Pasting in Photoshop

[Turgenev]

Alternatively ... after you are done placing all the forest you want.... you can hide all the other layers, then Shift+ctr+E to merge visible.... that at least makes the one extra step at the end so you don't have to keep merging....

Or alternatively, click on the little link box (next to the visibility eye) on the layers you want to merge. This not only allows you to move the many images as one unit but it also allows you to merge all those layers at once (instead of doing each layer one at a time or turning off visibility on layers to do a merge visible). I wouldn't know what to do without layers. They are a godsend.