Where is the forest core area? Gradients of flora in the ecotone of urban forests in Warsaw

Urban forests are very important ecosystems, performing key cultural services and having crucial regulatory functions. However, naturally valuable areas in cities continue to decline or undergo irreversible transformations. It is necessary to determine the extent of human influence on forest ecosystems in highly urbanized environments. Our study examined the indicators of diversity, naturalness, lability, and stability of flora along transects from edge to center of six urban forests. Furthermore, the shape of gradients of changes and the threshold distance and straightness between forest edge and core for these indicators were measured. Finally, the impact of the number of inhabitants, the degree of protection, and the size and shape of the forests on each indicator separately and their aggregation in the form of the Flora Transformation Index were analyzed. The findings revealed that the gradients of changes were rarely linear and showed a logarithmic or polynomial shape. The most regular gradients were observed for lability and naturalness of flora. The regularity of the shape of the thresholds differed between the studied indicators. The threshold distance was relatively far (93–113 m) and depended heavily on human influence, but forest shape and area were also very important. Our results showed that legal protection, even as a reserve, has no effect in the case of solid human penetration of the forest. Our findings are expected to help determine suitable indicators for defining threshold values, i.e., the size of the edge and core zones of urban forests, which is crucial to the protection of the native plant species pool.