Compensatory growth and competitive ability of an invasive weed are enhanced by soil fungi and native neighbours

Abstract

Compensatory responses to herbivory by invasive weeds may foil attempts to arrest their spread with biological controls. We conducted an experiment to study the effects of defoliation and soil fungi on interactions between Centaurea melitensis, an invasive annual from Eurasia, and Nassella pulchra, a native Californian bunchgrass. Defoliation of C. melitensis reduced its final biomass in all species–fungicide treatments, except when C. melitensis was grown with both Nassella and non‐treated soil fungi at the same time. In this treatment, the biomass of clipped C. melitensis plants was equal to that of unclipped plants, indicating that soil fungi and Nassella promoted a compensatory response in the weed. Overall, the biomass of C. melitensis was 44% lower when soil fungi were reduced. However, in soil not treated with fungicide, the total biomass of C. melitensis increased in the presence of Nassella, but decreased when it was grown alone. When stressed by defoliation, C. melitensis may benefit from a form of mycorrhizae‐mediated parasitism through a common mycorrhizal network, or Nassella may alter the fungal community in a way that enhances the positive direct effects of soil fungi on Centaurea.

Effects of Arbuscular Mycorrhizal Fungal Infection and Common Mycelial Network Formation on Invasive Plant Competition

“…For example, the invasive species Centaurea melitensis was larger when grown in competition than when grown alone, but only with AMF present (Callaway et al 2001), suggesting a direct invasive plant benefit from CMN formation through "mycorrhizal-mediated parasitism" (Callaway et al 2001, Bray et al 2003.…”

“…The mycorrhizal symbiosis is capable of altering competitive interactions between plants (Moora and Zobel 1996, Casper and Castelli 2007, Wagg et al 2011, including invasive plants, both with conspecifics and native competitors (Goodwin 1992, Marler et al 1999, Callaway et al 2001, Bray et al 2003. Potential mechanisms for competitive alteration include plant-soil feedbacks (Bever et al 1997) and either resource sharing or competition through common mycelial networks that connect multiple plants (Hart et al 2003, Selosse et al 2006.…”

Plant coexistence mediated by arbuscular mycorrhizal fungi

“…Conversely, removing AMF from tall grass prairie soil promoted plant coexistence because the dominant grasses were more mycorrhizal dependent than were subdominant species [21,22]. Similarly, experimental removal of AMF promoted coexistence by reducing the ability of mycorrhizal-dependent Centaurea maculosa to invade native grasslands of western North America [23,24], and of Medicago minima to invade an Australian grassland [25].…”

Con‐specific neighbours may enhance compensation capacity in an invasive plant

“…hasata, to compensate for herbivory through soil shading. Callaway et al (2001) suggested that the presence of soil fungi and the co-occurring native species Nassella pulcbra promoted a compensatory response of the invasive species Centaurea melitensis to herbivory. Parmesan (2000) reported that herbivory by the butterfly Euphydryas editha reduced fitness of the native montane annual Collinsia torreyi at low density, but not at high density.…”