MalteWhat version of Photoshop do you use? I'm using Photoshop 7.0 and it seems that I'm unable to select multiple layers (newer version might have that feature).
However, I can only add leaves either onto the trunk and branches of the tree
You might have the 'transparent pixels lock' enabled. It allows you to paint on an object on the layer you have currently selected without painting 'outside' from that object.

or you have a layer 'grouped' to that object:

It's similar to the transparent pixels lock, except you're painting outside the object too, which is not visible until you 'ungroup' that layer.
or onto the background leaving the tree itself uterly "untouched" by them
Hmm, that's new to me. :unsure: Do you paint on the "Background" layer or a layer that is just 'below' the tree layer?

Because the layers above are always 'untouched' from the layers below them. That's the only thing I can think of.
A few screenshots of your workspace might help me to understand better (if those problems aren't solved yet with these solutions).
