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element!
By the time the Silmaril was returned to Valinor, the time for healing the Trees was apparently long past. Yavanna said at the time of their death:
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The Light of the Trees I brought into being, and within Eä I can never do so again. Yet had I but a little of that light I could recall life to the Trees, ere their roots decay; and then our hurt should be healed, and the malice of Melkor be confounded.
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Silm Of the Flight of the Noldor
Surely the Trees were well into decay by the time of Eärendil. Though natural death was arrested by the power of the Blessed Realm, perhaps the poison of Ungoliant caused the roots to rot quickly, taking away any chance of reviving the Trees after much time had passed.