Possible Mechanisms of the Exclusion of Johnson Grass by Tall Grass Prairies

Abstract

Historically, plant distribution typically has been studied with the purpose of learning why a species grows and survives where it does; but why a species does not survive in a particular habitat has rarely been studied, although it may be just as important. According to the US Department of Agriculture, Johnsongrass [Sorghum halepense (L.) Pers.; formerly Johnson grass] is listed as an agricultural pest in most states south of the 42^nd parallel. Control of Johnsongrass in agricultural fields involves various labor intensive cultural, mechanical, and chemical means. Release of a bio-control agent has not been suitable for intensively cropped areas. An agriculturally important weed and prominent member of early stage secondary succession, Johnsongrass is not present in later stages of prairie succession. Various environmental factors (biotic and abiotic) that might be involved in restricting Johnsongrass survival were examined in this research. In two sites in Oklahoma, soil conditions were found to be more favorable for survival and growth of Johnsongrass in undisturbed prairie than in the disturbed areas in which Johnsongrass was found vigorously growing. However, even when its rhizomes were introduced into mature prairie, Johnsongrass did not thrive. In laboratory and field trials, presence of the living dominant prairie grasses or leachate from living or dead leaf blades seemed to influence growth and survival of Johnsongrass rhizomes. The prairie grasses, little bluestem [Schizachyrium scoparium (Michx.) Nash] and Indian grass [Sorghastrum nutans (L.) Nash], seem to play a similar allelopathic role in restricting the growth of Johnsongrass to outside of the prairies. Looking at this past study might lead to new methods for the future. (Semtner 2012)