Most landscape art is going to have trees either as a feature or incorporated somewhere within the compostion. One of the good points about trees is that there is little limit to their shape and so there is no formal idea of what a tree should look like. For instance Poplar trees are tall and thin, Oaks tall and broad and many other trees are everything in between.
The figure on the left is a sketch of a generic tree and I will discuss each point indicated by the arrows to aid you in understanding certain properties trees possess with which we must be aquianted and familiar.
1: The Tree Trunk As a general rule trees are not assymetrical, but we might be inclined to draw a perfectly tubular trunk from which to led into our branches, however this can look naive and unobservant. Study your tree and ensure you capture bulges, hollows and dissymmetry to produce a more pleasing shape.
Note too how light is dappled on the trunk due to the canopy and should not be shaded like a cylinder.
2: The Canopy Also like the trunk the foliage of the tree canopy is not assymetrical as we are not drawing a lollipop on a stick. This would suggest a solid mass of leaves through which we cannot see, but, depending on the type of tree, we should be able to see some sky or some other coloured form through parts of the foliage. In heavy foliage these areas are often painted last as little dabs of sky over the top to give an illusion of gaps.
Remember that light will play a great part in giving your canopy a sense of volume to its irregular and broken shape and gaps will give it a more natural look.
3: Sun and Shade On a sunny day the darkest parts of the tree are going to be on the underside away from the light direction. Ensure that you set the tonal value of these areas to give your tree its feeling of weight. Use a little reflected light in a mid-tonal range on areas such as the shadowed edge of the trunk to add volume.
4: Dead Branches Trees don't have to look perfectly healthy and a study of Gainsborough paintings will show details of dead or dying branches that many landscape artists today tend to omit. Such things as dead branches or twisted and gnarled forms can add drama to your scene.
5: Shadows A tree's shadow will also be dappled and can prove very useful for compositional elements such as lead ins, light direction indicators and breaks across paths, roads etc.
6: Subtle Details Don't forget to look for things like exposed roots, dead foliage and leaf litter.
Tip: Do not try to draw or paint every leaf on a tree, not only is it impossible, it leads to a poor result. Instead we must simplify matters and create the feeling or impression of leafs through pattern and through simplified brush strokes that lets the viewers eye create the detail.