Root System of Quercus macrocarpa in Relation to the Invasion of Prairie

Abstract

1. An ecotone between the grassland and the deciduous forest along the Missouri River occurs in southeastern Nebraska. The mean annual precipitation is 28-32 inches; humidity is relatively low, evaporation and wind movement are relatively high; the silt loam soils are deep but during drought contain only a small amount of water available for growth. Thus competition for water between the grasses and the invading shrubs and trees is great. 2. Quercus macrocarpa is the most xeric forest tree. Since the cessation of prairie fires it is invading the grassland either directly or in the wake of the Rhus-Symphoricarpos-Corylus chaparral. 3. The shrubs advance largely by means of rhizomes into the grassland. Their roots often extend outward and then upward under the grasses. Those of Symphoricarpos especially are so finely branched and abundant as to compete successfully with the excellently developed and extensive root systems of the plants of the prairie. 4. The oak seedling develops a deep taproot before the leaves are unfolded. The strong, finely branched taproot extends into moist soil 3-5 feet the first summer. 5. Mature trees 50-65 years old were 35-40 feet tall. 12-18 inches in basal diameter, and grew 10-40 feet apart in a pure stand. 6. The taproot gave rise to thirty or more large main branches, most of which arose in the first 2 feet of soil. It tapered rapidly and extended to a depth of 14 feet. 7. Most of the main branches, which varied from 1 to 7 inches in diameter, extended widely (20-60 feet) before turning downward. Some grew even deeper than the taproot. All branched repeatedly, and together they occupied a very large volume of soil. 8. Many branches of the main roots grew vertically downward 8-15 feet, each more or less resembling the taproot system of an oak sapling. Others extended obliquely or vertically upward and filled the surface soil with a mat of absorbing rootlets. 9. Ropelike roots, 0.5 inch or less in diameter, extended many feet without much change in thickness. A cordlike type, 3-5 mm. thick, was also abundant. A third type consisting of fine, much branched rootlets clothed the widely extending skeletal framework and furnished the bulk of the absorbing surface. Mycorrhizal mats were abundant. 10. The weight of the roots equaled that of the tops; the volume of the roots was about one-tenth less than that of the parts above ground. 11. Low water content of soil is compensated by a widely spreading, well branched root system. This may account for the wide spacing of the oak trees and the open forest canopy.