DISTURBANCE AND SEASONAL DYNAMICS OF MYCORRHIZAE IN A TROPICAL DECIDUOUS FOREST IN MEXICO

Mycorrhizal fungi were sampled in a deciduous tropical forest on the P acific coast of Mexico during different seasons and in natural treefal l gaps and pastures. All 12 plant species sampled in the forest were a rbuscular mycorrhizal. The percent root infection and spore production were closely related to the phenology of the plants. Most tree specie s and all herbaceous species had the highest infection in the summer r ainy season, bur two species, Opuntia excelsa and Jacquinia pungens, h ad highest infection in the dry season. Unusually high rainfall during the dry season was associated with increased infection bur not increa sed spore production. Spore density was low for all species at all sam ple times, except at the beginning of the July 1993 rainy season in, w hen we observed up to 28 spores/g soil. The percent cover of shrubs or herbs did nor increase in gaps after two years, and we observed no co lonizing seedlings. No plane species with cover higher than 2.7 percen t occurred exclusively in gaps or forest. The percent mycorrhizal infe ction did nor differ significantly between gaps and forest. Spore coun ts were as high in the gaps as in the forest in two of the three gaps but lower in the third gap. The lack of significant response of plants in these gaps after two years differed from the rapid response in tro pical rainforests. it is likely related to the small size of the gaps and to light infiltration to the forest floor. Pastures were dominated by two species of exotic grasses and one species of mycorrhizal fungu s, whereas forests had lj fungal species. The slow regrowth of vegetat ion in gaps was not limited by mycorrhiza fungi, since they were still abundant after the treefalls, bur recovery in pastures could be affec ted by low fungal diversity and dominance of grasses.