Effects of Excessive Natural Mulch on Development, Yield, and Structure of Native Grassland

An upland prairie, undisturbed by mowing, grazing, or burning for 15 years, was studied. About 80% was covered with a nearly pure stand of Andropogon gerardi. Similar stands of Panicum virgatum and Sporobolus heterolepis each occupied about 9%. 2. A dense natural mulch, 4.5-8 inches deep and weighing 6-9 tons per acre, covered the deep loam soil. The mulch intercepted much precipitation but promoted more rapid infiltration of water which reached the soil and greatly retarded evaporation. It increased the organic matter of the surface soil 1.5-2% and nitrogen content 0.1% of its oven-dry weight. 3. Roots and rhizomes grew thickly below the mulch and mellowed the surface soil. Pore space was 61.7%, and volume-weight was 0.97. 4. Soil temperatures 22⚬-28⚬ F. lower under the mulch in May delayed growth about 3 weeks compared with plants where the mulch had been removed. Production of flower stalks was considerably later in all mulched grasses and also less in Andropogon. Flower stalks were abundant on all unmulched grasses. 5. Yields in June, July, and August were 53, 26, and 29% less from mulched stands of Andropogon, and 57, 55, and 26% less in Panicum than those from unmulched stands. 6. Consequences of the effects of the mulch upon the environment were the production of a nearly pure, but some-what thinner than normal, stand of Andropogon. The understory characteristic of upland prairie had all but disappeared. The usual mid grasses of upland were few or none. Only a few of the taller forbs remained. 7. Dense stands of Panicum with forbs characteristic of very moist sites were typical. Between the large bunches of prairie dropseed the deeply mulched soil was usually free of vegetation.