Is it true that ...

YOU CAN GET THE HEIGHT OF A LEANING TREE BY JUST LEANING THE SUUNTO?

No. It would be nice of it were this simple, but it is not.

Leaning the clinometer by the same amount as the tree does increase the length over simply ignoring the lean, and that is at least the right trend, but it does not give the right answer.

This idea may be a hold-over from the use of the Biltmore stick, which had a "similar triangle approach" to measuring tree height as one of the additional scales along the side, (sometimes called a "hypsometer"). In this case, inclining the stick did correct for the lean of the tree, but the Suunto works on a slightly different principle. Sorry.

We think that the simplest approach is to measure the lean and correct the length that way.