Differential effects of tropical arbuscular mycorrhizal fungal inocula on root colonization and tree seedling growth: implications for tropical forest diversity

The potential for mycorrhizae to influence the diversity and structuring of plant communities depends on whether their affinities and effects differ a cross a suite of potential host species. In order to assess this potential for a tropical forest community in Panama, we conducted three reciprocal in oculation experiments using seedlings from six native tree species. Seeds w ere germinated in sterile soil and then exposed to arbuscular mycorrhizal f ungi in current association with naturally infected roots from adults of ei ther the same or different species growing in intact forest. The tree speci es represent a range of life histories, including early successional pionee rs, a persistent understory species, and emergent species, typical of matur e forest. Collectively, these experiments show: (i) the seedlings of small- seeded pioneer species were more dependent on mycorrhizal inocula for initi al survival and growth; (ii) although mycorrhizal fungi from all inocula we re able to colonize the roots of all host species, the inoculum potential ( the infectivity of an inoculum of a given concentration) and root colonizat ion varied depending on the identity of the host seedling and the source of the inoculum; and (iii) different mycorrhizal fungal inocula also produced differences in growth depending on the host species. These differences ind icate that host-mycorrhizal fungal interactions in tropical forests are cha racterized by greater complexity than has previously been demonstrated, and suggest that tropical mycorrhizal fungal communities have the potential to differentially influence seedling recruitment among host species and there by affect community composition.