Can you explain how the spreading of trees works? How can player influence it? I'm thinking about installing it. Thanks

It's pretty simple. It iterates over every loaded chunk, randomly selects a couple, and marks them for tree propagation. If any chunks have been marked for propagation, it grabs a random tree from that chunk and tries to spawn a new tree of the same type within 12 tiles of it.

Right now there isn't a way for a player to influence it in-game. This was made to somewhat mimic natural forest growth while still being somewhat useful.

Currently this happens every 12500 to 25000 ticks, or half-day to full day. I'm working on a version that is more configurable, and will allow other entity types to be registered either in configs or at runtime.

However, if you have any suggestions on how you would like player interaction with tree growth to work, I would love to hear it.

With the 0.13 ability to spread fire it becomes interesting
Do you check if the ground isa desert to slow down propagation ?

I like the mod. It makes the game evolve that bit more natural-like.
I don't think you need to integrate player interaction. That's something for treefarms or bio-industries or whatever.
Make the propagation a little more advanced like nyghtbrynger suggests. Make the growth naturaly bigger in green biomes than desert biomes. That brings this mod right on the missing-mod-spot.

I currently do not check, but I am definitely working that direction. :)
(Updates have now been posted)

Question is, will this one work in multiplayer? the other mod that does this, doesn't work very well, causing desynchs.

I haven't had a chance to test multiplayer, as none of my friends own Factorio.
I will have a GitHub repository for this up soon. So feel free to post any problems you have here until that is up.
EDIT: GitHub repository is now up