Pollinator habitat enhancement through ornamental plants, floral traits and urban green area features for planning biodiversity-friendly cities

Ornamental plants are chosen for aesthetics or suitability to urban conditions, while their contribution to habitat enhancement is usually not a priority. However, studies have shown that ornamentals could have potential for pollinators. It is a priority to rank ornamental plants, and to understand which floral traits or environmental conditions influence visitation by pollinators, for habitat enhancement and landscape planning. Here, we surveyed ornamental plant-pollinator interactions across seven Italian cities, in May, to quantify relative plant attractiveness in field conditions, elucidate flower trait-pollinator relationships, and assess habitat descriptors or green area type influences. By focusing on plant-pollinator interactions, 43 ornamental plants were ranked for the contribution to both pollinator abundance and species richness, or to specific groups or few insects. Single floral traits positively or negatively associated with specific insect groups: these diverse responses clarify that trait diversity is essential to support heterogeneous urban pollinators. Furthermore, more abundant and speciesrich pollinators were found on plants in heterogeneous area conditions, and distinctions emerged across parks, historic villas and botanic gardens. Therefore, apart from intrinsic plant features, the design and management of green areas should focus on adequate ornamental plants with diverse floral traits and in heterogeneous habitat contexts, to develop urban green infrastructures for pollinators. Landscape planning and management of green areas could integrate biodiversity-based strategies with priority ornamental plants for pollinator interactions, or use flower traits when lists are not available, to trigger urban ecosystem functionality and enhance habitat quality.