Prairie wanderings: Ecologic succession
By PAUL G. JANTZEN
Contributing writer
Plant and animal communities are not permanent. The combination of all species populations occupying a given area and the nonliving environment with which they interact is an ecosystem. Fires, climate changes, and earthquakes an create conditions better suited to organisms with different adaptations. The new combination of organisms, in turn, may so change the environment that the area can better support still another combination of creatures. A pond becomes filled with silt to become a marsh which becomes a meadow. This orderly replacement of one community by another over time is called ecologic succession.
The final stage of natural succession is determined by the climate and geography of the region. Fire, lumbering, drought, flood, overgrazing, and farming may interrupt succession. The region may then return to an earlier stage for a while.
Elroy L. Rice, professor of botany at the University of Oklahoma, has done extensive research into the succession that follows the depletion of minerals and the consequent abandonment of agricultural lands of central Oklahoma. He and his colleagues found that succession in the abandoned fields of that area included four steps: (1) weeds, (2) annual grasses, (3) perennial bunch grasses, and (4) true prairie.
The weed stage, composed of annual weeds such as pigweeds, ragweeds, horseweeds, and crabgrass usually last two to three years. The annual grass stage, dominated by triple awn grass, lasted from nine to 13 years. The perennial bunch grass stage, dominated by little bluestem was estimated to exist 50 years or more. The climax stage in that region is dominated by little bluestem, big bluestem, switch grass, and Indian grass, also characteristic of the Flint Hills.
The observations raised questions about the reasons for the sequence of species in the four stages of succession. Investigators discovered the fruits of little bluestem seldom were carried much more than six feet from the parent plant by the wind, while fruits, and even whole plants, of triple awn grass often were carried fairly long distances by the wind. This might be one reason for the earlier invasion of abandoned fields by triple awn grass, even while surrounding areas were populated with little bluestem.
In further investigations, fruits of triple awn grass, little bluestem, and switch grass were planted and grown in various concentrations of nitrogen, phosphorus, and potassium. In low nitrogen solutions, triple awn grass did better than either little bluestem or switch grass. Triple awn grass also did better than the other species in the low phosphorus solutions. But in the lowest phosphorus solutions, switch grass did very poorly. All species did about the same as their respective controls with potassium solutions. Apparently little bluestem has a slightly lower requirement for nitrogen and phosphorus than does switch grass.
So in abandoned fields of Oklahoma, the order of invasion by these species is triple awn, little bluestem, and switch grass. That also is the order of increasing need for nitrogen and phosphorus.
As long ago as 1915, Kansas soil had lost from 22 to 43 percent of its nitrogen. Similar losses of phosphorus also were demonstrated.
But can inorganic nutrition be the sole answer to all plant interactions? Since 1828, botanists have occasionally raised this question. In the 1920s, potato and tomato plants wilted when grown near black walnut trees In the 1940s, peach roots were found to be toxic to the growth of peach seedlings. And the natural thinning of pure stands of smooth brome was attributed to a toxic substance produced by the roots of smooth brome. Extracts of six different parts of buffalo-bur inhibited the growth of many different species including the buffalo-bur itself. Apparently, the production of plant inhibitors is another adaptation that helps a species compete with other species in its ability to obtain the minerals, water, and light required for growth and reproduction. And toxins that inhibit their own seedlings function to control overcrowding which can be important in areas or times of limited water or mineral supply. This is the case with smooth brome, stiff sunflower, buffalo-bur, and Johnson grass.
Knowing of the widespread production by seed plants of substances that affect other plant species, we should not be surprised that certain microorganisms also are involved in chemical warfare. In the 1960s, Rice and his associates reported numerous studies demonstrating the inhabitation of nitrogen-fixing and nitrifying bacteria by seed plants. Nitrogen-fixing bacteria inhabit the root tissues of legumes where they "fix" atmospheric nitrogen incorporating it into nitrates. Nitrogen-fixing also is done by lightning, blue-green algae, and certain free-living bacteria. Some nitrifying bacteria oxidize ammonia to nitrites, and still other further oxidize nitrites to the nitrates necessary for plants in their production of proteins. Rice has demonstrated the early invaders of abandoned fields in Oklahoma not only had lower requirements for nitrogen; many of them actually produced substances that inhibited the activities of nitrogen-fixing and nitrifying bacteria. This would have the likely effect of prolonging the first and second stages of succession.
But this inhabitation of nitrifiers also might increase the available nitrogen in the soil. Ammonium ions are attracted to soil particles while nitrites an nitrates are more easily leached to below the root zones of may plants.
The growth of deep-rooted forbs (non-grass herbs) in early stages of succession in abandoned fields may bring minerals from deeper layers to the surface and increase the availability of nitrogen and phosphorus in the surface layers of soil.
Thus weather factors, interactions of various plants, and microorganisms in the soil cooperate to return an abused field to a healthy, stabilized climax stage of tall grasses and its several hundred species of interacting creatures.
