Background Information Concerning Local Wetlands

Origination of Wetlands and Production Rates Most riparian wetlands of the high cold desert region of Wyoming probably originated in the early to mid Holocene Period. It is not clear if initial deposition of humic materials in these wetlands originated from forest or grassland sources in the higher areas. Based on the Fremont County, East Part and Dubois Area, Wyoming, Soil Survey, the majority of the riparian wetlands probably originated from grassland sources. Dr. Berg, University of Uppsala, Sweden, stated temperate forests may produce up to 80 kg/m2 (1,800 U.S. tons/acre) of humus, with the humus accumulating over period of up to 3,000 years (Berg, B. 1998). Grasslands accumulate even more humus, and have higher production rates than forests (Figure 2).

Temperature also plays an important part in the rate of humic accumulation. "In soils, every 10 ^oC increase in mean annual temperature results in the organic matter being reduced by ¹/₃ to ¹/₂ , if all other factors are constant" (Franzmeier et al. 1985).

Most riparian wetlands found in the cold desert region are found within stream systems at sites of underground flows, and occasionally springs on the surface are found immediately downstream from the wetlands themselves. Other wetlands are found close to where underground sources return to the surface. However, a few wetlands are undoubtedly dependent solely on overland flow for their main source of water.

Each year, the humus and humic material underlying the sedges, rushes, spike grasses, and other forms of vegetation produced in the wetland must be renewed by a slow process of decomposition. Detritus or other dead vegetative or animal matter remaining in the wetland from the previous season supplies material for decomposition. The decomposition of sedge may take five to eight years just to break down the least resistant portions of the dead plant to humic material. Secondly, the soil sponge of humus, humic material, and mychorrhizal fungi and bacteria underlying riparian wetlands, absorb, then slowly release water downstream.

Current humus production within the local cold desert region comes primarily from remains of sedges (Carex spp., Reed grass (Juncus spp.), Spike grass (Eliocharus spp.), and miscellaneous upland grasses, forbs, and detritus. According to a figure developed by Franzmeier et al., 1985, bunch and short grass areas of Wyoming contain approximately 400 U.S. tons of humic material per acre, to a depth of 39.4 inches. Because of more rapid and continuous growth of riparian wetland species, and the increased heights and lowered average temperatures of the local cold desert region, humic contents of undisturbed riparian wetlands are more than 400 U.S. tons.

A major problem with local riparian wetlands is that production rates like the ones mentioned above require years of vegetative input for decomposition to produce a few pounds of humic or humus material. Many pounds of dead grasses or other vegetative sources are thus needed. Soil types where riparian wetlands exist within the Sweetwater Drainage are mostly considered Mollisols, rather than Histosols. However, a caveat is that if the surface layer of organic material is so thick that the soil is recognized as a Histosol, the mollic horizon is considered to be buried and no longer meets the definition of a mollic epipedon. I believe this was the case on many of the riparian wetlands prior to the introduction of oxen and cattle. Waterlogged organic soils (Histosols) may have turnover times exceeding 2,000 years according to the University of Wisconsin Soils Handbook SS325 (Turnover = Total organic content of the soil / input of annual organic biomass). The turnover times for Mollisol soils, are placed between 200 and 1,000 years.

Portions of dead plant material with high cellulose content, such as grasses, decompose more quickly then lignin (found mostly in the stems of grasses). The varying rates of decomposition lead to various descriptors. Humic material is considered that part of a plant or animal that is only partially decomposed. The vast majority of the material underlying a riparian wetland falls within this category. Nitrogen plays an important part in initiating and then slowing decomposition. Initially, the presence of nitrogen speeds the breakdown of cellulose. Once the cellulose breaks down, the presence of nitrogen impedes, rather than stimulates, the breakdown of lignin and other more resistance portions of plants. Thus, the higher the nitrogen levels (normally high in grasses) the slower the remaining portions decompose to become humus. Decomposition rates usually range from about 45% to 100% decomposition, with grasses confined to the lower range (Berg, B. 1998).

Many other complex materials exist beneath a riparian wetland besides humus and humic material, for instance humic and fulvic acids. Complex interrelationships between these complex materials and mychorrhizal fungi and bacteria also exist below a riparian wetland, with many of their functions still unknown. A very reduced level of the true interactions taking place during plant decomposition is shown in Figure 3.

When riparian wetland soils begin to dry out in dry cold desert areas production rates of humic material rises, but the annual organic biomass input produced by riparian wetland species decreases, so no net gain may result. The drier riparian wetlands become, the more prone they are to erosional losses of much needed humic and humus material, especially when grazed by cattle. Much of the humus and humic materials that took hundreds of years to accumulate in riparian wetlands has been lost within the past 50 years because of cattle grazing.

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