Incipient woody plant encroachment signals heightened vulnerability for an intact grassland region

Question: What constrains Juniperus virginiana encroachment in semi-arid grasslands: precipitation-based constraints on establishment or dispersal-based constraints on spread?. Location: Sandhills grassland, Nebraska, USA. Methods: We tracked juvenile and adult stages of J. virginiana encroachment using field sampling and remote sensing across a network of 40 sites spanning a wide precipitation gradient (399–655 mm). Regional patterns of encroachment were then used to assess the relative support for precipitation-based constraints on establishment versus dispersal-based constraints on spread in a region transitioning to a more encroached state. Results: Woody encroachment was widespread and we found no evidence that low precipitation precludes encroachment in the Sandhills. Instead, encroachment patterns were best described by proximity to planted propagule sources. However, levels of encroachment were highly variable. Encroachment density was low at more arid sites that lacked nearby stands of planted J. virginiana, and encroachment tended to increase with proximity to plantings and higher mean annual precipitation, suggesting that both variables play a role in the rate of encroachment. Conclusion: Our results indicate that woody encroachment is constrained by dispersal in the Sandhills and that planted propagule sources increase grassland vulnerability to encroachment, regardless of mean annual precipitation. This may be true for other intact grassland regions where barriers to woody plant establishment have been altered or overcome. A key implication is that programs and policies need to consider encroachment risks from planted propagule sources and how to manage them to avoid fragmentation of intact grasslands.