Sorry, that would take all the charm out of the animated forest.

That makes no sense.

What does make sense is that the animated trees' natural habitat would be somewhere they could blend into the environment (forests)... and I really don't see how running into an occasional lone animated tree in the wilderness would take anything out of the animated forest any more than (for example) running into the occasional minotaur takes anything out of the minotaur maze now.

If you find animated trees prior to visiting the animated forest for the first time then you won't be as surprised as when you don't. Don't you remember the first time you discovered the animated forest.

I do. But that argument could be applied to a number of other things in ADOM as well, yet running into undead did not spoil the surprise when I discovered the CoC graveyard, running into blue dragons did not spoil the surprise when I discovered the blue dragon caves, running into assassins didn't spoil the assassin's guild... the list goes on. I don't believe running into animated trees in the forests where I'd expect them would have subtracted from the surprise of discovering them far underground where I wouldn't expect them.

If it helps, the level limit for the encounter could be somewhat higher, perhaps 20 or 25. That would make a new player managing to get to that level without having already discovered the animated forest very rare.

Maybe just make a condition. If AF_Visited=1 -> Make a chance to spawn random animated trees in the forested areas of wilderness map.
I think that PC's visit might be a trigger for the trees' curiosity to explore the outside world and some of them leave the dungeon to roam wilderness that naturally seems like their primal habitat.

Another idea would be to create a new wilderness location, perhaps an Animated Copse?
With conditions similar to quickling tree - you need alertness, woodcraft and survival at 80+ to discover it. Some stuff to kill, some items, some respawning animated trees a'la the natural respawn in the actual animated forest in CoC...
Perhaps a separate RFE is in order, though I doubt anything like this would be implemented. I'm just wondering about any potential improvements over this element of the game.

There's a thread on the main forum for community quests that TB will be browsing for ideas. That would probably be a sensible place to submit this.

A plant-based quest might be nice actually, given that there are plant slaying weapons and a dearth of monsters to use them on.

This is genius.

Here's an alternate idea, that would require even less coding...

You can only encounter an Animated Tree in the wilderness if you have killed an Animated Tree (in the Animated Forest or elsewhere - I'd like to see a small chance of an Animated Tree generating on river levels with Danger Levels above some number). Reasoning being that Animated Trees on the surface are generally docile (due to the effect of sunlight and regular rain), but they are angered by those who would kill their brethren. So the only thing to do is to check whether Animated Tree is on the list of killed monsters. It also ensures that the PC can't encounter them in the wilderness without first encountering them in a dungeon.

I'm storing this as an idea resource during the implementation of the missing encounters / levels from crowdfunding :-)