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 plants 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.