Influence of soil microorganisms, allelopathy and soil origin on the establishment of the invasive Acacia dealbata

Background: The invasive success of Acacia dealbata is partially attributed to the release of allelopathic compounds and changes in soil characteristics. However, both mechanisms have been studied separately, which does not reflect field conditions. Aims: To test whether interactions of allelopathy, soil origin and soil microbiota affect plant germination and growth of A. dealbata. Methods: Soil sampling and collection of natural canopy leachates were conducted in Coimbra, Portugal. Germination and seedling growth were tested in sterilised and control soils from native forests and invasive acacia stands, watered either with acacia or native canopy leachates. Sapling growth was further studied in soils from native forests and invasive acacia stands. Results: No interaction between Acacia leachates and soil microbiota was detected. Leachates of A. dealbata had a negative effect on the growth of its own seedlings in native soils. Soil microbiota reduced seedling growth in native forests. Saplings of A. dealbata grew bigger in the invaded soil than in soil from native forests. Conclusions: Soil microorganisms have a positive effect on the initial establishment and growth of A. dealbata. The effect of Acacia leachates depends on the origin of the soil. Soil previously modified by A. dealbata seems to favour its own growth.