CHANGES IN PRAIRIE VEGETATION UNDER EASTERN RED CEDAR (JUNIPERUS-VIRGINIANA L) IN AN EASTERN NEBRASKA BLUESTEM PRAIRIE

To assess changes in herbaceous species cover due to woody plant invasion, plant species composition was determined at different locations under and adjacent to isolated 10 to 22-yr-old eastern red cedar (Juniperus virginiana L.) trees situated in an eastern Nebraska native bluestem prairie. Little bluestem (Andropogon scoparius Michx.) dominated plots without trees (44% cover); Kentucky bluegrass (Poa pratensis L.), a nonnative species, dominated under trees (19% cover). This difference in dominant species represents a 20-yr change in herbaceous composition that occurred following establishment and growth of tree crowns over native prairie. In addition to little bluestem, 11 other common species (frequency >10%) declined under persistent tree cover including big bluestem (Andropogon gerardii Vitman; -10%) and white aster (Aster ericoides L., -5%). However, Kentucky bluegrass and carex (Carex spp.) cover increased 13% and 10%, respectively- No consistent correlation was found between photosynthetic pathway (C3 or C4) and species response to tree cover. Direction from the main tree stem was a significant factor affecting the response of 4 of 13 common species to tree crown development, percent cover of these species generally was lower to the N and E than to the W and S. Tree size significantly affected the response of 5 of 13 species. The data indicate that bluestem prairie species composition can be significantly affected by less than 20 yr of shading by invading trees, suggesting that protection from such invasion is essential to the continuation of this ecosystem.