Influence of resource availability on Juniperus virginiana expansion in a forest–prairie ecotone

Woody plant expansion into grasslands and savannas is a global concern. Rapid expansion of Juniperus virginiana, a tree native to North America, has profound ecological consequences. We used transplanted J. virginiana seedlings to investigate the role of resource availability on J. virginiana expansion following the removal of fire, the factor historically limiting range expansion of this fireintolerant species. We evaluated J. virginiana seedling survival and seedling growth, two important phases in woody plant expansion, relative to two belowground resource factors, plantavailable soil water (soil clay content, an index of plantavailable soil water) and plantavailable nitrogen (PAN), and an aboveground factor, photosynthetic active radiation (PAR). In three plant communities associated with an oak forest–tallgrass prairie ecotone, we transplanted 2-yr-old J. virginiana seedlings in a systematic grid design and measured J. virginiana seedling survival and growth 8, 20, and 30 months following the transplant. We also measured soil clay content, PAN, and PAR in 1m2 quadrats centered on each transplanted seedling. We employed path analysis at two spatial scales (144 and 2916 m2) to compare the role of resource factors in seedling growth and survival. Juniperus virginiana seedling survival was about 10% greater in tallgrass prairie and upland oak forest than in the old field, and seedling growth in tallgrass prairie exceeded the two other communities by about a factor of five. Tallgrass prairie in our study area is clearly more vulnerable to expansion of J. virginiana than the other two plant communities. Survival and growth were controlled largely by available light (PAR) and secondarily by plantavailable soil water, indexed by soil clay content. In all three vegetation types and at both fine and coarse scales, soil clay content also exerted an indirect effect, possibly by mitigating the influence of light. Results of this study suggest that soil distribution maps and associated ecological site designations might be useful for directing J. virginiana management in the oak forest–tallgrass prairie ecotone by identifying the potential hotspots of invasion.