Most tree trunks simply use regular brick such as slopes, and even the ones that are more inventive lack the detail of natural trees with their random markings and decals.
You can therefore turn to pieces that are moulded so they have those details incorporated into their design, and with enough other parts you can flesh the ideas out into proper trees.
Take a look at these designs by Marion on Flickr, who uses some fairly unique pieces such as brown legs, stud shooters and even the log minifigure accessory! Leaves could easily be added on to certain points, giving you a solid tree deign you can use in your MOC.
Check out Marion's post on Flickr here: https://www.flickr.com/photos/35385165@N05/29446623048/in/pool-839039@N23