title: Conditions Promoting Mycorrhizal Parasitism Are of Minor Importance for Competitive Interactions in Two Differentially Mycotrophic Species.
abstract: Interactions of plants with arbuscular mycorrhizal fungi (AMF) may range along a broad continuum from strong mutualism to parasitism, with mycorrhizal benefits received by the plant being determined by climatic and edaphic conditions affecting the balance between carbon costs vs. nutritional benefits. Thus, environmental conditions promoting either parasitism or mutualism can influence the mycorrhizal growth dependency (MGD) of a plant and in consequence may play an important role in plant-plant interactions. In a multifactorial field experiment we aimed at disentangling the effects of environmental and edaphic conditions, namely the availability of light, phosphorus and nitrogen, and the implications for competitive interactions betweenHieracium pilosellaandCorynephorus canescensfor the outcome of the AMF symbiosis. Both species were planted in single, intraspecific and interspecific combinations using a target-neighbor approach with six treatments distributed along a gradient simulating conditions for the interaction between plants and AMF ranking from mutualistic to parasitic. Across all treatments we found mycorrhizal association ofH. pilosellabeing consistently mutualistic, while pronounced parasitism was observed inC. canescens, indicating that environmental and edaphic conditions did not markedly affect the cost:benefit ratio of the mycorrhizal symbiosis in both species. Competitive interactions between both species were strongly affected by AMF, with the impact of AMF on competition being modulated by colonization. Biomass in both species was lowest when grown in interspecific competition, with colonization being increased in the less mycotrophicC. canescens, while decreased in the obligate mycotrophicH. pilosella. Although parasitism-promoting conditions negatively affected MGD inC. canescens, these effects were small as compared to growth decreases related to increased colonization levels in this species. Thus, the lack of plant control over mycorrhizal colonization was identified as a possible key factor for the outcome of competition, while environmental and edaphic conditions affecting the mutualism-parasitism continuum appeared to be of minor importance.
authors.affiliation: Department of Experimental and Systems Ecology, University of Bielefeld Bielefeld, Germany.
authors.forename: Martina
authors.initials: M
authors.lastname: Friede
authors.affiliation: Department of Experimental and Systems Ecology, University of Bielefeld Bielefeld, Germany.
authors.forename: Stephan
authors.initials: S
authors.lastname: Unger
authors.affiliation: Department of Experimental and Systems Ecology, University of BielefeldBielefeld, Germany; Ecosystem Physiology, University of FreiburgFreiburg, Germany.
authors.forename: Christine
authors.initials: C
authors.lastname: Hellmann
authors.affiliation: Department of Experimental and Systems Ecology, University of Bielefeld Bielefeld, Germany.
authors.forename: Wolfram
authors.initials: W
authors.lastname: Beyschlag
keywords: Corynephorus canescens; Hieracium pilosella; arbuscular mycorrhizal fungi; competitive interactions; mutualism; mycotrophy; parasitism