It is well known that Dryads live as long as their trees do - that's probably one of the reasons that they aren't too fond of woodcutters or fire in their Woods. There is also plenty of other evidence - Queen Xantha's tree is nearly dying by KotM, and so is she, plenty of references to this 'bond' between the Dryads and their respective trees (Ce'Nedra's cousin Xbel for instance).
Or is it? When Gorim went up to confront UL (*by the way, I found a place where they wrote Ul instead :) *) he went all the way up to Prolgu (or what would become Prolgu). From whatever references to Prolgu I can find, it's not exactly very hospitable to vegetation (it's high up on a mountain, and probably above the treeline (is that what it's called?)). When Garion and co went up there during MG, it was freezing cold with snow billowing around them. Buildings were made of stone, rather than wood. The gatherers of food for the caves had to go down to valleys to hunt/cultivate/grow/whatever they do. And for quite some time, the 'monsters' stayed in peace with the Ulgos in Prolgu.
Belgarath also says that the Dryads have lost much of their UL-given religion and had reverted to certain sacred groves and the like instead. I would take this to mean that originally, when UL consented to be their God, they sort of worshipped him. (well, the more intelligent 'monsters' did, anyway). And when the earth was wounded and maddened, so were they.
Now, it was said that the Dryads were closest to mankind compared to the other 'monsters'. I would surmise that they were also very close to nature before UL adopted them (hey, not having a God, you either turn to the heavens or the earth, and I don't think that Dryads have the mental concentration necessary for contemplation of the stars :)). From now on, this is pure speculation...
What if, at the cracking of the world, the Dryads also went mad, but not as mad as the other 'monsters'? Maybe their madness simply drove them away from the fellowship of UL and away from Ulgoland? Again; this is speculation, because I have no direct evidence that the Dryads /were/ at Ulgoland in the first place - monsters range everywhere, after all. But anyway, they eventually end up in the Borune Woods. This travel probably took some long time, and they'd probably became less mad as time went by. Having entered the Woods, they became more and more peaceful - their previous closeness to nature coming back to the fore.
From that is but a small step to one Dryad deciding to 'bond' with a tree (you all know what I mean, don't think dirty :)). Wouldn't be long before the other Dryads found out that one was living quite a bit longer than they were (lets just postulate that being close to mankind they had comparable lifespans). In time, more and more would have decided it was a good idea to bond, and before long, it's become institutionalised. As soon as a Dryad was born (or maybe reached a certain age, I don't know), the other Dryads gather round and decide to find a nice seedling/young tree to bond the Dryad to, and do it, until said Dryad was old enough to bond herself, voluntarily. (Sort of like infant baptism and then confirmation, etc).
Having that tentative hypothesis, which is completely dependent on the above speculation, I can now postulate that Ce'Nedra has never 'confirmed' her bonding with the tree. It's not automatic, and is externally imposed for the Dryad's 'formative' years. I mean, she's lived with her father in Tol Honeth for quite some time, she's gets out at 16 and goes with Garion and co. From that time on, her every moment is accounted for - more or less, anyway. She's has two explicit visits to the Woods, and no direct evidence of any other visit. Safe to assume that Ce'Nedra can still pick another tree - and no prizes for guessing which other tree she'd pick.
by Magus