What delays naturalization? how planting effort, traits and cold tolerance influence the lag phase in woody horticultural species

Horticulture is a major source of plant invasions in urban areas. There is often a delay, known as the lag phase, between the introduction of a non-native species and its establishment in the wild. Understanding the factors driving the lag phase and how it varies among species is key to predicting future plant naturalizations. In this study, we analyzed nursery and naturalization records of 128 naturalized tree and shrub species planted in Canberra, Australia. Canberra’s extensive documentation of introduced horticultural plantings since the city’s establishment in 1913 makes it an ideal location for assessing variable lag phases in woody non native species. We examined the association between planting history and lag phase by recording the year of first introduction, planting intensity over time, and the year of naturalization based on herbarium records. We also gathered data on plant traits, including height, seed mass, dispersal mode, and cold hardiness. Using univariate and multivariate models in a Bayesian framework, we found that propagule pressure, measured as the average intensity of planting over time, and plant height were key determinants of lag phase duration. Species planted in higher numbers per year had shorter lag phases, while taller species exhibited longer lag phases. We also found a potential relationship between the naturalization patterns of woody horticultural species and human population growth in urban areas. Collectively, these findings can help predict whether a non native plant species has the potential to naturalize in the future.