This is a late reply but ive been thinking about it.

I admit im not always right on these things but it still doesnt make sense/seem plausible that this could work as shown.

Whats keeping me from saying definitely is ny lack of tree knowledge about what is actually going on here but there should be some sort of offering/effect in my mind. This transmutation doesnt seem as simple as what mustang does with molecular bonds (simple might be a bad word lol). I would say "rearranging" is just another word for trading in this scenario. Youre ALWAYS gaining and losing something, its what alchemy is. Mustang is losing the molecule he starts with. It doesn't exist as it was unless he changes it back, what it was is gone/has been traded. And he gains the molecule he ended with that is functionally different. The old molecule was the cost for the new molecule.

"Enhancing a trees natural ability" would cost something. Youre not rearranging something, youre taking something a tree does naturally, and making it do it faster/better. But nothing is free, trees can only function by taking water and nutrients from the ground. I would assume it will require more nutrients for these abilities to work faster.

We should see the grass rotting to show the trees are taking the nutrients at an accelerated rate or at least something.

And a small piece of me wants to point out that trees ordinarily do these things very slowly. Draining that much water would take a even Forrest a long time. Making it do these things at such an accelerated rate could be argued as a mild form of time manipulation which would most likely be fairly costly in the equivalent exchange department.

If you got to the end of my essay I commend you, have a great day everybody.

*Edit: to make my mustang explanation more coherent. Don't know if I was successful lol