The trees are difficult to paint particularly if the trees are in the foreground. It is very difficult for you to decide whether to draw many details or not. The trees in the distance are easy to draw with little color change and clear contour. Whether you are painting the winter bare trunks or summer lush canopy, a certain degree of simplify is needed. Because of the distance, the trees are for you to make the simplification. Trees nearby are dispersed into small parts with independent-shape leaves, looking very complicated. Therefore, at the beginning of observation, half close your eyes to make the fuzzy line of sight and to observe its overall shape. This is a highly recommended way and will put your attention details out, which helps you build a large shape and color.
The first is to cover the screen with relatively thin paint brush which is not too stiff. It will dry more quickly. The whole oil painting can be acted as a template and you can add some branches and mass and remain the small details to the end and try to maintain the consistency of painting brush strokes as far as possible. Even if you are close to the object and can clearly see their details, don't even try to portray every leaf. It is enough to pick out crown edge branches to depict.
Attention should be paid to depict the light in the canopy and each cluster on the branches, so that the whole tree seems to have a sense of volume, but it is difficult for beginners to do. Another common mistake is to draw too deep edge line like the clip in the connection of the tops of trees and the sky, and then the leaves in the top of branches are thinner than those in the lower part. Light can penetrate them easily, so color in the canopy edge should be relatively shallow and should not appear in the thick edge line.