SINK AND SOURCE POPULATIONS OF THE INVASIVE HERB

Abstract

Microstegium vimineum is an invasive annual shade tolerant grass native to Asia that has spread from Tennessee to all states east of the Mississippi and south of New York. Controlling the spread of Microstegium has been difficult especially as it spreads into less assessable areas like the interior of forests. This study examined the population dynamics of the grass, in an attempt to identify a pattern of source and sink populations. Invasive plants tend to establish themselves in areas of disturbance such as alongside roads and trails. If Microsiegium growing along the roadside of a forested area were more likely to be source populations and patches growing in the interior were more likely to be sink populations, then the interior populations could conceivably be controlled by the carefully timed mowing of the roadside population of this annual. A total of five paired plots, each containing a roadside and interior sample 0.5 x 2 m2 in size, were set up. Four factors that influence the replacement rate, the j value, were identified and measured. These factors were the percentage of plants that survived the growing season, the percentage of plants producing seeds, the average number of seeds produced per seedbearing plant and the percent emergence of those seeds. These four factors were then multiplied together to determine the expected value of J. The overall J value did not differ significantly between the interior and roadside plots indicating that there is no difference in the likelihood that a roadside or interior population of Microstegium would be a source population. A correlation was found between seedling emergence, one of the J variables, and the roadside vs. interior location and a weak correlation was found between the average soil depth within a plot and the J value of that plot. The ramifications of this study for the control of existing populations M.icrostegium are not promising and a strategy of prevention becomes increasingly important. Early detection and removal and controlling roadside populations that have not spread to the interior by mowing the plants before they go to seed in the .fall may be effective. However, mowing roadside populations after interior patches have been established will not necessary control the interior populations.