1.
Coastal vegetated ecosystems, such as salt marshes, actively sequester large amounts of carbon from the atmosphere and can store this carbon in their soils for millennia. These ecosystems have been ...badly degraded from anthropogenic activity over time, with evidence suggesting that their stores of carbon can be released to the atmosphere as a result. There is a general lack of studies which report on the carbon storage in salt marshes over the full depth of the soil deposit, and it is not well established just how much carbon is stored in these systems. Correspondingly, 1 m is often used as a default in estimates. Thus it is largely unknown what geomorphic or environmental parameters drive differences in carbon storage between marshes. How much carbon is lost when drained for agriculture or other land use change has a similar paucity in studies, and those which do exist are geographically biased to warm temperate climates. This thesis reports on two projects which seek to address these gaps in research. The chapter two study sought to estimate the total carbon stock of four salt marshes along the coasts of New Brunswick, Canada, and Maine, USA using GIS interpolation and identify any spatial trends or relations to general climate and geomorphic conditions. The spatial distribution of soil in the marshes was similar to developmental models of developed for similar marsh types in literature, with soil depth the greatest in the center of the marsh and declining towards the upland and seaward margins. The average carbon storage and carbon densities of the marshes were lower than current global averages. Average carbon density with depth was very stable except a single notable decrease which occurred at a breakpoint at 50 cm depth – likely due to carbon losses from the rapid decay of labile carbon in the rooting zone. Thus a very strong linear relationship between soil depth and carbon storage was found, which would allow for estimations of carbon storage using just soil depth in marshes of similar characteristics. This also indicates that assuming a soil depth (such as 1 m) is not an acceptable method when estimating carbon stocks. Comparing interpolation results to simple averages of the cores indicated that a single transect of cores could acceptably estimate soil depth (thus carbon storage) in marshes with simple morphology, and that one transect per axis of soil depth variation may work in more complex marsh systems. The chapter three study measured carbon stocks and calculated losses for a series of drained marshes and paired undrained marsh along the Kamouraska region of the St. Lawrence River, Quebec, Canada. The estimated rate of loss averaged 459 g C m-2 yr-1, with overall losses varying between 15% and 39% of the original amount since drainage. The rate is lower than the IPCC default emission factor and most current rates reported in literature, which are from warm temperate climates, and suggests that rates of carbon loss may vary in different climates. Using the average rate of loss for the St. Lawrence region, the total estimated carbon loss in the region since 1987 (further dyking was banned after this date) is 1,673,055 t C.
PDF
