Citing checkpoints would be inconsistent given the wild design of the course, so simply navigate through the track until you reach the point where the two different paths rejoin on solid ground. Before you drive onto solid ground, you'll find yourself on top of some wooden planks resting on some tires. Sit on the first plank and slowly drive forward so that it moves away, then jump to the second plank and slowly fall into the gap that you've created. You'll fall a good distance downward, hopefully landing on a small landing ramp at the bottom. You'll immediately be facing another wall; drive up to it and it should fall out of the way, opening the path into the tree room.
Note that the tree will viciously attack you the moment that it sees you trying to jump over that little fence.
InvestigationEditIt has been suggested (and the images seem identical) that the design here is that of Charles Darwin's 'Tree of Life'. This image was taken from Charles Darwin's notebooks that were writen around July 1837 showing his first sketch of an evolutionary tree.
Details about the 'Tree of Life' can be found here.
This section is reserved for Confirmation/comments about this investigation. Investigations will remain open until advised by Redlynx that the final solution for each secret is found.