The pyroclastic flow is filled with toxic gasses, the island is probably scrubbed clean. By just like Mt St. Helen, we might be surprised how quickly life takes hold again.
Those flows are also super-heated.
I climbed Mount St Helens 5 years after the blast. You never saw such desolation. Inch tall trees that would be buried by the wind, a few grasshoppers blown in by the wind, and some spiders.