Wow, really wonderful pictures and even if "simple" for you, great technique! I'm really terrible with pictures of my trees, but I promise I will study in this new year!!!
What Is the distance between the camera and the tree more or less?
it depends on the lens, in his photo, hes using a telephoto which requires you to stand back atleast 5 feet. Also it depends on your camera body, some dslrs have crop sensors which add to the distance so if you have a 35mm lens on a nikon dx camera (which has a 1.5x crop)then your true focal length(mm): 35x1.5=52.5mml; a telephoto lens thats 70-300mm, is actually 105-450mm
if you have a fullframe camera whatever lens is on there is your focal length.
the lower the focal length the closer you need to be towards your subject.
I usually stand around 4-5 feet from the subject with my telephoto lens set at 105-110mm, with my 52.5mm I stand closer around 3 feet away. I also don't have massive trees so I can get in a little closer.
As for aperture setting Joe Noga likes to shoot at f8, however he has a $60,000 camera and 3 softbox lights that act as an external flash. I like to shoot around f9-f11