Changes in microhabitat, but not allelopathy, affect plant establishment after Acacia dealbata invasion

Aims The tree legume Acacia dealbata Link is an aggressive Australian invader that severely affects abiotic and biotic compartments of ecosystems worldwide. This invasive species outcompetes native plant communities through direct competition, changes in microhabitat and soil properties under the canopy and the release of allelopathic compounds. However, these effects are usually studied separately and under controlled conditions. The objective of this study was to evaluate the combined effect of these modifications exerted by A. dealbata on the establishment of native and invasive species in the field. Methods A full factorial experiment was performed in order to test the combined effect of microhabitat, soil type and allelopathy on the establishment of the invasive A. dealbata and three native species in the field (North-West Spain). We sowed seeds of native or invasive species in pots with different soil type (soil collected under A. dealbata or Pinus pinaster), allelopathy status (soil treated or untreated with activated carbon) and microhabitat (under the canopy of A. dealbata or P. pinaster). The number and total biomass of established plants were evaluated after 8 months. Soil abiotic properties were determined in all soils. Important Findings The establishment of native and invasive species was significantly affected by the microhabitat, with a higher number of seedlings under the native forest (P. pinaster). The establishment of A. dealbata seedlings was also significantly promoted by its own soil. A negligible effect of allelopathy was detected on the establishment and growth of both invasive and native species. We conclude that (i) the main factor affecting seedling establishment was microhabitat rather than changes in soil properties or allelopathy, (ii) soil modifications by A. dealbata promote the establishment of its own seedlings, contributing to the progress of invasion and (iii) allelochemicals released by A. dealbata do not seem to play a key role for the establishment of native and invasive plants under field conditions in the European non-native range.