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The way I feel this could and should be done is along the lines of alien nest expansion except with alien nests, they have a 7 chunk radius in which if there is a nest or building nearby it is less likely to expand in that chunk, and with the trees it would have a 4 block radius and it has a chance to drop a seed (entity with 1 hp) that acts the same way a alien that is spawning a nest. Seeds are collectable and can be placed in any grassland biome.

it randomly creates an item that must be picked up and planted? not all that realistic, and the forest won't expand on it's own.

You could just have new trees pop up next to existing ones every now and then... and have seeds be created when say, chopping down existing ones so you can plant them elsewhere.

Unfortunately, trees that "Grow" over time just add to the number of things that the game has to constantly do, which is probably the main reason why it hasn't been done already.

It is done in the Natural Evolution Tree Expansion Mod.

* Natural tree expansion mod: https://mods.factorio.com/mods/aster26/ ... eExpansion
* Hardy fire-resistant bush mod: https://mods.factorio.com/mods/ToadRoach/AussyBush