Shifts in fire regime and grazing pressure have contributed to range expansion of woody native species into grasslands and prairies, thereby reducing grassland biodiversity and species richness. Numerous studies have investigated the effects of woody range expansions into grasslands along expansion fronts. However, we still lack a mechanistic explanation of which factors may reduce the pace of range expansion at an early stage. We applied field microsite manipulations at three sites along a soil gradient to test the effects of intra- vs. interspecific competition and grazing on Juniperus virginiana, a vigorous range expanding tree species in North America. Concurrently, we investigated the spatial distribution in naturally expanding populations of J. virginiana in three sites that differ in soil properties, disturbance and management regimes. Spatial statistics were used to investigate population dynamics: aggregated distributions are usually an indication of facilitative interactions, whereas regularly spaced individuals are likely to experience competitive interactions.
We found a highly site-specific response to microsite manipulations (site*treatment: p<0.05). Overall, the presence of conspecifics had a positive effect on target plants (i.e. intraspecific facilitation) leading to 2.6 times larger growth rates compared to interspecific competition, which had a markedly negative effect on growth. The outcome of grazing treatments changed across soil types: plants growing in favorable soil achieved 2-4 times higher growth rates than plants growing in unfavorable soil.
Spatial statistics indicated facilitation from conspecific individuals at early life stages and little or no effect of neighbors (regardless of species identity) in adult trees. Our preliminary results indicate that interspecific competition strongly hampers the growth of Juniperus virginiana, an aggressive woody encroacher, in its early life stages, and that grazing can play either positive or negative roles on successful establishment depending on abiotic stress levels and soil properties. Future studies will investigate whether J.virginiana can hinder grassland species via soil feedback.