Predicting patterns of spread and impacts of the invasive woodland species, Japanese stiltgrass
Angie Shelton (postdoc, IU Dept. of Biology and IURTP), Dan Johnson (grad student, IU Dept. of Biology), S. Luke Flory (postdoc, IU Dept. of Biology), Keith Clay (professor, IU Dept. of Biology), Burney Fischer (professor, IU School of Public and Environmental Affairs), Cindy Huebner (research scientist, US Forest Service Northern Research Station, Morgantown, WV)

Fig 1. A patch of Japanese stiltgrass. Fig 2.Microstegium litter floating on floodwaters. Fig 3. Biomass and seed harvests. Fig 4. Invasion in old log yard. Fig 5. Invasion along roadside. Establishment in new log yard, <1 year after clearing

Invasive species can significantly alter natural communities and are considered one of the greatest threats to the integrity of natural ecosystems. Japanese stiltgrass (Microstegium vimineum) is an invasive species new to southern Indiana. It was first documented in Tennessee in 1919 but has only recently been recognized as invasive. It arrived in Monroe County, Indiana, around 2001 and has rapidly spread through the county.

Microstegium primarily invades forest understories and areas with intermediate light levels and some soil disturbance. It spreads remarkably fast and can quickly expand its range each year. The plants produce two types of seeds that together create very efficient dispersal to new areas. Open-pollinated (chasmogamous) seeds are produced in clusters at the tips of stems, quickly fall from the plant, and are transported by wind, vehicle tires, boots, or animals. Self-pollinated (cleistogamous) seeds are produced between the leaves and grass stem and often stay within the dead litter of the plant. The plants are very shallowly rooted and are easily uprooted, especially by flooding. In this case, entire plants and their seeds are transported on the water to new sites.

As part of a Joint Research Venture with the Northern Research Station of the U.S. Forest Service, we are monitoring invasions in the field to predict patterns and rates of future invasions and investigating the impact of Japanese stiltgrass on tree seedlings. The project has three primary components:

1. Characterize invaded sites and monitor the process of natural and experimental invasions
2. Determine the impacts of Microstegium invasion on tree regeneration and growth in natural systems
3. Develop spatially-explicit predictive models for Microstegium invasions and forest impacts

In order to understand the impacts of Microstegium on native tree regeneration, we planted seeds of six tree species in areas that have been invaded by Microstegium in three different types of forest: human disturbance, natural disturbance and no disturbance. We have paired invaded plots with plots where we have removed Microstegium with a grass-specific herbicide. The goal is to quantify the impacts of Microstegium on native tree regeneration and determine the potential for habitat restoration.

To characterize and monitor invaded sites, we are mapping several sites invaded by Microstegium in Monroe, Brown, and Clark Counties of Indiana. These sites differ in disturbance history (undisturbed, logging disturbance, natural flood, or storm disturbance) and cover a range of environmental conditions of light, aspect, slope, and moisture levels. We are mapping the Microstegium populations at each site using handheld GPS units and GIS software that allows us to create maps of the populations in the field.

We use these maps to determine the probability of invasion in relation to slope, aspect, distance to dispersal vectors, and distance to other patches of Microstegium. We then combine the map data with field data on light levels, biomass, and seed production of the plants. From this combination of data we will generate spatially-explicit computational models to predict the future spread of Microstegium populations.

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Revised: November 4, 2008