Few topics in ecology are as intriguing or bedeviling as comparative studies of different types of mycorrhizas formed in the same plant species. Attempts to determine the relative benefits from each fungal type to the host plant are fraught with difficulties (Jones et al., 1998), and for this reason plants that form tripartite associations with arbuscular and ectomycorrhizal fungi make ideal experimental systems. Just such a tripartite system is explored in a report in this issue by Chen et al. (pp. 545–556).

Why compare mycorrhizas? Although both ecto- and arbuscular mycorrhizas are generally known to increase the uptake of nutrients such as phosphorus and nitrogen in infertile soils, their functions and the benefits they provide to their host plants may not be equivalent (Jones et al., 1998). Hence the ecological benefits of the two types of symbiosis are of great interest to plant community ecologists and researchers studying nutrient cycling. Arbuscular mycorrhizas are formed as a symbiosis of plant roots with primitive fungi of the order Glomales (Zygomycetes), while ectomycorrhizas are formed with higher basidiomycetes and a few ascomycetes. Only a few plant genera and species are capable of forming both types of association, sometimes on the same root tip (Fig. 1), but it is these that are so useful in comparative studies.