Abstract
The net effects of competition and facilitation on plants are determined by the interactions of environmental stressors. Juniperus virginiana is a range-expanding species that is native to the eastern United States. J. virginiana can tolerate a wide range of biotic and abiotic conditions. However, their growth rates depend on site quality, including temperature, precipitation, and competition. We tested the stress-gradient hypothesis along a short latitudinal range created by lake-effect snow. We conducted a field experiment at three different sites in northeast Ohio (Geauga, Portage, and Tuscarawas) at increasing distances from Lake Erie. We examined the effects of the site (temperature, precipitation, and soil nutrient differences) and intraspecific competition (zero, one, two, and four competitors) on the growth and performance of J. virginiana saplings. We measured the saplings’ growth (relative growth rate, total biomass, and nitrogen concentration), and water status and gas exchange (midday water potential, assimilation rate, transpiration rate, and stomatal conductance). We found partial support for the stress-gradient hypothesis. J. virginiana at Tuscarawas, which also had the highest soil phosphorus content, had the highest performance and gas-exchange activity. Saplings at the Portage site (intermediate) had the lowest overall performance, which can be related to the site’s poor drainage and low nutrient content. Contrary to our prediction, we found no effect of intraspecific competition on J. virginiana performance at all three sites. Our results suggest the importance of considering the effects of local abiotic conditions, such as soil properties, to understand variation in J. virginiana survival, performance, and encroachment.