Someone suggested, in another thread, that you can help Mourns-for-Trees by taking him the Decanter of Endless Water before freeing Ignus. Well, I tried this and didn't get a dialogue option to do so (yes, I had already learned the activation word). Can you indeed somehow help him with his trees, and if so, how?

Tell your party members to want the trees to do well. If you have enough party members who do that, then Mourns-For-Trees will report the trees are doing better the next time you ask about them.
This can't be accomplished early on in the game because you don't have enough party members.

What do you need for this? WIS, INT, CHA? I played Fighter so I didnt make it, but than i raised everithing, but it wes too late, so i'm thinking about taking tatoos (on 2nd run)

First *you* have to agree with Mourns that the fact trees are dying is a sad thing. Then you can say to him that you'll help him and earn XP500.

Now you can ask Morte to promise the same thing (no stats important).
Dak'kon will also do it without questioning your stats.

But Annah won't help him if your CHR is low. Use Friends spell if your CHR needs boost.

That should finish this "quest".

A side notes:
You can't ask from Ignus to help Mourns, but you can ask it from Nordom. Be aware that if you haven't patched the game, Nordom's dialog with Mourns will turn into unbreakable loop.

I wont question your knowlege, i played that a long time ago and i remember i saved and reloaded few times and didnt make it. But yes, i tried it now with more wise and int. character (magelike), and made it?!? Btw. what number is the latest version?

There is only one patch, v1.1.

I think Morte is lying the first time he agrees. If you have sufficient wisdom(?), you get a dialog option that says something to the effect of "Come on, Morte, I'm serious." After that, Morte agrees for real. If your wisdom(?) is too low, you don't notice that Morte wasn't serious the first time.