Leto into a tree???

***SPOILERS***

Leto II does not become a tree, he is becoming a symbiote; a half human, half sandworm entity. The "bark" like substance you see on his skin is the sandtrout (baby sandworms) he allowed to connect and bond to him in Jacurutu; they give him great physical power and near immortality, but in turn they make him no longer fully human, and he can't turn back, they're a part of him now and are changing him into a new life-form.

Here is a picture of what he will eventually become, 3,500 years after Children of Dune;

http://www.duneinfo.com/michael/images/leto2nd.jpg

He allows this thing to happen to him because he foresees that it will put the universe on "The Golden Path", which is essentially a delicate plan to ensure the survival of the human race (which both Paul and Leto II foresee a need for; the human race will eventually be extinguished without it, for more read the novels from Dune through to Chapterhouse Dune).

Because he was transforming and had those powers, read the book and you'll find out.