Also this technique would work with photo trees but not as well. We tend to steer clear of those in plan. Also as to layer masks, you really should be only using it instead of the erase tool. It can save you a ton of time in the long run if done right.
Our office uses a single layer for trees of one type. You essentially have a blank tree shape and use the blending tools to apply texture, shadow, depth, or outline depending on your needs. Then you use the marquee tool to copy the shape around the same layer. This minimizes your file size as well as joining outline so they don’t overlap as they would if you just copied layers. Put multiple kinds of trees (colored, different size) on different layers.
And places like Land8 are a great resource. Where my book budget falls short I have a huge library of just image clippings and articles that are tagged with things like “great pedestrian path” or “interesting take on a plaza.” Forms and arrangements from graphic artists and painters are often applicable to my designs. Often Ive been presented with a site and thought that some totally unrelated element I saved on my harddrive a year ago could help me come up with an idea.