Biodiversity conservation and restoration in cities is a global challenge for the 21st century. Unlike other common ecosystems, urban landscapes are predominantly covered by gray, artificial structures (e.g., buildings and roads), and remaining green spaces are scarce. Therefore, to conserve biodiversity in urban areas, understanding the potential conservation value of artificial structures is vital. Here, we examined factors influencing the distribution of ferns in building gaps, one of the more common artificial structures, in urban Sapporo, northern Japan. We observed 29 fern species, which corresponds to 30 % of all fern species previously recorded in Sapporo. The four dominant species were Equisetum arvense, Matteuccia struthiopteris, Dryopteris crassirhizoma, and Athyrium yokoscense. Statistical analyses showed that their distribution patterns in building gaps were associated with both local-and landscape-scale environmental factors. Although ground cover type and distance from continuous forests were the most important determinants, other factors such as the amount of solar radiation, habitat age (years after building development), and urban district type also affected fern distribution. These results suggest that building gaps act as an important habitat for ferns in highly urbanized landscapes. Policy makers and city planners should therefore not overlook these cryptic habitats. Clarifying the ecological functions of artificial structures will both further our understanding of novel ecosystems and develop a new framework for conserving and restoring biodiversity in human-modified landscapes.

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