Invasive Prunus serotina vs. Robinia pseudoacacia: How does temperate forest natural regeneration respond to their quantity?

Invasive trees negatively impact forests, by making the vegetation more homogeneous when invaders are present than when they are absent. Here, we aim to more deeply understand the effects of invasive trees on forests with a focus on seedlings and saplings and how they respond to continuous variation in aboveground biomass of invaders rather than presence/absence. Our findings are useful for close-to-nature silviculture, as they elucidate how much natural regeneration will change under particular biomasses of invasive species. Specifically, we evaluate the relationships of two invasive tree species: black cherry Prunus serotina Ehrh. and black locust Robinia pseudoacacia L. with natural tree regeneration in temperate forests. We established 160 circular 0.05 ha plots in western Poland managed forests, in two different habitat types: nutrient-poor with Pinus sylvestris L. and nutrient-rich with Quercus spp. We assessed natural regeneration by counting all trees < 1.3 m in height, within four circular subplots (r = 3 m). Relationships between invader biomass and regeneration of other tree species were idiosyncratic. Natural regeneration of dominant forest-forming tree species ( P. sylvestris , Quercus petraea ) decreased with increasing invader biomass, while shade-tolerant, nitrophilous tree and shrub regeneration increased with invader biomass. The most negatively correlated were P. sylvestris in nutrient-poor habitats and Q. petraea in both nutrient-poor and rich habitats. We observed increased density of other non-native species as R. pseudoacacia abundance increased, in line with the invasional meltdown hypothesis.