Weed co-occurrence patterns in revegetation plantings: Implications for managing multiple weed species

Most weed research has focused on single species, whereas managers of conservation habitats often deal with multiple co-occurring species. Effective multi-weed management in revegetation plantings is crucial to prevent unintended responses to the removal of single species, which can change the successional trajectory of the planting project. Predicting positive and negative weed associations can help managers prioritise weed removal to prevent unintended responses from co-occurring weeds. Seedling plots were used to identify species co-occurring under each of three target woody weeds in revegetation plantings in Auckland, Aotearoa – New Zealand ( Solanum mauritianum , Paraserianthes lophantha , and Ligustrum lucidum ) and compare them with seedlings growing under native plants. Twenty-four pairs of weed species were significantly correlated with each other’s occurrence, either positively (often found together) or negatively (rarely found together). Mature, shade-tolerant L. lucidum was significantly correlated (both positively and negatively) with the most weed species ( n = 9). Nitrogen-fixing P. lophantha had one positive and one negative pairwise association with other weed species, while allelopathic S. mauritianum had two negative associations. Shared bird dispersal and shade tolerance were associated with an increased probability of co-occurrence for seedlings. While bird-dispersed seedlings arrive at the site together, their associations are subsequently shaped by interspecific interactions, such as competition and facilitation. Higher housing density, resident native species richness, and vascular vegetation cover were all associated with higher weed richness, whereas higher human population density was associated with lower weed richness. Woody weeds, particularly shade-tolerant and bird-dispersed species, should be prioritised by managers of urban revegetation projects.