Well, I did some searching for specific tree species paired with "dragon" and found these on deviantart. The second isn't particularly well-executed, but the idea is sound:

http://ironshod.deviantart.com/art/Oak-dragon-33316896

http://rowanwatersprite.deviantart.com/art/Colourful-Dragon-158513905

http://baardk.deviantart.com/art/Little-leaf-dragon-34002135

and then I found these in the Elfwood:

http://www.elfwood.com/~mikewaters/Maple-Leaf-Dragon.3044449.html

http://www.elfwood.com/~mikewaters/Royal-Paulownia-Leaf-Dragon.3044485.html

http://www.elfwood.com/~trefly/Leaf-Dragon.3542367.html

this one might be a bit on the menacing side, but I liked it:

http://www.epilogue.net/cgi/database/art/view.pl?id=87702

There's also this, which I found interesting - an artist might be able to stylize it to bring out the dragony aspects of the tree:

http://artofbonsai.org/galleries/images/halloween_2007/small/halloween_2007_Jupp_grape_elm_dragon.jpg

Maybe some helpful ideas in there, if nothing else. :)