April 7, 2008

Prairie ecosystems evolved along with great herds of grazers until each came to depend on the other for existence. Animals like bison and cattle, when managed so they graze an area hard for a short time then move on, will do wonders to enhance the soil and the prairie matrix (even though when they are moved off the area, it looks torn up). The seemingly violent event causes plant roots to die back (because their photosynthesizing tops have been mowed off), which deposits organic matter directly into the soil. The root dieback also creates additional channels in the soil matrix that enhance water and gas transport.

Brief mob grazing events have the added benefit of chopping the dry, dead and oxidized plant material left from previous growing seasons into small pieces, and incorporating them into the soil. Make no mistake about it. The rich deep prairie top soils are the direct result of regular and cyclical organic matter deposits caused by root dieback and mulching of the tops. Without regular grazing, they would likely have never formed.

In an artificial situation, where ungulate herds don’t have access to unlimited grassland acreages, they will have a tendency to visit their favorite areas frequently enough that they will cause damage (soil and plant) in the long term. Thus, it is important to force animals (especially domestic) to move around so that the pasture can recover sufficiently after each grazing event. Follow a herd of bison with access to hundreds of thousands of acres for a year, and it will be doubtful that they will visit the same patch of prairie twice. Unless, of course, it’s a prairie dog town. But that’s a story for another time.