Abstract
The outcome of competition for nitrogen (N) between native and invasive tree species is a major concern when considering increasing anthropogenic N deposition. Our study investigated whether three native (i.e., Fagus sylvatica, Quercus robur, and Pinus sylvestris) and two invasive woody species (i.e., Prunus serotina and Robinia pseudoacacia) showed different responses regarding morphological and physiological parameters (i.e., biomass and growth indices, inorganic vs. organic N acquisition strategies, and N allocation to N pools) depending on the identity of the competing species, and whether these responses were mediated by soil N availability. In a greenhouse experiment, tree seedlings were planted either single or in native-invasive competition at low and high soil N availability. We measured inorganic and organic N acquisition using 15N labeling, total biomass, growth indices, as well as total soluble amino acid-N and protein-N levels in the leaves and fine roots of the seedlings. Our results indicate that invasive species have a competitive advantage via high growth rates, whereas native species could avoid competition with invasives via their higher organic N acquisition suggesting a better access to organic soil N sources. Moreover, native species responded to competition with distinct species- and parameter-specific strategies that were partly mediated by soil N availability. Native tree seedlings in general showed a stronger response to invasive P. serotina than R. pseudoacacia, and their strategies to cope with competition reflect the different species’ life history strategies and physiological traits. Considering the responses of native and invasive species, our results suggest that specifically Q. robur seedlings have a competitive advantage over those of R. pseudoacacia but not P. serotina. Furthermore, native and invasive species show stronger responses to higher soil N availability under competition compared to when growing single. In conclusion, our study provides insights into the potential for niche differentiation between native and invasive species by using different N forms available in the soil, the combined effects of increased soil N availability and competition on tree seedling N nutrition, as well as the species-specific nature of competition between native and invasive tree seedlings which could be relevant for forest management strategies.
Highlights
Biological plant invasions have consequences on plant interactions in native communities, severely affecting ecosystems in the future given that plant community composition and structure are largely determined by the outcome of plant– plant interactions such as facilitation and/or competition for limiting resources (Goldberg and Barton, 1992)
Evidence points in the direction that species in native plant communities have evolved different strategies to avoid competition for N via preference of different N sources to effectively avoid competition via niche differentiation (Näsholm et al, 2009; Hodge and Fitter, 2013; Simon et al, 2017), via N acquisition at different times during the growing season (Simon et al, 2017), and/or via the interaction with soil microorganisms to enhance N acquisition (e.g., Hodge and Fitter, 2013), these strategies might no longer be effective in competition with non-native plant species because of their higher competitive ability compared to native species (Gioria and Osborne, 2014)
The higher competitive ability of Prunus in our study is in accordance with earlier work by others; previous studies investigated the effects of Prunus and Robinia when competing with native species mainly considering plant growth and biomass (e.g., Closset-Kopp et al, 2011; Robakowski and Bielinis, 2011; Kawaletz et al, 2013, 2014), whereas we found competition effects on growth and biomass and on N acquisition and N allocation to different N pools depending on the species
Summary
Biological plant invasions have consequences on plant interactions in native communities, severely affecting ecosystems in the future given that plant community composition and structure are largely determined by the outcome of plant– plant interactions such as facilitation and/or competition for limiting resources (Goldberg and Barton, 1992). Evidence points in the direction that species in native plant communities have evolved different strategies to avoid competition for N via preference of different N sources to effectively avoid competition via niche differentiation (Näsholm et al, 2009; Hodge and Fitter, 2013; Simon et al, 2017), via N acquisition at different times during the growing season (Simon et al, 2017), and/or via the interaction with soil microorganisms to enhance N acquisition (e.g., Hodge and Fitter, 2013), these strategies might no longer be effective in competition with non-native plant species because of their higher competitive ability compared to native species (Gioria and Osborne, 2014) Considering their higher capacity to exploit limited resources, better resource use efficiency, inhibition of growth, as well as establishment of potential competitors (DAntonio and Vitousek, 1992; Gioria and Osborne, 2014), non-native plant species have a large potential to become invasive (Keller et al, 2011) and as a result induce profound changes in forest ecosystem structure and functioning (Holmes et al, 2009; Vilà et al, 2011; Aerts et al, 2017). In general, non-woody species are investigated preferably in plant interaction studies, the understanding of the mechanisms underlying tree interactions is rather scarce (Trinder et al, 2013; Pommerening and Sánchez Meador, 2018), resource acquisition and internal allocation are important for resource-storing and -remobilizing in longliving trees
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