The role of arbuscular mycorrhizal fungi in pasture colonization by the tropical forest tree species Terminalia amazonia

Abstract

At three sites in Siete Colinas, Coto Brus, Costa Rica, I examined differences in composition of arbuscular mycorrhizal (AM) fungi between forest fragments and pastures, and consequences of these differences for seedling establishment in pastures by the tropical forest tree Terminalia amazonia. I estimated species composition of AM fungi through spore counts from field soils and greenhouse cultures. The AM fungal community differed between forest and pasture. While some AM fungi were common in both habitats, others were abundant in one and rare or absent in the other. To assess the importance of the change in community composition for pasture colonization by T. amazonia, I planted seedlings inoculated with either forest or pasture soil in forest and pasture, and compared survivorship, growth, and root colonization by AM fungi. Seedlings inoculated with forest soil experienced lower mortality, and greater initial growth rates and colonization by AM fungi, than seedlings inoculated with pasture soil. Differences in growth and survivorship may have resulted from differences in the AM fungal community or other differences between forest and pasture soils. To determine whether AM fungal communities differ in their benefit, I compared the growth of seedlings inoculated with either spores obtained from forest or pasture, or with spore-free media. Seedlings inoculated with forest AM fungi had the highest root colonization, shoot phosphorus, and biomass. Seedlings inoculated with pasture AM fungi differed little from controls. I used molecular analysis to identify which AM fungi colonize forest T. amazonia, pasture-grown seedlings, and dominant pasture grasses. AM fungal communities of seedlings planted into pastures did not reflect those of the other plants, suggesting the importance of both environment and host in determining AM fungal community composition. Conversion of forest to pasture alters the AM fungal community. Seedling survival is higher for seedlings inoculated with forest than with pasture soil. At least some pasture AM fungi reduce mycorrhiza formation and seedling growth of T. amazonia relative to forest AM fungi. Seedlings colonizing pastures do not form mycorrhizas with the same symbionts as in forest. The significance of these differences for forest regeneration in tropical pastures merits further exploration.