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.)