In Dr Who Season 08 episode 10, "In the Forest of the Night", are the
glowing things trees or are they aliens possessing the trees?
Basically, what are they?
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They claim to be the "lifeforce of the trees". From Tardis Data Core wiki:
The lights become still and use Maebh to communicate; they tell the Doctor that they are the lifeforce of the trees, which have been and will always be there, and that a powerful solar flare is coming.
So we also learn that they will always be on Earth and that they can predict solar flares.
We know that they can (must?) communicate to humans through other humans, or at least through one of Clara's students, Maebh (Wikipedia).
Beyond that, we do not know.
Let's hear it in their own words (from the transcript of the relevant episode):
HERE: We are Here. Here, always, since the beginning and until the end.
DOCTOR: Here? That's it?
HERE: We are the green shoots that grow between the cracks, the grass that grows over the mass graves. After your wars are over, we will still be Here. We are the life that prevails.
DOCTOR: Why now? Why are you here now?
HERE: We hear the call and we come, as we came before to the great North Forest, where we lie still in a great circle. As we came to the vast Southern Forest.
DOCTOR: Who is calling you now?
HERE: The sun that creates. The sun that destroys. You are hurting us. Let us go.