Mycorrhizal fungi and reproduction of field populations of Abutilon theophrasti Medic. (Malvaceae)

The effects of inoculation with mycorrhizal fungi and phosphorus amendment on reproduction of Abutilon theophrasti Medic, were documented over two years in the field. In Year 1 all plots were thinned to a common and relatively low density (12 plants m⁻²). Mycorrhizal infection significantly increased capsule and seed production. Phosphorus amendment had little effect on these characters, possibly due to low soil moisture content. In Year 2 following natural recruitment, inoculated (M) plots were much more dense than uninoculated (NM) plots. In contrast with the results from Year 1, however, individual plains in M plots produced fewer capsules than those in NM plots in Year 2 because capsule production in Year 2 was density‐dependent. Based on these results and others, we suggest that in monospecific stands of Abutilon, mycorrhizal infections have the greatest effects on fecundity at low densities. At higher densities, density‐dependent forces may reduce the importance of mycorrhizal infections to reproductive effort of a single plant. Our results highlight the necessity of examining the effects of mycorrhizal fungi in natural populations and communities.