We performed two glasshouse experiments to determine whether the
presence of arbuscular mycorrhizal
symbiosis reduces the intensity of intraspecific competition at low concentrations
of available phosphorus (P), and
whether this effect is modified by a reduction in light intensity. In the
first experiment, Trifolium subterraneum cv.
Mt. Barker was grown at different densities in controlled conditions of
light and temperature, and half of the pots
were inoculated with spores of the arbuscular mycorrhizal fungus, Gigaspora
margarita. In the second experiment,
the plants were grown in similar controlled conditions but the light intensity
received by half of the pots was
reduced by >50%. The biomass and P content of individual mycorrhizal
plants and the biomass response to
mycorrhizal infection were drastically reduced as plant density increased.
The effects of density on percentage
infection, shoot and root P concentrations, and root: shoot ratios were
inconsistent. Generally reduction in light
intensity did not alter these effects. Mycorrhizal symbiosis increased
intraspecific competition intensity through
an increase in the availability of soil P. This increase in competition
was reflected in the greater size inequality of
low density mycorrhizal treatments. Our results emphasize that the main
effects of mycorrhizas at the individual
level cannot be expected to be apparent at the population level, because
they are overridden by density-dependent
processes.