After trying to implement a minor controlled burn to tame the wild forests surrounding my home, I managed to create what I shall heretofore refer to as The Burning Land.
It occurred to me that we do not properly understand fire and its ability to spread from treetop to treetop. I am quite certain that I witnessed fire jumping from leaf block to leaf block over a span of at least two blocks of air, so clearly trees can catch fire even if they are not adjacent to a blaze.
So what is the minimum safe spacing between trees?
(A similar question was raised in Properties of Fire?, but I thought this question could be focused on forest fires, which the other question doesn’t appear to address in sufficient detail.)
Going by this post Firefighting post, the minimum safe distance between trees is 4 blocks. If I find a more concrete source I will edit this answer.