K wrote:
By coincidence, I was at a talk yesterday by one of the country's
leading mycologists. The fungus grows steadily outwards, fruiting around
the outside, hence the expanding ring. You can form an estimate of the
age of the fungus but the diameter of the ring of fruiting bodies.

Since the spores aren't propelled in any way by the fungus, they are
dependent on air currents for their dispersal. It's hard to envisage a
mechanism whereby they would be carried outwards only.
If putting a plastic sheet down killed the fungus, then it wasn't by the
mechanism described.

There's something to do with the nutrients that the mushrooms seek in
the soil hence them moving outwards and spores falling on depleted soil
wouldn't do well. I think that's the mechanism for the fungis to move
outwards. The spores collecting are an option, an organic one, which
works. Off course there's the wind, hence the 1m sheet outwards.