Some trees have prominent vertical cracks in their bark. When you draw trees, you can use a simple observation to help show the dimension and roundness of the trunk.
Evenly spaced vertical lines on a column will seem to get closer together as you near the edges of the column. The same is true of cracks in a tree. On either side of the tree you will see closely spaced, narrow vertical cracks. Toward the middle of the tree, the cracks are more widely spaced. Lets apply this to a drawing of a Coastal Redwood. Click on the first image to start a step-by-step sideshow.
With only edges drawn in, the tree looks flat.
Add closely spaced cracks along the edges. Vary the size and spacing of the cracks.
Now add a few big cracks in the center and you are good to go. Note that you do not need to make a careful gradation of big to small cracks. Trees are variable the the viewer’s imagination will fill in the gaps.
Add lighter and less detailed trees in the background and you have a sunlit tree in the middle of the forest.