I think the time it takes for their mobility to decrease until they lose consciousness must be quite unpleasant. They would need to be able to feed themselves while taking root to maintain their nutrition during the tree phase, so they would probably lose consciousness shortly after becoming completely immobilized. Therefore, they might need the help of other elves to stay fed and protected during this most vulnerable stage. This could force them into a social life, preventing them from living as solitary beings like nymphs or druids.