The Root of Light by Anérea  

A few extra words on this idea ...

This is the general gist I wrote up initiall  in other, unTolkienesque, words, before I managed to convince my brain's creative-writing hamster to run for me that morning:

The two Trees synthesise photons, while the plants of Middle-earth need photons to grow. Light from Varda's stars may be sufficient for them to maintain a hibernating state under the sleep Yavanna has put on them, but certainly not enough to grow. Fully aware of the cycles of balance in the nature she has created in her living things, of death and decay nourishing life, she's certain that the Children, when they come, will need thriving plants in order to thrive themselves.

The Trees don't only have light in their leaves, flowers, and fruit, it runs through their roots too. Perhaps it's inherrent or maybe it's because they reabsorb the glowing dew that falls? Either way, light runs through their roots. And their roots burrow deep into Aulë's rock, and deeper down into Ulmo's caverns. (*I was delighted to later discover that there is a similar description of Yavanna herself: "Some there are who have seen her standing like a tree under heaven, crowned with the Sun; [...] but the roots of the tree were in the waters of Ulmo, and the winds of Manwë spoke in its leaves." so I gratefully added it, albeit for the Trees rather than Yavanna.) And Ulmo's waters run throughout the world. So she seeks out Ulmo and asks for his assistance in transporting the light from the Trees, under the sundering sea to the dark lands, that those plants may absorb photons that way, and still maintain life and grow — slower than they had under the lamps, and much slower than they would under the Sun, but enough to sustain the life of the kelvar (Quenya for animals, living things that move) and, when they arrive, the Children of Illuvatar.

I envision the light being soaked up through a network of mycelium that spreads underground throughout the land, thus feeding even those plants far from streams and underground sources of water.

This flow of light ceases, of course, after Ungolient saps the Trees of life, but for a while there is still a little light left in Ulmo's waters and the mycelia, just enough, as it turned out, to sustain life until Arien was launched into the sky. And indeed, some mycelia retain the magic of transporting light, even today, which is why we have bioluminescent fungi!