Comment by floatrock

Huge amount, but maybe not in the way you intended.

Many of California's ecosystems have evolved to expect fires. Humans can't stand fires and aggressively put them out. So fuel that would be regularly burned off in mild wildfires instead builds up into megafires that exceed the limits of what the ecosystem can handle (a lot of California trees are fire-tolerant, but there's a point where the flames get too high and too intense).

So yeah, the human activity that affects these cycles is caused by our cognitive dissonance and fear to phrases like "mild wildfire".