There are two reasons you might be asking this, jackwryan.

One, you want shadows cast by trees etc on your scene, even when the object is positioned outside the camera-view of your scene. In which case you might use a gobo light that has a pattern on it, and thus casts things like tree shadows. Wikipedia: "A gobo is a stencil or template placed inside or in front of a light source to control the shape of the emitted light." Various sets of these are available, or you can roll your own. You could also do it in post-work, though that might be a bit tricky if the shadow traverses a complex scene.

Second, you might be asking because you want to be sure that stuff outside your scene is not slowing down your render times. In which case just hide/delete the items and do some test renders. Utilities such as SceneToy can give you a handy scene-list, which enables you to mass delete stuff quickly and easily.

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