Well, it's awkward to discuss anything involving plant and animal survival in a world with no Sun, so perhaps it's good not to draw any attention to it. I think Galadriel was speaking of the inevitable final fading of Lothlorien. Once she finally rejects the Ring, she knows what Elrond had already accepted -- either Sauron will conquer their realms at last, or they will have to sacrifice everything they struggleed for millennia to protect in order to finally defeat him. She cannot continue to defend Lothlorien either from Sauron or from Time, and she finally admits it. But I think that Lorien had a normal spring and summer in 3018, same as every previous year at least since Galadriel arrived with Nenya. Possibly there will immediately be no spring for the mellyrn. Their life cycle, what with keeping their broad leaves all winter, is not at all appropriate for central Europe and they'll probably die of thirst pretty quickly once Galadriel can no longer do... whatever she was doing. I suspect she made the forest skip over winter, and filled it with some essence of the winterless life of Aman. (Then again, mellyrn managed to grow in Gondolin too, so... I dunno. Ulmo blessed it with mild weather? Keeping their leaves all winter was Galadriel's doing and not inherent to the species? Tolkien was not a biologist. In any case, mellyrn are from Aman and Eressea (who brought them to Gondolin, I wonder?) and like the athelas they don't fully belong in Middle-earth, or are doomed to fade away.) Arwen arrived in winter either because it was literally winter in the whole northern hemisphere (she did die late in the year, I think) or figuratively because the forest was fading and the mellyrn were dying.