How do terrain-spread-decay ticks work?

[shubla]

I have planted trees.
I plowed some tiles nearby, and then I saw some wald-terrain spreading from the trees.
What causes this? Does the wald spread faster if I plow near my trees often?

My theory is that tiles accumulate chance to turn into wald over time, and periodically a decay tick comes and checks if the tile should turn into wald.
And by plowing, we can force the decay ticks to execute nearby. Thus it might not matter if I plow near my trees or not, as the chance to turn into wald tiles is basically the same over time, even if decay ticks happen less often.

I wonder if some other people have better idea of how they work? (and should I plow frequently near my trees to make wald spread faster)

[Sevenless]

Iirc:

Decay ticks hit the ground randomly and it does a check to see what the tile should be. If no trees > grass, if trees it copies the nearby forest biome or picks a generic one which I forget. However if the tile update happens while you're in view, the terrain doesn't pop until you force a graphical update by plowing a tile.

[NeoBasilisk]

unless it's changed I believe that every time a tree grows to a new stage it will convert a nearby non-forest tile to forest
if the tree is growing on a forest tile then it will spread that
if it is not a forest tile then it will spread wald

this info may be out of date though since I did not play most of W10