Allelopathic Effect of the Invasive Species Acacia dealbata Link and Hakea decurrens R.Br., subsp. physocarpa on Native Mediterranean Scrub Species

Invasive species can profoundly alter ecosystems through mechanisms such as allelopathy. This study evaluates the allelopathic effects of <i>Acacia dealbata</i> and <i>Hakea decurrens</i> subsp. <i>physocarpa</i> on two dominant Mediterranean native species, <i>Cistus ladanifer</i> and <i>Lavandula stoechas</i>. Germination bioassays using aqueous extracts (1:10 <i>w</i>/<i>v</i>) at concentrations of 1, 1/2, and 1/4 of leaves collected in March and September were used to evaluate germination, hypocotyl emergence, and root development compared to control values (water) and between treatments. The phenolic composition of the solutions used was also analyzed. Significant inhibitory effects were observed across all parameters, especially at high concentrations, with responses modulated by the invasive species, the native target, and seasonal variation. <i>A. dealbata</i> showed stronger phytotoxicity in March, while <i>H. decurrens</i> subsp. <i>physocarpa</i> was more active in September. Phytochemical analysis revealed a higher load of phenolic compounds in <i>A. dealbata</i>, which may be related to the greater allelopathic activity of this species. These findings confirm the allelopathic potential of both invasive species and their ability to interfere with the establishment of native plants while facilitating their own, potentially impacting the colonization success of invasive species and altering vegetation succession in Mediterranean ecosystems.