Frigid Alluvial Flat

Mechanism

Using Drakesbad Meadow as the example illustrating the transitions that can occur in this ecological site, the transition was initiated in the early 1900’s when the meadow hydrology was altered and shrubs were manually removed. Non-native grasses were seeded to improve forage for livestock. A threshold was crossed when the hydrology of the site was altered, and non-native plant species established. Spring flow was diverted down a road, away from the meadow. Ditches were created in the meadow to drain the wet areas and redirect flow to drier areas. The ditches captured surface water flow, subsequently causing the water table to drop in the surrounding areas. The lower water tables allowed other plant communities to establish on the recently dried fen habitat. Additional drainage channels may have formed when the dominant vegetation shifted from sedges, which form dense stable root mats, to grasses, which offer less protection against soil erosion. In low slope meadows dominated by sedge, surface flow generally sheets across the surface and infiltrates as subsurface flow without true channels, but in areas where grasses are dominant and the soil is less tightly bound, the surface flow can scour channels. Shrub removal has left the meadow dominated by graminoid species and now lacks the valuable wildlife habitat provided by shrub cover. Willows have not reestablished their former cover in the meadow. The physical impact from grazing horses and cattle is not documented, but it’s conceivable they may have altered species composition due to selective herbivory and/or soil compaction. Introduced grasses have created plant communities that did not exist prior to European settlement. It is possible for fen soils to dry to a sufficient depth and duration to allow for the decomposition of the organic material in the upper soil horizons. This is a real concern, but soils here do not indicate that this process is occurring at this time. As stated earlier, the development of organic fen soils takes thousands of years, but they can decompose in a decade when dried out. When the organic material decomposes the fibrous peat disintegrates and becomes amorphous. This causes the peat to shrink, reducing hydraulic conductivity and increasing bulk density (Patterson, 2005). The process of decay is a combination of complex chemical and biological processes. Many of these processes are inhibited in anaerobic conditions (lack of oxygen). When water tables drop in fen habitats, peat is exposed to oxygen and begins to decay. The exposure of the peat to air can increases carbon mineralization, which creates a loss of carbon into the atmosphere in the form of carbon dioxide (CO2). The process of nitrogen mineralization (decay) is also affected by changes in the water table, but the process seems to be complex and dependent upon the depth of the water table and other variables. Nitrogen mineralization (decay) creates ammonium ions (NH4+) that attach to the negatively charged clay particles. The ammonium can be utilized by plants or processed further to form nitrate via nitrification. Nitrification requires oxygen and creates negatively charged nitrate ions (NO3-) that are leached out of the soil, leaving positively charged hydrogen ions (H+) which, in turn, decrease the pH of the soil. The extent of the impact from these processes in the areas where the peat has been drained for almost 100 years is unclear and could use more study.