Prairie Wanderings: Can the prairie restore itself?
By PAUL G. JANTZEN
Contributing writer
What happens to a field when it is abandoned because it has lost its fertility?
I heard part of the answer back in 1965 at the dedication of the Sand Prairie Natural History Reservation in Harvey County. Following the dinner, Elroy L. Rice, professor of botany at the University of Oklahoma, addressed the assembled group on his investigations into the steps of ecological succession in abandoned fields in central Oklahoma.
Succession is the replacement of one community by another community through time. The pioneer species are replaced by a succession of species each adapted to the changes brought about by its predecessors, until a relatively stable community results.
Back in 1937, ecologists found that fields abandoned 40 years earlier had not yet reached the same plant species composition as those climax prairies of the region.
In central Oklahoma, succession in fields abandoned due to loss of fertility generally followed four stages: (1) annual weeds, (2) annual grasses, (3) perennial bunch grass, and (4) climax prairie. The weed stage, lasting two to three years, was often composed of pigweed, ragweed, horseweed, crabgrass, and Japanese brome. The annual grass stage, dominated by triple lawn grass, lasted 9 to 13 years. The perennial bunch grass stage was dominated by little bluestem for 50 to 100 years.
The actual time spans of various stages are determined by many factors, including climate, soil conditions, natural seed supplies, and the effects of various insects, birds, mammals, and soil organisms. The climax community of the region is dominated by little bluestem, big bluestem, switchgrass, and Indian grass. The same climax is found in the tallgrass prairie of eastern Kansas.
Dr. Rice searched for causes of the sequence of stages in abandoned fields. Various workers found that the abandoned fields in Oklahoma were severely deficient in nitrogen and phosphorus. Dr. Rice found that the sequence of stages followed the order of increasing requirements for nitrogen and phosphorus. Apparently, some plants of the earliest stage root deeply enough to gradually bring nitrogen and phosphorus from deeper layers of soil to the surface. But this takes time.
Dr. Rice also found that the pioneer species in old field succession contained inhibitors of nitrogen fixing and nitrifying bacteria. Nitrogen-fixing bacteria combine nitrogen from the air with hydrogen to form ammonia or ammonium ions. Nitrifying bacteria then oxidize those products into the nitrites and nitrates required by plants to make proteins. So, another reason succession takes so much time is that the weedy species interrupt the nitrogen cycle, prolonging stage (1), giving them an advantage over plants of later stages of succession which have higher nitrogen requirements.
Prairie ecologist David Costello points out that, as the dominant plant species change from one stage of succession to another, associated animal populations also change. For example, the number of harvester-ant colonies increased in weedy and annual grass stages to as many as 57 per acre. In the final mixed prairie the number of their colonies diminished to one or two mounds per acre. Apparently, their numbers were associated with food supply. These ants, in turn, influenced succession by their careless loss of seeds as they collected food for storage.
Also living among the roots and rhizomes in prairie soils are other important insects, nematodes, and earthworms, plus bacteria and fungi such as mycorrhizae. Big bluestem inoculated with mycorrhizae in a greenhouse experiment produced more than 80 times as much tissue as plants grown in sterile soil.
In contrast to the natural process of nature, most agriculture attempts to halt succession at an early stage. It imposes an annual monoculture (single species) on a system that tends toward the diversity and stability found in the climax of a perennial polyculture. But even the climax prairie is not without change. There were, and are, always disturbances such as prolonged drought, flooding, wind erosion, insect invasions, rodent burrowing, bison wallowing, and overgrazing. Even a buffalo chip or a heap of cattle dung are localized disturbances.
Costello observed that "all the plants that become conspicuous during [various stages of] succession actually persist on the prairie at all times. They are able to sustain themselves in suitable microhabitats made by gopher mounds, badger diggings, anthills, drifted soil, and mouse runways." These few early-stage plant species are ready, in the event of a catastrophe, to quickly renew the succession process until it again reaches the climax tallgrass prairie with its bluestems, switchgrass, Indian grass, and the myriads of broadleaf plants and associated animals, bacteria, and fungi.
So an abused prairie can restore itself, if given time and if appropriate microorganisms are still available in the soil.
