Here are some things to consider when working with values in a landscape. Sometimes when observing the play of light it can be hard to decipher subtle value differences. These are  not hard and fast rules, but hints that may be helpful in creating the illusion of light and shadow. First, I'll start with a simple illustration:

1. The sky is the source of light, so it is usually the lightest area of the painting.
2. The flat plane of the ground is usually the 2^nd lightest value in the landscape, because it is the earth element most exposed to the light source (except when in cast shadows.)
3. Slanting planes (like the slopes of mountains) are typically the 3^rd lightest value. They still get a lot of light, but they are more upright than the flat earth planes.  Mountain values can have undulating cast shadows as well. They are still fairly light in value however. They are also cool in color temperature, due to normally viewing them from greater distances.
4. Cast shadows on a flat earth plane will be the 4^th lightest value. Shadows generally appear cooler and lower in value than lighted areas, but be wary of painting shadows too darkly (a common problem when painting from photographs.) Cast shadows in nature will still have light and color in them because in most cases they are reflecting the light from the sky.  Also note: Overcast days when the light is generally cooler and more diffused tend to have shadows that appear warmer and more diffused. Sunny days when the light is warmer tend to have shadows that appear cooler.
5. Upright structures, such as tree trunks and certain trees like cypresses will generally show some of the darkest values because there is less surface area for the light to hit. Be aware though that most trees will still have some slanting planes in the foliage on the side where the sun shines. There may also be darker values than the upright ones occurring on the undersides of your trees and bushes. 
6. Values can get tricky once you start working with color, so it can sometimes help to squint your eyes in order to check your values. Making quick thumbnail value drawings is extremely helpful as well, as a preliminary to your painting. These “hints� are especially helpful when working from photographs. Note that there are always exceptions (like rising and setting suns), so use your observing eye! Nature often has her own ideas. When working from Nature, be open to exceptions, but also be prepared to use your “hints� if you find it helps you to make a more convincing painting!  

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