Naturalization of ornamental plants in the United States depends on cultivation and historical land cover context

Abstract

Cultivation by humans is the primary mode of introduction for naturalized plants and an important driver of naturalization, a critical step in the invasion process. Historical records of cultivated plants can represent introduced species pools and propagule pressure, allowing for tests of how species' traits and environmental context affect naturalization while accounting for human influence. Ruderal traits, which generally promote naturalization, may not be universally advantageous across closed versus open landscapes (forest versus grassland/shrubland) or different agricultural land use conversion types, though such context dependence has not yet been demonstrated at a broad scale. We analyzed the naturalization of 3949 cultivated ornamental non‐native plant taxa that were for sale in nursery and seed catalogs in the conterminous United States during a period over 200 years to test for context dependence between traits associated with ruderality (short lifespan, shade intolerance, and self‐compatibility) and estimates of historical forest/grassland cover and agricultural land use change. We found that present‐day naturalization was closely tied to longer cultivation duration and greater cultivation extent. While ruderal traits tended to promote naturalization, perennial lifespan and shade tolerance favored naturalization in US states with higher forest cover, which is consistent with an alternative invasion strategy in closed‐canopy systems. Land use conversion to pasture and succession of abandoned agricultural land promoted naturalization of disturbance‐adapted plants in both forest and grassland landscapes. Our results emphasize the central role of cultivation in plant invasion and provide spatially and temporally extensive evidence that, while ruderal traits are important predictors of naturalization, they are dependent on the landscape context into which plants are introduced. Our work demonstrates the importance of integrating historical cultivation and land use/cover data for a nuanced understanding of the ecological factors that drive plant naturalization.