In one hectare of forest, around 280 kilos (about 620 pounds) of carbon can be moved between trees, which is about 4% of total photosynthetic activity.
It seems that the fungi are moving sugars around in order to protect their own carbon stores in the trees.
Something else has just occurred to me. From one perspective, can we just see mycorrhiza-tree symbiosis as glorified lichens? A podcast I watched last night (not sure if it's been edited for general viewing yet) noted that the earliest trees evolved at the same time as the early tree-lichens.
This shows just how important it is to get out and get inoculating.