Make sure you can see the entire top of the tree in your field of view. You need to be, approximately, three times the height of the tree away from it, or more. A tree's aura can be huge, depending upon the size of the tree and how strong it is. At the top of the tree's aura you can sometimes see a fountaining effect, as if it's aura were slowly spraying up out of it. I am not sure what causes this, and I have not seen this with every tree I have observed.
Some people say it is the tree's spirit, or a nature spirit living inside the tree, that causes it.
Some trees do seem to have a kind of consciousness within them, so this may be true. Try and observe them when they are resting. Animal auras are not as colourful as human auras, but they are still very good practise. Keep in mind, that the colour of the animals coat will affect the colour of it's aura. Disease can be detected in the aura of an animal, the same as with a human subject.