Successions of sheathing mycorrhizal fungi.

Many fungi capable of forming sheathing (ecto-) mycorrhizas are associated with trees. But what are the rules governing their occurrence? Evidence from first generation woodlands/forests, where trees grow on sites that have been treeless for many years, indicates that species of fungi occurring at early stages of woodland development ('early-stage fungi') are, in due course, superseded by others ('late-stage fungi'). There is an ordered succession which seems to reflect, in large measure, the different abilities of early- and late-stage fungi to form mycorrhizas on roots growing in soils with accumulations of recalcitrant leaf litter. In second generation woodlands/forests there seems to be little evidence of early-stage fungi. This is not so surprising as it may seem, bearing in mind that their soils are already likely to have accumulations of recalcitrant litter. Instead of classifying sheathing mycorrhizal fungi by referring to the temporal stage of woodland development, it now seems more meaningful to judge them by their abilities to colonize roots in soils with or without accumulations of different types of litter.