Often, when we are sketching en plein air, we have very limited amount of time to put down on canvas what we want to take home. There's never enough time to faithfully copy all the intricate shapes that we see before us. (Who wants to copy anyway?)
So we have to simplify, and still manage to capture the character of whatever it is we're painting. In school, the instructors talk about visualizing a complex object as made up of fewer, simple geometric solids. For many a student, this is a puzzling concept. Sometimes the trees become oversimplified and end up looking like lollipops - you know what I'm talking about.
I think that in order to do this well, first you have to have an understanding of how light, shadow and form are related. You should be fairly adept at painting the simple solids like spheres and cones out of your head. Otherwise, translating a complex tree form into a simpler one without looking simplistic, is difficult, if not impossible.
Once you have command of painting spheres out of your head, practice painting full, densely foliaged trees. The denser the foliage, the the easier it is to see form.
Imagine the entire tree as one form, and noting where the light source is, indicate the lit side and the shadow side. It's like painting a sphere in the color of the foliage. Make sure the color of the light side is warmer and lighter than the shadow side.
If you are mixing your lit side color by lightening the value of the shadow side, be careful; if you just mix white into your shadow color, it'll get lighter, yes, but it won't be warmer, so you won't have a "sunlit" tree; the light will be too cool, and will appear "chalky". Make sure to reach for your yellows first, then if you need to lighten up further, add tiny amounts of white at a time.
Now comes the good part. Just as you imagined the entire tree as a simple solid, you want to imagine smaller masses within the tree in the same way. imagine a clump of foliage as a lopsided sphere (chances are, foliage masses won't look like perfect spheres) and indicate light and shadow on it.
Repeat the process on another clump of foliage, and keep going until you have sufficiently broken up the tree into a more natural, complex looking mass than when you started.
You don't want to neglect the overall silhouette of the tree, for that has a lot of impact on the character of the tree, but that's another day's post.
With practice, and using what you actually see as reference only, you'll be able to suggest light and shadow on tree masses pretty convincingly pretty quickly, freeing up your time for other important things like... the rest of the painting.
In the examples I'm showing on this post, I hope you can see the lumps of foliage as I picked them out and lit them. I hope you can see that it's just a matter of identifying the lumps, and simply identifying the lit side and shadow side of each lump.
Just where are these lumps on a tree? You can use the tree in front of you as reference, you can use your imagination, or you can use a combination of both.
If you get adept at using your imagination to identify these lumps, you'll find that you can paint trees out of your head - which is obviously a very handy skill.
I have to warn you, that in order to go beyond the generic, you really have to study and be familiar with specific species and their characteristics. The deeper your knowledge about the subject, the better you'll represent it.
The same exact technique can be used to paint clouds. Not the thin, wispy ones with no discernible shadows, but the dense thunderheads with clear light and shadow patterns.
So as you can see, this technique not only helps to quickly indicate light and shadow patterns on trees and clouds, but it really is a fundamental skill in painting stuff out of your head. (Or if you want to sound more impressive, paint from memory)
But in order to design clouds and trees to suit your composition (because again, copying will only get you so far) , you have to be able to re-shape them and still light them convincingly. Knowing how to reduce complex forms into simpler solids will make this possible.
Homework for ya; Try painting a few eggs out of your head. Then try painting them green. Then try making the shape of the egg a little irregular, with broken and soft edges. Try putting that in a landscape. Let me know how it goes~
If you want me to show you, why not come join me in Michigan for a three day workshop in September? We'll be painting on the beautiful grounds of the Franciscan Life Process Center in Lowell, MI. I'll be talking about this, and everything else I can think of about the art of landscape painting!
You can sign up by going to the Center's workshop page.