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

Abstract

The western border of the deciduous forest, extending along the Missouri River, meets the prairie in south-eastern Nebraska. The mean annual precipitation is about 32 in.; the humidity is relatively low, average day ranging between 50-80% in yrs. of greater rainfall, but falling to 40-50% during drier yrs. Average night humidity is frequently 20% higher. Evaporation is usually 20-30 cc. daily during the growing season; wind movement is relatively high. The silt loam soils are many feet in depth but during drought contain only a small amount of water available for growth. Hence competition for water is great between the grasses and the shrubs and trees, which are invading since the cessation of prairie fires about 60-70 yrs. ago. Quercus macrocarpa is the most xeric forest tree and invades the grasses either directly or in the wake of certain shrubs, chiefly Rhus glabra, Symphoricarpos vulgaris, and Corylus ameriana. Invasion occurs on well drained lowlands where Andropogon furcatus, Sorghastrum nu-tans, and other grasses 6-10 ft. tall and of similar depth dominate. It also occurs on uplands where Andropogon scoparius, A. furcatus, Poa pratensis, etc., are 0.5-4 ft. tall and 3-6 ft. deep. The shrubs advance largely by means of rhizomes into the grassland. Their roots often extend outward and then upward under the grasses. Some are so finely branched and abundant as to successfully compete with those of the prairie grasses. Bur oak develops a taproot about 9 in. deep before the leaves are unfolded above ground. The strong, finely branched taproot extends into moist soil 3-5 ft. the first summer. Mature trees 50-65 yrs. old were 35-40 ft. tall, 12-18 in. in basal diam. and grew 10-40 ft. apart in a pure stand. The taproot gave rise to 30 or more large main branches, most of which arose in the first 2 ft. of soil. It tapered rapidly and extended to a depth of 14 ft. Most of the main branches, which were 1-7 in. in diam., extended widely (20-60 ft.) before turning downward. Some grew even deeper than the taproot. All branched repeatedly, and together they occupied a very large volume of soil. Many branches of the main roots grew vertically downward 8-15 ft., 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. Ropelike roots, 0.5 in. or less in diam. extended many ft. without much change in thickness. A cordlike type, 3-5 mm. thick, was also abundant. A 3rd type consisting of fine, much branched rootlets clothed the widely extending skeletal framework and furnished the bulk of the absorbing surface. Mycorhizal mats were abundant. The wt. of the roots equaled that of the tops; the volume of the roots was about 1/10 less than that of the parts above ground. 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. They do not obtain moisture from the water table but depend upon the direct supply furnished by rainfall.