When drawing trees, the first consideration should be to the outline shape. Be aware of the spaces in the foliage. Attention should then be given to the arrangment of lights and darks. Look for negative shapes, they are just as important as the positive ones. The shadow on a tree trunk will be darkest next to where the light strikes it.
Tree trunks are often a warmer colour 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, dark behind, will give the tree depth. The limbs can be painted dark against the light foliage and light against the dark to enhance the feeling of depth.