What is "sheltered site" (as opposed to "exposed site")? What is "sheltered site" (as opposed to "exposed site")?

It's used in context of location of trees in bumpy ground areas and weather damage.
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.