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.
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