Historical frequency of plants in nursery catalogues predicts likelihood of naturalization in ornamental species

Ornamental horticulture is the major pathway of non-native plant species introductions worldwide. Historic nursery catalogues capture a long-term view of introduction effort arising from garden plantings and are a powerful resource for understanding why some introduced ornamental species subsequently jump the garden fence. Analyses of historic nursery catalogues can help us understand the reasons for failed invasions as well as why some species successfully naturalize or invade. We used New Zealand nursery catalogues from the 1860s to the 1990s to understand the patterns of failed invasions, as well as successful naturalization and invasion. Together with data on several horticulturally relevant plant traits, we used a boosted classification model to predict the likelihood of naturalization and invasion. A species' maximum height, its frequency in nursery catalogues, and the family-level global naturalization rate were the most influential variables. Naturalized species were generally taller, more frequently offered for sale, and belonged to families with higher global naturalization rates than those that had not naturalized. Other traits such as cold hardiness or shade tolerance were not significantly different between naturalized and non-naturalized species but contributed somewhat to the overall model fit. By contrast, our predictions of the likelihood a species would become invasive were poor, without any robust relationships with any of the covariates. This indicates that factors that drive the transition from naturalized to invasive species may be determined by the characteristics of the habitats that they invade. Species incorrectly predicted to be naturalized may not have had sufficient opportunity to do so and may pose a greater risk of naturalizing in the future. This provides an avenue for identifying future naturalized species and enabling proactive management or monitoring of these species of concern.