Sunday, August 1, 1999

Climate Change in Wetland Areas: Carbon Cycle Implications

Climate Change in Wetland Areas Part II: Carbon Cycle Implications
From Acclimations, July-August 1999
Newsletter of the US National Assessment of
the Potential Consequences of Climate Variability and Change

By Jon Kusler, Institute for Wetland Science and Public Policy

Wetlands affect the levels of atmospheric carbon in two ways: First, many wetlands, particularly boreal and tropical peatlands, are carbon reservoirs. Carbon is contained in the standing crops of trees and other vegetation and in litter, peats, organic soils and sediments which have been built up, in some instances, over thousands of years. The magnitude of storage depends upon wetland type and size, vegetation, the depth of wetland soils, ground water levels, nutrient levels, pH and other factors discussed below. These carbon reservoirs may supply large amounts of carbon to the atmosphere if water levels are lowered or land management practices result in oxidation of soils. Second, many wetlands also continue to sequester carbon from the atmosphere through photosynthesis by wetland plants; many also act as sediment traps for carbon-rich sediments from watershed sources. However, wetlands also simultaneously release carbon as carbon dioxide, dissolved carbon, and methane. Deposited sediments are, in some instances, dislodged during floods and hurricanes. The net carbon sequestering versus carbon release roles of wetlands are complex and change over time although net, gradual sequestration occurs over time for peatlands and certain other types of wetlands. Land use practices also affect sequestering.

Wetlands as Carbon Reservoirs

In carrying out photosynthesis, wetland trees and other plants convert atmospheric carbon dioxide into biomass. Carbon may be temporarily stored in wetlands as trees and plants and the living animals which feed upon them, and detritus including fallen trees and plants and the animals which feed upon them.

Carbon may stored in the longer term in organic-rich soils, peats, and various forms of coal, shale, sandstone, and other sediments. It is long term storage that makes some wetlands effective as carbon reservoirs. Wetlands often provide longer term carbon storage than other ecosystems systems because decompositional processes are hindered by the saturated conditions, high acidity (bogs), and low temperatures (tundra). Many organic "flats" wetlands are underlain by deep layers of peat; permafrost wetlands may be underlain by more than a meter of organic rich
soil; and it is not uncommon to find ten or more meters of unconsolidated organic matter in peat lands. Significant quantities of carbon from both wetland and nonwetland sources may also be trapped and stored in wetland sediments.

The long-term effectiveness of some wetlands in storing carbon is demonstrated by the extensive coal deposits throughout the world. These were formed in wetland or wetland-like conditions, in many instances hundreds of millions of years ago.

Like wetland forests, upland forests sequester carbon in standing vegetation and to a lesser extent in debris and the upper layers of the soil. However, long storage in soils is often limited due to rapid decompositional processes and re-release to the atmosphere. Rapid decomposition and re-release also occurs in some types of wetlands such as rice paddies.

The total amount of carbon in wetland standing vegetation, debris, peats and other soils is large, and it has been estimated that wetlands hold 35% of the total terrestrial carbon. Drainage of peatlands, tundra, and other wetlands acting as carbon reservoirs results in oxidation of the organic matter, releasing it to the atmosphere as CO2, methane, and other greenhouse gases. Conversely, enhancement, restoration or creation of certain wetlands may provide important additional carbon sinks.

Wetlands as Active Carbon Sinks (Sequestration)

Photosynthesis by wetland plants converts atmospheric CO2 into biomass. Wetlands are, therefore, net carbon sinks if the rate of plant production exceeds the rate of decomposition for fallen trees, litter, and wetland soils (e.g., peats) and net export through release of gases or water transport of dissolved carbon or sediments. Wetlands often store more carbon than other ecosystems despite their low productivity due to low decompostion rates. In addition, wetlands may act also as net carbon sinks if they trap carbon-rich sediment from upland sources and such accumulation exceeds losses. Many riverine, estuarine, coastal and estuarine wetlands trap large quantities of sediment from natural and anthropogenic watershed sources.

Rates of photosynthesis in wetlands, of course, vary. Some wetlands (e.g., coastal flats, playas) have little vegetation with resulting limited production of plant biomass; some (e.g., salt marshes, tropical forests) have much vegetation and high rates of production. Trees and other vegetation grow quickly in tropical and temperate wetlands with ample sunlight, nutrients, water and warm temperatures. In contrast, the growth of trees and other vegetation is slow for high latitude wetlands (e.g., peatlands) with less sun, nutrients, and water and colder temperatures.

Rates of decomposition also vary and fluctuate over time, depending upon a variety of interrelated factors such as temperature, water levels, hydroperiod, flow of water and nutrients. In addition, removal of carbon by physical processes may occur quickly in some wetlands and very slowly in others. For example, litter, peat and carbon rich sediments may be quickly removed from some coastal wetlands by frequent coastal storms; riverine flood flows may scour some riverine wetlands. In contrast, organic mater in bogs may remain undisturbed for hundreds or thousands of years (e.g. bogs) in others.

Research on peat lands indicates that photosynthesis and decompositional processes are complex and fluctuate in a specific setting, depending upon ground water levels, temperature, substrate availability, nutrient levels, methanogene population and other factors. Research suggests that, overall, peatlands are net carbon sinks. However, releases of carbon dioxide and methane may exceed photosynthesis in some circumstances. In addition, peat lands may convert carbon dioxide to methane-a more active atmospheric gas. It has been suggested that wetlands are a source of 15% to 20% of atmospheric methane.

Processes vary at different levels with a peat deposit. The lower levels of peat (catotelm) produce larger amounts of methane while the upper levels (acrotelm) produce carbon dioxide and at least partially oxidize methane released from the lower levels. The output of methane is determined by the production of methane by methanogenic bacteria and its removal by methanotrophic bacteria. Studies suggest that if the water levels are lowered in the upper levels due to drainage, decreased precipitation, or increased evaporation and transpiration, carbon dioxide and methane production may exceed sequestration. However, this may not continue once the upper levels of peat are oxidized to the level of the new water table.

Impact of Climate Change on Wetland Carbon Sequestering

Climate change will likely affect the ability of wetlands to sequester carbon, but the results will vary and are difficult to predict. Increased CO2 in the atmosphere will result in increased plant growth in most if not all wetlands, and the potential for increased carbon sequestration will increase under certain circumstances. Increased rainfall may also result in increased sediment deposition in some wetlands. Other the other hand, increased temperatures may result in decreased ground and surface water levels for many wetlands due to increased evapotranspiration where precipitation decreases, remains steady, or only slightly increases. Decreased ground and surface water levels and increased temperatures may result in increased decomposition. The carbon storage and sequestering role of peatlands could also be reduced by the melting of permafrost. Certainly, responses will be complex. Eville Gorham wrote in 1991 that, "given the diversity of possible responses by boreal and subartic peatlands to climatic warming, it is impossible at present to predict their future contributions to the global carbon cycle" - and others have recently endorsed Gorham's conclusion.

Management Strategies for Protecting and/or Enhancing Carbon Reserves and Wetland Carbon Sequestering Capabilities

A variety of strategies are available to protect and or enhance carbon reserves and wetland carbon sequestering. Some would be compatible with broader biodiversity protection goals and other goals to protect wetland functions; others would not. Some strategies include:

  • Protect natural wetlands systems.
  • Conduct regional inventories and prepare management plans for wetlands of greatest importance as carbon reserves and for carbon sequestering.
  • Control fires.
  • Protect low flows and residual water.
  • Install water control structures.
  • Plant trees, other vegetation.
  • Restore, enhance, and create wetlands.

Conclusions and Recommendations

There is broad agreement that certain types of wetlands contain large historic, reservoirs of carbon in above ground biomass, litter, peats, soils and sediments. There is also agreement that land management practices such as drainage may release at least a portion of the carbon. However, accurate estimates are not available for total carbon reserves in wetlands the U.S. or other countries. And, the impacts of various land management practices such as forestry upon such reservoirs are also only partially known.

Similarly, there is broad agreement that wetland plants continue to convert atmospheric carbon into biomass and carbon-rich sediments continue to be deposited in wetlands. Net carbon sequestration occurs as long as rates of conversion exceed decomposition and external transport of materials from wetlands. However, it is difficult to evaluate the net carbon sequestering role of wetlands because decomposition of organic matter, methanogenisis and sediment fluxes are extremely complex and there are gaps in scientific knowledge.

What is needed to better evaluate generically and in specific settings the roles of wetlands as carbon reservoirs and for carbon sequestering and to guide protection, enhancement, restoration or creation efforts. A combination of literature surveys, scientific consensus-building measures (workshops), field measures and laboratory studies are needed. Some priority topics for such evaluation efforts include: evaluating wetlands as carbon reservoirs; estimating sequestration rates in wetlands; and enhancing, restoring and creating wetlands.

For more information, contact:

Jon Kusler, Director, Association of State Wetland Managers, P.O. Box 269, Berne, NY 12023-9746 (518-872-1804).