What is the minimum safe spacing between trees to contain forest fires?

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.

So I did some tests and it appears as though three spaces is the absolute minimum, meaning fires can jump across two air blocks to ignite other flammable materials. However as you can see in the attached image, grass will also sometimes catch fire. So, to be safe when doing controlled burns, I definitely recommend a four-space buffer. If grass between your trees catches, that turns a safe three-space gap into an inferno friendly two-space gap.

(Thus, Rapida was correct.)

SCIENCE!