A sketchy website claims that this mod makes trees spread organically, without player involvement...

The mod will add one realistic feature to minecraft - now the trees will spread around the world by themselves. That is, a seedling of trees can sprout next to one tree and after a while it will become a full-fledged tree and so on - the trees will slowly but surely spread around the world themselves.
[translated from Russian]

...which is definitely not the case. Might be a fun feature though.