Submitted to: Journal of Environmental Horticulture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/17/2001
Publication Date: N/A
Interpretive Summary: The nursery and landscape industry in the United States has long advocated the introduction of non-native woody plants to lend diversity and interest to our gardens and urban landscapes. Unfortunately, a small proportion of non-native species, such as common buckthorn and multiflora rose, have become serious pests, causing some nursery professionals and government agencies to adopt policies that strongly favor (or exclusively use) native plants. Ideally, a balance should be struck between prohibiting all non-native species and the uninformed introduction of new, potentially invasive woody plants. In this research, we found that a geographic analysis, including climatic comparisons and a mapping of the native ranges of cultivated trees, shrubs and vines that have naturalized in Iowa as well as those that have not, was a valuable predictive tool for identifying specific high- risk regions in Europe and Asia that may serve as future sources for naturalizing woody plants in Iowa. This information can be used by horticulturists and land managers to assess the tree and shrub species used for future plantings in Iowa. Geographic risk analysis is no substitute, however, for field monitoring and early identification of plants escaping cultivation. A vigilant and conscientious team of land managers, botanists, horticulturists, and nursery professionals is still the best safeguard against the introduction and spread of invasive landscape plants.
Technical Abstract: The objective of this study was to determine whether patterns of native distributions of naturalized woody plants and their relationships to climatic analogs can serve as a sound basis for geographic risk analysis to help identify high- and low-risk regions for the introduction of new woody plants to Iowa. We compared the native ranges of 28 non-native species naturalized in Iowa with those of 72 non-native species widely cultivated in Iowa, but with no record of naturalization. From this comparison, we tested two related hypotheses: 1. Regions with the highest number of native species that have naturalized in Iowa have a higher proportion of naturalizing species than predicted by the overall ratio of naturalizing species to the total set of species studied; 2. Regions identified as climatic analogs to Iowa conditions, based on important determinants of woody-plant adaptation, have a higher proportion of naturalizing species than predicted by the overall ratio of naturalizing species to the total set of species studied. We discovered that the two regions with the highest number of naturalizing species (in southeastern Europe and northeastern China) have a significantly higher proportion of naturalizing species than predicted by the overall ratio of naturalizing species to the total set of species studied. Two of the five regions identified as climatic analogs to Iowa conditions (in northeastern and central Asia) also displayed significantly higher proportions of naturalizing species, while a third (in southeastern Europe) was statistically significant only at the 10% level.