The extent to which plant species become established outside their native ranges is of great interest to ecologists. Although most attention has focused on inter-continental invaders, data on intra-continental movements are also highly valuable. First, these occurrences, whether the result of natural dispersal or human translocation, can provide new information on species’ climatic tolerances when they occur under novel climate regimes. Second, naturalizations may represent emerging shifts in species’ distributions in response to recent climate change. Third, the establishment of adventive populations can inform our understanding of possible consequences of controversial management strategies, like “assisted colonization”, being considered to conserve species threatened by climate change in their native ranges. Here we evaluate intra-continental naturalizations in New England, and ask how often such adventive occurrences are ephemeral, persistent, or expanding and fully naturalized. We used herbarium specimen databases, e.g. Global Biodiversity Information Facility, to find recent herbaria records (post-1980) for a sample of plant species native to the US but adventive in New England. In 2015, we surveyed potential naturalization sites in the field. We ask whether region of origin within the US, or life history traits are related to the likelihood of adventive occurrences persisting or spreading in the region.
Results/Conclusions
A review of recent floristic data indicates that about 374 US native plant species have been reported as adventive in New England. Our sampling in 2015 focused on a subset 49 species. Of these, 37 were relocated, suggesting that many adventive species have occurrences persistent across multiple years. A significantly greater proportion of woody (32/38 or 84%) than herbaceous species (5/11, or 45%) were relocated. Twenty-five species (65%), all woody, were found to be spreading. Twenty species (54%) were spreading into unmanaged areas, and there was a non-significant trend for herbaceous species to do so more than woody species (p = 0.096). Six species (13%), all woody, occurred in monotypic stands with >100 individuals/stems, and may be considered “invasive”. The region of origin within the US was not related to the probability of relocation, or spread. At the landscape scale, most species occurred at only a small number of unmanaged sites, with very few at >10 sites. There was a significant positive relationship between species’ average abundance within sites and the total number of located sites, as would be expected if the plant traits resulting in large local populations also enable colonization at the landscape scale.