Mapping the permeability of urban landscapes as stepping stones for forest migration

Large-scale urbanisation has become a significant barrier to the migration of trees, which is being exacerbated by accelerated climate change. Maintaining and increasing landscape permeability is expected to be an effective strategy to facilitate the process of forest migration through the city. This study develops a new methodology to map the permeability of urban landscapes as stepping stones from the perspective of seed dispersal. Since seed dispersal agents experience their landscapes as hierarchical mosaics of patches, two spatial scales—habitat and home-range scales—are simultaneously considered in the study. The proposed method combines a least-cost path model and a graph theory-based approach. The least-cost path model is applied to map the potential movements of dispersal agents, based on which two graph theory-based indices—the probability of connectivity index and the integral index of connectivity—are used to quantify the accessibility of the landscape at habitat and home-range scales, respectively. This method is demonstrated by a case study in the Greater Manchester area, UK. Eurasian jay, Eurasian siskin, coal tit and grey squirrel are selected as the main dispersal agents in the study area. The results provide a comparison of the landscape permeability maps generated from different dispersal agents and identify key areas likely to facilitate the process of forest migration through Greater Manchester. Recommendations regarding landscape design and management for improving permeability are also discussed.