Trees are like the human figure, they have a particular anatomy to them. (Read John Carlson's Guide to Landscape Painting chapter 9.) The trunk is the skeleton of the tree, it sets up the motion and movement. The trunk should always gradually taper so the wider the trunk, the taller the tree. Also, make the trunk and branches more angular when you paint them, don't round them off, that tends to make the trunks and branches look like wet noodles. Angular brushstrokes look stronger, like they can hold up a lot of weight. Like most objects in painting, the trees have to look solid and 3-dimensional and are generally darker because they are an upright plane. Making trees too light gives them a feeling of floating, not heavy.