Rebecca Utley
The Role of Symbiosis with Frankia on the Invasiveness of the Shrub Elaeagnus umbellata.
BSES Senior Research 2004

The objective of this research is to test the role of a symbiotic bacterium in limiting the spread of an invasive shrub. Elaeagnus umbellata, Autumn-olive, is a deciduous shrub that was introduced to the U.S. from East Asia in 1830 for re-vegetation of disturbed areas, food and cover for wildlife, and as wind and soil erosion preventive measures. It escaped cultivation and has become one of the most problematic invasive plants in eastern North America. Elaeagnus’ symbiotic relationship with the actinomycete genus Frankia contributes to its success. Frankia colonizes the roots of Elaeagnus, forming coralline roots containing actinorhizas. N-fixation occurs in the actinorhizas and gives Elaeagnus the ability to establish in N-deficient soils.

The specific objectives of this research were to determine 1) if Frankia is present in various soil communities, 2) how its degree of presence or absence affects the ability of Elaeagnus to establish in those areas, and 3) how that establishment changes the nitrogen composition of the surrounding soil. Cuttings of young stems were dipped in root growth hormone and placed in sterile potting soil in order to develop Frankia-free roots.

The first experiment tested the presence or absence of Frankia for soil from two areas, one with an abundance of Elaeagnus and the other an open field without Elaeagnus. Half of the soil from the Elaeagnus field soil was sterilized; the other half was left as a control. 25 rooted cuttings were planted in each of the three soil types, and the speed of root colonization and the number of actinorhizas indicate the density of Frankia in the soil. In the second experiment, 25 cuttings were placed in six different soil types covering a successional gradient to determine which areas are more susceptible to invasion by Eleaegnus. Dry biomass weight of the plants was compared by soil type and found to be statistically significant for each experiment. Plants in soil taken from an area with a presence of Eleaegnus had greater total and nodule biomass values than plants in Eleaegnus-free soils. Areas with a greater density of Frankia in the soil are more susceptible to invasion by Eleaegnus.

Establishment of one Eleaegnus plant in an environment has a positive feedback response in which it increases the growth of Frankia, in turn increasing Frankia’s availability for other Eleaegnus plants. Thus, this creates a snowballing effect and allows for more invasion by Eleaegnus.