Alien woody plants on the retaining walls of the Vistula boulevards in Kraków, southern Poland

Introducing alien plants for urban horticulture and forestry may lead to their naturalisation and invasion, bringing new challenges to protecting native biodiversity and cultural heritage in urban areas. Many old buildings and monuments are threatened by invasive trees and shrubs, which can damage their structure by physical and physiological root processes and by increasing biomass of above-ground parts. The study aimed to determine the floristic composition and abundance of alien woody plants occurring on the historic retaining walls of the Vistula boulevards in Kraków, southern Poland. A total of 11 alien woody plant species were recorded in 2022, including some invasive species in Poland, such as Acer negundo, Ailanthus altissima, Fraxinus pennsylvanica, Juglans regia, Parthenocissus quinquefolia, Robinia pseudoacacia and Rosa rugosa. Most of the species showed low abundance (less than 50 individuals) in each of the distinguished age classes (seedling, juvenile and mature individuals), except vines, which in some places occupied relatively large surfaces of the walls (more than 25 m2). Regular control and monitoring of trees and shrubs are recommended to properly protect the retaining walls of the Vistula boulevards in Kraków.