The shadow on a tree trunk will be darkest next to where the light strikes it.

Tree trunks are often a warmer color nearer the ground due to reflected light.

When painting groups of trees, vary tone, shape, size, direction, and texture to keep the painting interesting.

Painting foliage in clumps, light in front and dark behind, will give the tree depth. Keep the edges fairly rough and varied (hard and soft). The limbs can be painted dark against the light foliage and light against the dark to enhance the feeling of depth.