Edit: this is wrong, see comments below.

The correct answer is water though, not air. Air only accounts for the Carbon content of the tree, which is a big part, but still a minority.

First, a live tree contains a lot of water (30-50%), that's why you need to let wood dry a lot before burning it.

And there's something else:

> Next we might look at water, but water is only made of hydrogen and oxygen, and the oxygen is released.

Not all oxygen is released. All oxygen contained in a plants molecule (mostly cellulose and hemicellulose, which are polymers of glucose, and lignin) comes from water. And there's a lot of it: there's as much Oxygen as Carbon in a glucose molecule.