Trees would have probably adapted to high levels of oxygen. They'd still find fire based fertility(scrub pine) useful, but would probably have a different composition of woody stem material to slow down combustion and allow other organisms to burn.
This assumes that the only way to get energy is by burning stuff in oxygen. That may not be a safe assumption. Imagine an atmosphere with some amount of fluorine, for instance.
The spice extends life. The spice expands consciousness. The spice is vital to space travel.