The invasion process of Acacia dealbata is partially mediated by releasing allelochemicals that affect native plants and soil microbes. However, non-volatile chemical compounds responsible for the allelopathic effect remain unknown, even though the allelopathic potential of this species has been studied under Europe conditions. We examined the allelopathic potential of aqueous extracts obtained from different plant materials of A. dealbata: litter from leaf, flowers, pods and seeds and fresh leaves and also litter extract's phytotoxicity evolution on germination and early seedling growth of Lactuca sativa L. Bioassays based on aqueous extracts and direct effect of plant materials showed that radicle length (RL), degree of root necrosis and germination percentage were the variables that gave the best prediction for assessing the inhibitory allelopathic effect. Chemical compounds present in all types of litter significantly reduced the hypocotyl and RL of L. sativa. GC–MS analysis reveals the presence predominantly of resorcinol, maculosin and moretenone in leaves; stigmasterol, d-alpha-tocopherol quinone, and lupanin in pods; and methyl p-anisate, p-anisyl alcohol, stigmasterol and anisal were identified in flowers. The inhibitory effect induced on L. sativa by the different plant parts supports the hypothesis that A. dealbata exerts its allelopathic potential throughout its phenological cycle.

Plant species that growth close to or under the canopy of Acacia dealbata Link (Fabaceae, subfamily: Mimosoideae) within its non-native range, survive with difficulty or not at all, especially if they are native. This phenomenon has been attributed to allelopathy; one of the strategies used by A. dealbata to trigger an invasion process. Native species Quillaja saponaria Molina (tree) and Helenium aromaticum (Hook.) H.L. Bailey (herb), share A. dealbata’s range in South-central Chile. This study was performed on the Mediterranean Biobío Region of Chile. We evaluated the effect of leaves, flowers, pods and seeds of A. dealbata on the germination and early growth of these native species. Biological assays were carried out under laboratory conditions, based on aqueous extracts and the direct effect of plant material. Leaf litter prevented the germination of both species and seeds of the invasive species impeded the germination of Q. saponaria. Other plant parts from A. dealbata also induced reductions of hypocotyl and radicle lengths in the native species, reaching over 50 % in some treatment values. All plant parts caused radicle necrosis, preventing the formation of root hairs and, consequently, jeopardizing the survival possibility of the recipient species. The results show that A. dealbata can interfere with the establishment of pioneer herbaceous species in ecological succession and can also affect trees if they are reached by the invasion front.

Summary: The distribution ofAcacia dealbataLink (Fabaceae) in its non‐native range is associated with disturbed areas. However, the possibility that it can penetrate the native forest during the invasion process cannot be ruled out. This statement is supported by the fact that this species has been experimentally established successfully under the canopy of native forest. Nonetheless, it is unknown whetherA. dealbatacan express shade tolerance traits to help increase its invasive potential. We investigated the shade tolerance ofA. dealbataunder the canopy of two native forests and one non‐native for three consecutive years, as well as its early growth and photosynthetic performance at low light intensities (9, 30, and 70 μmol m⁻²sec⁻¹) under controlled conditions. We found manyA. dealbataplants surviving and growing under the canopy of native and non‐native forests. The number of plants of this invasive species remained almost constant under the canopy of native forests during the years of study. However, the largest number ofA. dealbataplants was found under the canopy of non‐native forest. In every case, the distribution pattern varied with a highest density of plants in forest edges decreasing progressively toward the inside. Germination and early growth ofA. dealbatawere slow but successful at three low light intensities tested under controlled conditions. For all tested light regimes, we observed that in this species, most of the energy was dissipated by photochemical processes, in accordance with the high photosynthetic rates that this plant showed, despite the really low light intensities under which it was grown. Our study reveals thatA. dealbataexpressed shade tolerance traits under the canopy of native and non‐native forests. This behavior is supported by the efficient photosynthetic performance thatA. dealbatashowed at low light intensities. Therefore, these results suggest that Mediterranean forest ecosystems of South America can become progressively invaded byA. dealbataand provide a basis for estimating the possible impacts that this invasive species can cause in these ecosystems in a timescale.