What is "sheltered site" (as opposed to "exposed site")?
Depends on the context. Examples can be: In depressions instead of lonely on the plain, inside of the forest instead of at the edge, along a river bank or lakeside instead of on dry ground.

Or, in short: exposed to wind &amp; weather instead of sheltered from the rigour.

Also depends on the tree. Some pioneer trees can grow individually in adverse conditions, others like it in groups and warm and wet.