Todays's Nature contains a News Feature by me (behind a paywall, I'm afraid), looking at the possibility that mycorrhizal fungi transfer nutrients between plants, and in the process undercut the above-ground competition between plants by robbing the rich to feed the poor - subsidizing plants less able to photosynthesize.
How, why and whether they do this is still uncertain and occasionally controversial. But everyone seems to accept that mycorrhizal networks do exist - namely, that a single fungus can link many plants, potentially of different species, creating a arena for a rich range of ecological interactions. It's just we're not sure what they are. But a bunch of people are doing their damnedest to find out. Mycorrhizal ecology looks like a funky (and fungi) field right now.
For a more technical (than my piece), but freely accessible introduction to this field check out this TREE paper.