Chandrasekhar said:
I'd say give it to them, but make their forest improvements slighty more poopish. As I've mentioned in my tree huggery thread, I really don't see the elves as being almost as much of an economic powerhouse as gilded dwarves. They really ought to get something else from forests...
well your tree-bashery is not the answer.
the simple facts have already shown they are not economic powerhouses (they don't receive a single

more than a vanilla civ, and they don't have a financial leader; and if anything, given their penchant for woods, they lose out on any river

).
something that would help bring others up to par (most civs still need work, the only area elves really excel in is their amount of unique units) would be giving city defender bonuses when a unit is on cottages/hamlets/villages/towns so that others can similarly defend their important holdings (though it shouldnt apply to elven villages since their villages and such already get a forest defense bonus). archers then on a town would get use of their +50% city bonus and any city defender promotions. i mentioned this in the fort thread.
about using treants as siege engines, im sure they could, but worrying about it as a balance issue is crazy, as its a high end tech and you could just as easily get meteors for the same effect (and isnt the treant summoning spell specific to leaves? removing their ability to get siege in any form at any time is silly, itd be like not allowing them to get Pact of Nilhorn or mages when the lack of siege engines in reliable massproduced ways is the true crippling).
though, if they were to get collateral damage they should have their strength reduced to around 6 id think, a 10 strength summon with collateral damage and bombardment would be a bit much for any level spell.