Barking at Trees
This is not necessary but I do it on all of my trees in case I want to use them for something else later on. It does not take much longer to do.
The idea is to roughen up the shaped piece of dowel with small lengthwise grooves to simulate bark on the bottom of the tree. Unless the tree is going to be barren, you only have to detail the bottom half of the trunk.
Using the threads of an old bolt and rubbing it hard up and down the trunk will give a nice bark effect. So will using a wood rasp.
If you hold your wood rasp at a slight angle and rub it up and down the trunk. It will give a nice bark look.
Like I said the basic idea is to roughen it up. And really you only need to do the first couple inches.
Detailing the Bark
A drill also works wonders for detailing the trunks after. Run a wire wheel up and down the dowel lengthwise.
If you want to speed things up... Kind of dangerous though as you loose finger skin every now and then. Lock your drill in a vice with one of these wire wheels on. Power it up and run the dowel on it length wise. This gives a very realistic effect. But as mentioned... it hurts when you make a mistake.
If you are making a rather large tree. You can use a small piece of real bark here and there, glued in to place.