Impacts of early- and late-seral mycorrhizae during restoration in seasonal tropical forest, Mexico

Disturbance of vegetation and soil may change the species composition of arbuscular mycorrhizal fungi (AMF), which may in turn affect plant species responses to AMF. Seasonal tropical forest in Mexico is undergoing rapid conversion to early-successional forest because of increased wildfire and may require restoration. The responses of six early- and late-successional tree species were tested using early- and late-successional AMF inoculum. The plants were germinated in the shadehouse and received three inoculum treatments: (1) soil from a two-year-old burned site, (2) soil froth a mature forest site, or (3) uninoculated controls. They were transplanted as seedlings to a site prepared by burning, and their growth was measured from September 1997 to November MOO. All six species had the greatest growth response to early-seral inoculum, but the response to late-seral inoculum varied. Two tree species, Ceiba pentandra and Guazuma ulmifolia, were smallest with late-seral inoculum, even smaller than the uninoculated. plants, and the other species, Brosimum alicastrum, Havardia albicans, Acacia pennatula, and Leucaena leucocephala, had intermediate growth with late-seral inoculum. Of these, Brosimum, Havardia, and Ceiba occur in late-successional forest, and the others are early seral. Of the several growth measurements (height, cover, biomass, stem diameter), stem-diameter responses to inoculum were still significantly different into the third year for four of the species. The uninoculated plants became infected by residual inoculum in the burned experimental site within three months of transplanting, yet mycorrhizal responses persisted. The treatment, size differences may be due to different species composition of the inocula. The early-seral inoculum was dominated by small-spored Glomus spp., while the late-seral inoculum had a higher density of large-spored Gigasporaceae. The latter are known from greenhouse experiments to promote a smaller plant-growth response than Glomus. Mature forest trees may withstand the carbon drain from Gigasporaceae better than establishing seedlings, so the growth patterns we observed with inoculum source are consistent with a rapidly growing successional forest, followed by slower-growing mature forest. The results suggest that early-seral AMF should be used when seedlings are inoculated for restoration, even for late-seral tree species.