Unusually high tree stunting rate

[Alayah]

Arguably if there is a critical failure condition, which you cannot really control by your actions - such as stunting - there should also be a chance for critical success. Similar to how wroughtmaking works. Basically if there is risk, there should also be some form of reward. Even withouth stunting, there is already some kind of risk involved when treeplanting. Apart from that trees might not sprout if you are not skilled enough, you invest materials and time. The risk is in a decrease in quality of your endresult and is balanced by potential gains.

[Omnipotent]

A 5% decrease in quality will make them useless (at some point in the world development).
Speed reduction would be enough, in case it's (say) a reduction of 20% of current speed (factor of .8) it could even be applied repeatedly...

A q100 tree getting stunted to q95 sounds perfectly acceptable. 5 quality isn't going to make a big difference at that point, and is certainly not 'useless' as you claim. You would still be able to get seeds and logs from it once it has grown. That's far better than having a tree/bush stunt at <30% and only getting blocks/branches. When you compare it to a tree being permanently stuck at a certain level (which actually makes no sense), this is a huge advantage.

5% will only be a big difference way further down the line, and at that point the numbers aren't weighted as heavily. Plus planting another tree is far more trivial when you have ample resources and parent trees.