Similar drivers but distinct patterns of woody and herbaceous alien plant invasion

The extent of alien plant invasion and numbers of invasive species are increasing, exacerbating invasion impacts. Effective and efficient management requires understanding the drivers and distribution of plant invasions at the landscape scale. In this study, we used a species distribution modelling approach to determine whether the patterns and correlates of alien invasion vary by plant growth form. Focusing on the occupancy and abundance of forbs, graminoids and woody vegetation, we used boosted regression trees (BRTs) to characterise alien plant invasion risk in two major catchment regions in Victoria, Australia. Of 7,630 quadrats surveyed between 1970 and 2019, 69% contained alien plants, with forbs being the most prevalent. Alien plants constituted 22% of the total number of plant species recorded. Alien species cover varied widely, with forbs and graminoids showing higher mean cover compared to woody plants. Abiotic conditions, particularly temperature and precipitation, had the greatest influence on alien plant invasion overall, explaining 41-76% of observed variation. Summer mean maximum temperature was a strong predictor across all growth forms. Forbs and graminoids showed increased occupancy with higher vegetation cover but lower proportional cover, while woody plants had a negative relationship with their own cover type. High levels of invasion were predicted in areas with intensive land use, such as urban and agricultural zones. Forbs had a high probability of occupancy throughout the region, even in higher elevations, while graminoids and woody vegetation were more restricted to lower elevations and areas with human activity. The study highlights that alien plant invasion is influenced by a complex interplay of abiotic factors, propagule pressure, human activity and biotic conditions. The findings underscore that while there are common drivers across growth forms, specific patterns and influences vary. For instance, forbs are more widespread but less dominant in high vegetation areas, while woody plants were less common and more constrained by existing vegetation. Management strategies should prioritize maintaining and restoring native vegetation to limit the dominance of alien species and controlling invasive plants after disturbance. Although single-species models remain valuable, our study shows that species distribution models based on growth form offer a practical approach for assessing plant invasions across diverse landscapes.