Wetlands

Note: As mentioned in the Release notes, we have a small staff and therefore have had to postpone some important material to the next release, scheduled for January 2018.  This will include a more thorough exploration of the importance of wetlands in addressing climate.

Wetlands only cover only a small proportion of the terrestrial surface area, with estimates ranging from 5-8% [Mitsch 2007]. Despite this, they store a disproportionate amount of Earth’s soil carbon, with estimates ranging from 20-30% [Lal 2008]. Soil carbon in wetlands can be as high as 40% [Vepraskas and Craft 2016; cf. Nahlik 2016: 2], compared to typical estimates for agricultural soils that range up to 2% [Lal 1995; see the Croplands section of this Compendium for a detailed discussion of agricultural soil carbon]. High carbon storage in wetlands is the result of anoxic conditions in wetland soils that lead to slowed decomposition and a resulting increase in stored organic matter. Wetlands have significant potential to act as carbon sinks under the right circumstances. Potentially reducing the carbon sequestration potential for wetlands are the methane emissions that they produce. Notwithstanding the uncertainty of methane emissions, wetlands globally likely serve as a significant net carbon sink.  With the complexity of processes that contribute to wetland carbon exchange and resulting function as GHG sinks or sources, management decisions may be a critical factor in determining the carbon sequestration outcome of wetland ecosystems.  [See Nahlik 2016 for review of quantitative data]

Overview

The importance of wetlands cannot be overstated. They exist in all biomes ranging from the tropics to the tundra, and on every continent except Antarctica . Wetlands also tend to host much higher concentrations of biological diversity than other ecosystems. The fact that the oldest intergovernmental environmental convention, the Ramsar Convention of 1971, was focused explicitly on wetland conservation is a testament to this importance [Mitra 2005].

The comparison of carbon dioxide storage to methane emissions is a source of uncertainty in determining the carbon sequestration potential for wetlands. Given methane’s stronger radiative forcing as a greenhouse gas, the IPCC currently uses a ratio of 25:1 to indicate the methane to carbon global warming potential.  Despite the high global warming potential presented by methane, it has a relatively short atmospheric “lifetime”, often reported in the range of 8-12 years. Although methanotrophic (methane metabolizing) bacteria are not typically calculated into the equations regarding the atmospheric lifetime of methane, bacteria that metabolize atmospheric methane (e.g., high-affinity oxidation methanotrophs) provide an additional mechanism hastening methane attenuation [Jardine 2009].  

Several trends appear to be instructive in wetland carbon storage. Tropical and temperate wetlands generally hold more carbon than northern boreal peatlands [Mitsch 2012: 7-9]. Freshwater inland wetlands may hold more carbon than coastal saline wetlands, though this may be based primarily on their surface area extent. More carbon storage occurs at depths from 30 – 120 cm (1-4 ft), at least as reported by an analysis of U.S. conterminous wetlands in which 65% of the total carbon was deeper than 30 cm (1 ft) [Nahlik 2016: 2-3].  A correlation exists between increased anthropogenic disturbance and decreased carbon storage, although this may be an artefact of settlement patterns rather than a causal relationship [Nahlik 2016: 4]. One study reported high carbon sequestration and low methane emissions in constructed wetlands [Mitsch 2012]; this is an encouraging result considering the many wetlands created as mitigations to compensate for human development impacts.

Wetland Article Summaries

Apfelbaum 1993.  Steve Apfelbaum of Applied Ecological Services, Inc. is a restoration ecologist with several decades of experience around the world. This brief paper, “The Role of Landscapes in Stormwater Management,” describes the historical condition of wetlands in the upper midwest, the degrading effects of agriculture and urbanization on water cycles, vegetation and the resultant pollution.  Included are recommendations for restoration of healthy wetlands and methods for slowing the movement of water so that it may keep soils hydrated and feed local ecosystems. In addition, restoration of wetlands includes high-capacity carbon storage due to low-oxygen conditions in wetland soils, with subsequent low rates of oxidation and loss of carbon to the atmosphere.

This paper presents evidence that many existing streams did not have conspicuous

channels and were not identified during pre-settlement times (prior to 1830s in the Midwestern United States). Many currently identified first, second, and third-order streams were identified as vegetated swales, wetlands, wet prairies, and swamps in the original land survey records of the U.S. General Land Office.

The data presented show that significant increases in discharge for low, medium, and high flows have occurred since settlement. Stream channels have formed inadvertently or were created to drain land for development and agricultural land uses. Currently, discharges may be 200 to 400 times greater than historical levels, based on data from 1886 to the present for the Des Plaines River in Illinois, a 620-square-mile watershed. Historic data document how this river had no measurable discharge or very low flow conditions for over 60 percent of each year during the period from 1886 to 1904.

This study suggests that land-use changes in the previous upland/prairie watershed have resulted in a change from a diffuse and slow overland flow to increased runoff, concentrated flows, and significantly reduced lag time. Preliminary modeling suggests the following results: reduced infiltration, reduced evaporation and evapotranspiration, greatly increased runoff and hydraulic volatility, and increased sediment yields and instream water quality problems caused by destabilization of streambanks.

The opportunity to emulate historical stormwater behavior by integrating upland landscape features in urban developments and agricultural lands offers stormwater management options that are easier to maintain, less expensive over time, attractive, and possibly more efficient compared with many conventional stormwater management solutions and the use of biofiltration wetlands.

and

Diverse and productive prairies, wetlands, savannas, and other ecological systems occupied hundreds of millions of acres in presettlement North America. These ecological systems have been replaced by a vast acreage of tilled and developed lands. Land-use changes have modified the capability of the upland systems and small depressional wetlands in the uplands to retain water and assimilate nutrients and other materials that now flow from the land into aquatic systems, streams, and wetlands. The historical plant communities that were dominated by deep-rooted, long-lived, and productive species have been primarily replaced by annual species (corn, soybeans, wheat) or shallow rooted non-native species (bluegrass lawns, brome grass fields). The native vegetation was efficient at using water and nutrients, and consequently maintained very high levels of carbon fixation and primary productivity. Modern communities, in turn, are productive but primarily above ground, in contrast to the prairie ecosystem where perhaps 70 percent of the biomass was actually created below ground in highly developed root systems. These changes in the landscape and vegetation coupled with intentional stormwater management have changed the lag time for water to remain in uplands and consequently increased the rate and volume of water leaving the landscape.

Mitsch et al 2012. This study evaluated the carbon storage and methane exchange potential for seven wetlands based on field data collected over several years, and used field data collected at 14 other wetlands globally to model the carbon sequestration and methane emission potential out to 300 years. A total of 21 wetlands were examined. The modeling accounted for the anticipated half-life of methane oxidative degradation in the atmosphere. Results indicated that methane emissions become unimportant within the 300 year model simulation time range, with most wetlands making the shift to net carbon storage by year 100 of the model. The study supports the potential for wetlands as carbon sinks.

Nahlik and Fennessy 2016. The objective of this article was to quantify the carbon stocks present in wetlands of the conterminous United States. To do so, the authors examined empirical field data collected during the 2011 National Wetland Condition Assessment conducted by the U.S. Environmental Protection Agency, and used this to quantitatively extrapolate to larger scale carbon estimates. These estimates were developed at regional and national scales. Results were evaluated by region, wetland type, freshwater or tidal status, and level of anthropogenic disturbance. The article indicated that 11.52 gigatons of carbon are present in the U.S., much of which is in soils deeper than 30 cm (1 ft). Freshwater wetlands located inland held nearly ten-fold as much carbon as intertidal wetlands overall, although this is at least partly due to the much greater aerial extent of inland freshwater wetlands; tidal wetlands still had higher concentrations of carbon storage. The authors also indicate a possible relationship between anthropogenic disturbance and carbon stocks, wherein less disturbed sites store more carbon. Insufficient data was available to determine whether this was a causal effect or an artefact of some kind, such as human preference in settlement patterns. The authors conclude that, due to the substantial carbon stocks that wetlands represent and the potential for anthropogenic impacts, existing intact wetlands should be protected to avoid the risk of further contributing to climate change.