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 proportional cover 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 growth form. Alien plants constituted 22% of the total number of plant species recorded. Alien species cover varied widely, with alien species contributing between 0.2% to 100% of vegetation cover in a plot. Alien forbs and graminoids had higher mean cover compared to alien 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. Areas with higher vegetation cover (native + alien species) were predicted to have higher occupancy, but lower proportional cover, of alien forbs and graminoids. In contrast, alien woody plants had a negative relationship with woody vegetation cover. 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 characterized by 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, alien forbs were more widespread but less dominant in areas with high vegetation cover, while alien woody plants were less common and more constrained by vegetation than the other two growth forms. Management strategies should prioritise 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.