Different growth strategies to invade undisturbed plant communities by Acacia dealbata Link

The leguminous Acacia dealbata is one of the most aggressive invasive tree species worldwide. The invasion process of A. dealbata is mainly favoured by environmental disturbances. However, this species is currently invading unaltered plant communities and environmental factors influencing its spread in these ecosystems remain unclear. In this study, we evaluated the A. dealbata growth related to environmental factors in non-altered shrublands and pine forests in North-Western of the Iberian Peninsula. Permanent sampling plots were placed along the invasion edge in each plant community and height and stem diameter increments were recorded on labelled A. dealbata individuals throughout two years. The number of new A. dealbata saplings was recorded at the end of the study. Environmental factors such as physicochemical soil properties, native vegetation cover and plot features were evaluated in each sampling plot in order to determinate their effect on the A. dealbata growth. Our results showed a different growth pattern of A. dealbata depending on the studied plant community. Pine forests had A. dealbata individuals with significantly higher height and diameter increments and higher number of new A. dealbata individuals than shrublands. Additionally, results also showed a clear seasonality effect on A. dealbata growth that was stimulated in spring in pine forests, while it did not variate along the year in shrublands. Regarding environmental parameters influencing the growth of A. dealbata, we found a negative effect by the photosynthetically active radiation and plot orientation, but also a positive effect by soil potassium content and plot elevation. However, our results indicated that other environmental factors not evaluated here might be also influencing. We conclude that A. dealbata can invade unaltered areas and the intensity of the invasion depends on the ecosystem. The natural shrubland could be an effective barrier to slow down the rapid invasion of A. dealbata. These results contribute to new knowledge of A. dealbata colonization without help of environmental disturbances in the field, which may help to define hot-priority areas and stablish new control strategies. Thus, we suggest that the management of A. dealbata in pine forests should receive preferential treatment on South-Western Europe.