Eastern red cedar is a common invasive tree in tallgrass prairie. Although native, its abundance has increased in the past century probably due to fire suppression. Eastern red cedar encroachment leads to a decrease in native prairie species diversity and abundance because it inhibits the growth of plants in its understory. We studied Eastern red cedar invasion at the University of Nebraska-Omaha’s T. L. Davis Preserve near Elkhorn, NE. We asked the general question of whether soil quality or light limitation was responsible for the lack of understory growth. A greenhouse bioassay did not provide a strong indication that Eastern red cedar soils discouraged growth of other herbaceous species. Thus, we set up a field study to see whether light limitation could explain variation in understory plant cover beneath Eastern red cedar stands. We measured photosynthetically active radiation (PAR) and understory stem density in 40 different stands, spanning a variety of light conditions on both N-facing and S-facing slopes.
Results suggest that PAR levels affect the understory stem density, with plots that receive more PAR generally having more understory plants. We found that PAR was especially important in promoting understory growth on north-facing slopes, where light is likely more limiting than on south-facing slopes. We also found that on south facing slopes, where light is less of a limiting factor, understory success was dependent on the height of the lowest tree branch. Trees with low branches tended to have less understory growth. Our results suggest that light limitation is a key factor in explaining the variation in, and often complete lack of, understory plant growth in Eastern red cedar stands in tallgrass prairie