Abstract
Invasive tree species decrease ecosystem resilience with negative impacts on natural regeneration. The influence of alien tree species on ecosystems is unevenly recognized and does not always account for different habitat specificity. We assessed the impacts of the three most frequent invasive tree species in European forests: Prunus serotina Ehrh., Quercus rubra L., and Robinia pseudoacacia L. on natural regeneration diversity, species composition, and density. We hypothesized that invaded forest types, in comparison with non-invaded, will differ in terms of species composition, will have lower taxonomic, functional, and phylogenetic diversity of natural regeneration, and will have lower densities of native tree species. We used a set of 189 study plots (200 m2) in a systematic design, established in various forest types in Wielkopolski National Park (West Poland). We analyzed impacts of forest type, accounting for soil C:N ratio, soil pH, and light availability on natural regeneration (woody species up to 0.5 m height) species composition, diversity, and density. We found an overlap of species composition among invaded and non-invaded forests and low impacts of invasive species on taxonomic diversity and functional richness. We found no impacts on phylogenetic diversity and other functional diversity components. In contrast, we found that the natural regeneration of forest-forming tree species reached lower densities in invaded than non-invaded forest types. However, sub-canopy and shrub species reached higher densities in invaded than non-invaded forest types. We confirmed that invasive tree species affect natural regeneration by decreasing the regeneration density of native tree species (in eight of nine tree species studied), species composition homogenization, and supporting natural regeneration of sub-canopy and shrub species. Therefore, the restoration of invaded forests requires eradication of invasive tree species to decrease propagule pressure and to stop decreases in the abundance of native tree species’ natural regeneration.
Highlights
Natural regeneration is a crucial process maintaining persistence of forests and emergence of new generations of trees [1,2]
Forests with R. pseudoacacia mostly overlapped the area represented by Quercus-Acer-Tilia forests, as well as P. sylvestris plantations, which were similar to almost all forest types
C. betulus, E. europaea, F. sylvatica, F. excelsior, P. sylvestris, Q. petraea, S. aucuparia, and T. cordata, we found positive but low influences of sapling densities of the invasive species studied on natural regeneration density
Summary
Natural regeneration is a crucial process maintaining persistence of forests and emergence of new generations of trees [1,2]. Natural regeneration is necessary for tree species composition changes and forest development [12,13,14]. The role of emerging tree species is crucial in the persistence of old-growth forests [15,16,17], in post-disturbance resilience of ecosystems [18,19,20], or in human-mediated transition of ecosystems [21,22]. Invasive tree species are considered to be one of the most serious threats for native ecosystems [23,24]. Alien tree species influence dependent native organisms: soil microbes [31], invertebrates [32,33], fungi [34,35], bryophytes [36,37], and vascular plants [38,39,40]
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