If you create a tree or any other piece of art then you automaticly own the copyright to that work, even if you do not register it. Makes no differece if you sculpt one out of clay or draw it with a brush & paint. The tools you use to create your work are immaterial. Would Stanly Tool own the rights to a wood sculpture you made because you used a chisle made by them? Does Grummbacher claim any right to any painting created with their oil paints? There are some plug-ins that create trees and lightning that will not allow you to sell (or distribute) their output. It makes sense in that the "plug in" really creates the object. All the user does is push a couple buttons to call out some presets.