Sorry for the delay, I was shooting a movie this weekend, and then got sick...
Thank you for your questions!  I had to think for a bit on how to answer some of them.  Here goes...
Griffin wrote:
When do they unattach themselves from their tree, and when they do, how do they get nutrients?  You said they are bipedal, does that mean they have hands, or do they only have one pair of limbs?
They can't detach until they reach full maturity, which takes about twenty years.  At that time, they have a reservoir of nutrients that can last them for nearly a month when detached from their host tree.
Once they're old enough, wild Bark Riders will only detach if their tree is threatened, or when they want to find a mate.  The tamed Riders (see below) will detach whenever they're summoned by a Delantrim (a resident of Delantare).
Re: Bipedal, They have four limbs, but only the rear two legs are used for walking/running.  The two forelimbs are mostly used for balance and gripping onto their host tree.  They also have a long, thick tail for balance and steering when running.  Their posture is like that of a raptor-type dinosaur.
6stringedsignseeker wrote:
I'm guessing they have a color equal to that of the tree, since the tree bark becomes their skin. I would guess the hard bark like skin would be a natural defense as well. (...) Does the bark grow on their undersides as well, or is the skin weaker there?
 Correct, their skin color matches that of the host tree.  Their undersides are not covered by bark, which makes them vulnerable there when they're detached.  In battle, the Delantrim make up for this by making special armor for their Bark Riders.
Sarah Sawyer wrote:
Interesting. So how long can they be unattached from trees? And how are they ridden? I'm having a tough time imagining how one might ride an upright creature with the body of a salamander. 

 But really, this is a neat idea! Certainly these would make great battle mounts, with their tough skin.
As I mentioned before, they can be detached for nearly a month, but they seldom do.  Their backs lay almost completely horizontal, and the Delantrim make special saddles that allow for an easier ride.
I have to go now, but I'll return and answer the rest of the questions soon.