Land use effects on mycorrhizal fungi: A meta-analysis and field survey

Background/Question/Methods   The carbon-limitation hypothesis suggests that removal of above ground biomass should limit growth of mycorrhizal fungi due to reduced photosynthate availability. We conducted a meta-analysis of 33 publications containing 99 independent experiments to explore the effects of above ground herbivory on root colonization by mycorrhizal fungi. We also explored how the type of mycorrhiza, fertilization, experimental setting, type of host plant, duration of the experiment, and year of publication moderated any effects. In order to identify mechanisms responsible for the patterns observed in the meta-analysis, we collected bulk soil samples in 27 grasslands managed with different combinations of grazing, mowing, and fertilization. We measured root length and mass, length of extraradical arbuscular mycorrhizal fungal (AMF) hyphae, and AMF colonization of roots.

Results/Conclusions   In the meta-analysis we found that herbivory reduced mycorrhizal colonization by about three percentage points. Treatment, host plant type, and year of publication were the only significant moderators, with real or simulated herbivory of leaves and real herbivory of shoots and leaves together suppressing colonization while simulated removal of shoots and leaves together tended to increase colonization. Herbivory did not affect colonization of annual crops, perennial forbs, and evergreen trees. Colonization was reduced in perennial grasses and deciduous trees following herbivory. Colonization of mixtures of perennial grasses and forbs increased following herbivory. Effect size increased with year of publication, likely due to shifts in experimental designs towards systems more likely to show positive effects of herbivory on mycorrhiza. Based on our field survey, grazing and fertilization did not consistently affect plant or AMF abundance. Root length and mass were reduced with increasing frequency of mowing, but AMF colonization of roots was not affected. The length of extraradical hyphae of AMF increased with mowing intensity, suggesting that following extensive removal of above ground biomass, plants may actually increase resource allocation to their mycorrhizal fungi. Our results challenge the carbon-limitation hypothesis, and suggest that resource delivery by AMF may be important for plant recovery following intense herbivory or mowing.