Urban vegetation decline under Brachychiton discolor f. Muell. tree

Brachychiton discolor F.Muell. (Malvaceae) is a deciduous ornamental tree native to Australia, which has gained ecological significance in urban ecosystems worldwide. This study evaluates the impact of leaf litter from B. discolor canopies on ground cover and plant diversity in urban environments. Areas under B. discolor canopies exhibited significant reductions in the cover of Cynodon dactylon (L.) Pers., Euphorbia hirta L., Malva parviflora L., Medicago polymorpha L., Melilotus indicus (L.) All., Poa annua L., and Sisymbrium irio L., along with Shannon-Wiener diversity (H') and evenness indices (E) declined. Moreover, leaf litter from B. discolor increased soil organic matter by 18.6% and enhanced the concentrations of certain nutrients, including potassium (16.24%) and copper (11.07%), in the affected urban soils. Greenhouse experiments demonstrated that leaf residues of B. discolor significantly inhibited C. dactylon germination (by 37.1%) and reduced the establishment of C. dactylon (by 54.6%) and M. parviflora (by 58.64%). This tree much reduced the light intensity under the tree canopy. High-performance liquid chromatography analysis identified key phenolic and flavonoid allelochemicals, including ellagic acid, pyrogallol, and benzoic acid among phenolics and kaempferol among flavonoids. These results suggested that reduced vegetation cover and diversity under B. discolor are primarily driven by its leaf litter and shading effects. The allelopathic impact of its chemical compounds appeared more pronounced than its mechanical effects. Consequently, B. discolor significantly alters urban vegetation cover and diversity, highlighting the need for conservation strategies to mitigate its impact and protect urban ecosystems.