I'm big into bonsai trees but like Rich said a tree would die quickly under water. I've done like mentioned above where you use java moss on a piece of wood to make it look like a bonsai. I found using the tap root and flipping it over so the actual tap roots became the upper limbs of the tree gave a very real bonsai tree effect. I did one in one of my 29g tanks a number of years ago like that were it looked like a bonsai tree and then a path into a mountain and I used the tap root I mentioned. I'll see if I can find a pic of it to show ya, I think I have some on my XHD just have to plug it in and look.