Trees are complex overlapping shapes. Your drawings will have a greater sense of depth if you incorporate several layers of overlapping branches. The secret is drawing front to back. Start with the parts of the tree that are closest to you, then move progressively back. With each successive layer, draw more lightly and add less detail. Lets use this technique to draw the trunk of an oak. Click on the first drawing to start a step-by-step annotated tutorial on how to draw tree trunks.
Start with the part of the trunk that is the closest to you. Use greater line pressure and detail.
Now add the second layer of branches behind the first. Use lighter pencil pressure.
Add branches behind the previous ones. Note how much lighter the background branches are compared to those in the foreground.
A frame sets the drawing off nicely and emphasizes the interesting negative spaces between the branches.