How does non-native Robinia pseudoacacia L. affect urban forest biodiversity?

Abstract

Robinia pseudoacacia L. is one of the most widespread non-native tree species in Europe, especially in human-transformed areas, yet its impact on urban forest communities has remained poorly understood. We aimed to determine whether R. pseudoacacia-dominated stands in fertile urban forests influenced species composition and vegetation functions similarly to those dominated by the native Pinus sylvestris L. We compared species composition, analyzed alpha diversity, and assessed the invasional meltdown hypothesis, by evaluating non-native species abundance in both stand types. We collected data from 68 sample plots in urban forests in two Polish cities, recording species occurrence and vascular plant cover, using the Braun-Blanquet scale. Our results showed that R. pseudoacacia locally increased biodiversity by promoting nitrophilous, ruderal, and edge species without negatively affecting typical forest species. R. pseudoacacia further facilitated shade-tolerant species such as Acer spp., confirming that mixed stands with R. pseudoacacia in fertile habitats represented only a temporary successional stage. Since R. pseudoacacia and P. sylvestris had similar effects on habitat conversion and non-native species occurrence, we found no support for the invasional meltdown hypothesis. Our findings suggested that R. pseudoacacia could be tolerated in fertile urban forests as part of a mixed forest structure rather than prioritized for removal. We recommend that forest management focus on site-specific assessments of non-native species impact and emphasize increasing forest resistance to invasions, particularly by maintaining high canopy cover.