Many assets contain items. like trees or buildings, that will never appear in the final rendered image. Will deleting these unseen items have a noticeable effect on the rendered image? Thanks. - Jack

IF you are looking for realism ... the shadows, ambient and reflective light that comes from these items may effect your scene, if these features are unimportant. It doesn't really matter. Most sets have an option of either loading the complete scene or using just the props you want ... if you prefer that option.

As I understand it, anything that is outside the boundary of your render camera is not considered in the scope of your final render. You could delete them, but it won't impact on your render time.

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.

If you open the Hierarchy window then you have the option to either hide (click on eye) or delete an object/figure. On a Windows machine you can click and Shift+Click to select a group of objects, then right click to get the delete or Visibility menu