The effect of Juniperus virginiana on plant species composition in an Oklahoma grassland

We established transects under twenty Juniperus virginiana trees that invaded an unburned central Oklahoma grassland within the last 20 years to determine their effects on plant species composition. Species richness and stem density increased as distance from the trunk increased. Stem density was also higher towards the south side of trees. Graminoid, forb and total cover were related to distance from the trunk and transect direction. Ordination revealed weak compositional gradients related to “openness” and compass direction. Woody species tended to be most abundant underneath J. virginiana canopies whereas grass and forb species were most abundant in the prairie. Woody and shadetolerant species preferentially occurred in north transects and quadrats underneath the tree. Conversely, forbs had highest abundance on edge quadrats whereas graminoids dominated south transects and prairie quadrats. Thus, invasion of grasslands by J. virginiana influences species composition in a spatially complex manner. Nomenclature: USDA, NRCS (2004). Abbreviations: DBH – Diameter at Breast Height; pCCA – partial Canonical Correspondence Analysis. A number of studies have demonstrated the effects of J. virginiana on grasslands. Engle et al. (1987) and Smith and Stubbendieck (1990) have shown dramatic reductions in standing biomass underneath J. virginiana trees. Other studies suggested that J. virginiana adversely affects species at specific locations (Jameson 1966, Smith and Stubbendieck 1990, Gehring and Bragg 1992). There is a shift in the dominant photosynthetic pathway as J. virginiana canopies close (Briggs et al. 2002a). The result is a larger proportion of C species outside of the canopy as compared to more C species under J. virginiana. Linneman (2004) demonstrated how J. virginiana influenced physical, chemical, and micrometeorological aspects of the environment. In the current study, we examine species richness and community composition under Juniperus virginiana trees in contrast to the open prairie in a central Oklahoma grassland.