Ecology and Relative Importance of the Dominants of Tall-Grass Prairie

Abstract

1. A study of the tall-grass prairie from southern South Dakota to northern Kansas and eastward into Iowa reveals several patterns or types of vegetation. The big bluestem type is the most extensive and the most important of those found on the lowlands. It is characterized by the single species, Andropogon furcatus, which alone often constitutes 80 or more per cent of the vegetation. Sorghastrum nutans is a close ecological equivalent which associates with it, but it is in general much less important. 2. Spartina michauxiana covers extensive areas of wet, poorly aerated soil with tall, dense, and often nearly pure stands. Panicum virgatum and Elymus canadensis form an intermediate lowland type of much less extent than either of the preceding. 3. Andropogon scoparius constitutes the most extensive upland type and easily exceeds in importance all other upland species combined. Stipa spartea is the dominant of another upland type found on steep slopes and ridges in the central and especially the northern part of the area. Sporobolus heterolepis exhibits a minor, dry upland pattern. Bouteloua racemosa is a minor constituent of all the grassland types, except Spartina, occurring less abundantly in the most xeric ones. Koeleria cristata is intermixed with A. scoparius, S. spartea, and S. heterolepis, thus showing a preference for dry uplands. 4. The lowland species, except Elymus and Bouteloua, are sod formers; those of the upland are bunch formers, although Andropogon scoparius regularly forms sod-mats under favorable conditions. Elymus and Bouteloua form either bunches or a poorly developed sod. 5. The chief "weapons" of competition of the tall, lowland dominants are their ability to endure shade and the rapid development of rhizomes resulting in the formation of a dense sod. Upland species rely upon successful competition for water and ability to withstand drought. 6. The seedlings of all species develop rapidly, tiller freely, and under favorable conditions without competition may produce seed the first summer, although under natural conditions two and often three years are required. 7. Root depths of 1.5-3 feet are attained the first season by various lowland species and 1-1.5 feet by those characteristic of upland. Mature root systems of lowland species are likewise in general deeper (4-10 feet) than those of the drier upland soil (1.5-5.5 feet). Those of lowland are much coarser and usually penetrate more vertically downward, however, while those of upland are fine, profusely branched, and widely spread. 8. Stipa and Koeleria begin growth in March, blossom in late May or June, and make a renewed growth in autumn. Elymus likewise starts growth in March; the other species about the middle of April. Anthesis begins in late July and reaches its height in August or early September. 9. Three general types of leaf structure are evident: the bluestem type, including Elymus; the wheat-grass type exhibited by Stipa, Koeleria, and Spartina; and the grama-grass type as shown by Bouteloua, Panicum, and Sporobolus. All show pronounced xeric structures. 10. Osmotic pressures in the various grasses varied from 7.5 to 14 A. during the spring and early summer when the water supply was abundant and growth vigorous. No consistent differences between upland and lowland species were found, but during a severe summer drought the more xeric species showed about twice the osmotic pressure of those of lowlands. These ranged from 22.4 A. for Stipa, 24.2 A. for Koeleria, and 28.1 A. for Sporobolus to 30.4 A. for Andropogon scoparius. Panicum gave a pressure of 13.7 A. and Andropogon furcatus of only 14.5 A. Other lowland species gave somewhat similar results, relict Spartina reaching a maximum of 16.4 A. Thus there seems to be a clear relationship between water content of soil and osmotic pressure of the cell sap, species of the drier soils developing the higher osmotic pressures. 11. All of the dominant grasses have excellent forage value, especially if they are grazed or cut for hay before the coarse flower stalks of the late-blooming species become hard and fibrous.