SPECIAL FEATURE Fungal Symbionts of Tropical Trees

The lush green vegetation of moist tropical forest is not what it appears. Dissolve away all the plant matter from the dense foliage, giant buttressed trunks, tangled lianas, and sinuous roots, and a ghostly fungal shadow of the forest will remain. These fungi—diverse symbionts that include mutualists, commensals, and parasites—play critical roles in the dynamics, diversity, structure, and functions of tropical forests. The last two decades have led to an appreciation for the ubiquity and diversity of fungal symbionts, and to much speculation about their impacts. Understanding the nature and scope of their impacts on tropical diversity, plant growth and recruitment, and species interactions requires untangling the connections that fungi establish between their host plant and other species and the environment. This Special Feature explores these themes through analysis of the patterns and roles of tropical fungal symbionts along several dimensions: from mutualists to commensals to pathogens; from the forest canopy to the dark understory to the soil; from a postagestamp sized piece of leaf to ecosystems; from seeds to leaves to roots. Through studies of an array of endophytes, epiphytes, and pathogens, we are better able to see tropical vegetation as a plant–fungus chimera and to critically explore the complex roles of fungi in tropical forests. Nearly all plants form symbioses with a diversity of endophytic (within the plant) fungi. These endophytes colonize leaves (foliar endophytes), roots (mycorrhizal fungi), or other plant parts. Arnold and Lutzoni used molecular sequence data to compare foliar endophyte communities along a latitudinal gradient from the Canadian arctic to lowland tropical forests in Panama. They found that foliar endophytes increase in incidence, diversity, and host breadth from arctic to tropical environments. Arctic fungal assemblages comprised a low number of fungal species from a broad diversity of higher taxonomic groups, whereas tropical assemblages included a broad diversity of species from a narrower subset of higher taxa. The nature of the interaction between endophytes and their hosts varied from mutualistic to parasitic, depending on species and abiotic conditions. Experiments by Herre et al. show that both foliar endophytes and arbuscular mycorrhizae in Theobroma cacao (chocolate) reduce disease caused by the foliar pathogen Phytophthora palmivora. They demonstrate both the importance and the feasibility of incorporating endophytic fungi into studies on plant defense, physiology, and genetics of plants in tropical forests. Most plant roots are linked in symbiosis with mycorrhizal fungi, which receive a significant fraction of fixed carbon from the host plant in exchange for critical services in resource foraging in the soil. The degree of host specificity for these fungi is important in the amount of ecological connectivity they may mediate. Trees in species-rich tropical forests are mostly associated with arbuscular mycorrhizal (AM) fungi, which are traditionally thought to lack host specificity. AldrichWolfe used terminal-restriction fragment length polymorphism (T-RFLP) to analyze the AM assemblages associated with the canopy tree Terminalia amazonia in both forest habitat and pastures undergoing reforestation. Seedlings planted into the pasture developed mycorrhizal associations different from those in forest habitat, but neither did they share the AM community of the dominant pasture grasses. These results suggest that host specificity is not likely to be a prime determinant of the AM assemblage that develops on forest plants, but that neither is there great opportunity for establishing a common mycorrhizal network. In contrast, McGuire studied the importance of common mycorrhizal networks formed by ectomycorrhizal fungi (ECM), which are common in monodominant tropical rain forests but rare in species-rich forests. Seedlings with access to a common mycorrhizal network had significantly greater growth and survivorship than did seedlings excluded from the network. Seedlings showed positive survivorship effects of being near to conspecific adults, suggesting that the negative density-dependent effects thought to help maintain