Sometimes your photos of aspens will not have much contrast, so you have to add it by adding some dark shapes behind the aspens. The background of this watercolor didn’t have hardly any darks in the photos I worked from, so I had to add them. Weave the darks through in an interesting and varied pattern. Another idea is to make the background a solid dark with variety in the darks. Even spatter or salt dropped into a wet background will add interest. Another idea is to suggest pine tree trunks and some reds or red-oranges (I like Burnt Sienna) for variety in the dark greens.