Elimination of black locust Robinia pseudoacacia L. as a non-native species in Bielański Forest (Las Bielański) in Warsaw, Poland – success or failure?

•Elimination of non-native species will increase the naturalness of the urban forest.•Effects of the action limiting young and old black locust trees were investigated.•An inadequately informed society does not understand the goals of protective measures.•Due to social conflict, forest conservation objectives were only partially achieved. We sought to determine the outcomes of 25 years of implementation of the plan for the elimination of the black locust from Warsaw’s Bielański Forest. Forty years was the planned duration of the work to eradicate the non-native species, through the removing of individual trees from stands, repeated every 8–10 years. We presumed that the effects of these silvicultural treatments were positive if during the 1992–2017 study period, it would be possible to note declines: in density (number of individuals per hectare) of black locust trees (even as density of maple trees increased); in the abundance of saplings of the species (as opposed to maple); and in the rate of advancement of the species from the low to the ‘high sapling’ layers (again as opposed to maple). Empirical data were collected from 29 permanent sampling plots every five years. However, just 7 years into the eradication effort, 1999 brought protests from people living close to the Forest, who were left uninformed about the intention to fell black locust trees. These protests proved so effective that they resulted in an almost-complete abandonment of the relevant silvicultural measures. Thus, 25 years on, the conservation goal set for the Forest had only been achieved in part, as the decrease in density of black locust trees in the forest observed for 1992–1997 was followed by a slower process of decline that by definition reflected natural processes, rather than the now-abandoned tree-felling measures. Furthermore, black locust trees were able to continue their advance up from the ‘low sapling’ to ‘high sapling’ layers. The 1999 event thus emphasised, not only the need to secure the participation and understanding of a local community in regard to forest management; but also the requirement that people be educated about their area’s valuable natural attributes, and about the need for nature conservation measures in general.