I wonder what could cause a yearly up and down, but maybe necessary to test.
There are initial branches when starting a map and then trees constantly spawn it. Young trees don’t until half matured. It spawns independend of temperature, so should not come in waves. And it wastes away after a year.
Bogs spawn birches and also branches as you noticed, which is meant as deadwood not making it through the swampy conditions. Also constantly.