Plenty of dead stuff naturally on the ground in a mesolithic forest...
You don't grow crops in a forest, you grow them in fields. They weren't slashing and burning in forests, they were doing it in open fields so they could get rid of the weeds and refuse that had grown since the prior year's crop. Once that dried they set fire to it leave a clean field for sowing their crops.
Many farmers burn their fields and pastures today, it's the same principle.