Well, understanding that I wouldn't consider myself an expert, I first
"got" tree riding in the middle of last season.

I think the big part is being able to make slow skidded turns on slopes
as steep as you need.

Next is being able to control how much you side slip down the hill
compared to how much you move forward in the direction the board is
learning. If you can learn to control these seperately, then you can
angle your board however you need to get through the gap, while still
just sideslipping downward at an angle so you don't pick up warp speed.

And yeah, because you tend to automaticly go wherenver you look, look
at the gaps, not the trunks.

Ohh, and the trees are NOT your friends. Keep your hands in fists,
don't reach out to tap them or anything as you go flying by... I had to
learn that the hard way... frozen trees are hard as rock.