Trump is not entirely correct, but there is actual fire science he is kind of referencing that is correct. Don't throw out the baby with the bathwater. Trees aren't necessarily always good, and cutting down trees and removing fuel load (fallen limbs, trees, leaves technically but not as big a factor) ... removing fuel load is an extremely important part of preventing huge wildfires. There are ecological reasons to remove trees, including fire prevention, and also including prevention of succession (sometimes trees can take over grasslands/shrublands ... grasslands are a much rarer and more threatened ecotype than many forests).