Effect of competition, with exotic invasive Bromus inermis and native Quercus stellata, and soil amendment on the performance of native invasive Juniperus virginiana

Juniperus virginiana is a native species to the eastern United States that typically establishes in old fields and open woodlands. Recently, this species has been encroaching into western U.S. grasslands, altering native plant communities and ecosystem functioning. Across the expanding range of this species, different competitors and soil nutrient content are likely to have contrasting or compounding effects on J. virginiana’s performance, leading to complex establishment patterns. We designed a greenhouse experiment to investigate factors that affect the encroachment of J. virginiana into new habitats that differ in their soil and competitive interactions. We tested the effects of competition with the invasive grass Bromus inermis and native tree Quercus stellata, as well as soil amendment with lime and fertilizer. We measured J. virginiana’s mortality, relative growth rate, biomass, root length, biomass allocation, nitrogen concentration, and midday water potential. Our results suggest that B. inermis negatively affected J. virginiana’s performance and survival. In contrast, Q. stellata did not significantly affect J. virginiana, and the effect of soil amendments was small compared to that of B. inermis. There was a minor interactive effect of lime + fertilizer, implying that J. virginiana tolerates rather than prefers limestone-rich soil. In addition, our results suggest that J. virginiana within the historical range of the Eastern Woodland habitats establishment is likely enhanced by low competition rather than soil nutrient content. In contrast, encroachment into disturbed areas of Western Grasslands should be promoted by the cultivation of J. virginiana but should be inhibited by both abovegsround and belowground competition with grasses.