Why are there places where trees don't grow? I can understand places like a desert where it's too dry or tundra where it's too cold, but why are there places like meadows or fields in the middle of the forest where trees don't grow? Or even large areas like prairies?
A non-conclusive list:


frequent wildfires
not enough water
too much water
bad soil conditions
average temperatures too high or low
not enough sunlight
animals
human intervention
other vegetation is better adapted to local conditions and thus suppress growing trees