Well, I am not only using Xfrog to model the plants I give away, or the few I am offering on nwda or on turbosquid for example. Apart from that: If you know my real name (which is not hard to find out) and compare that to the splash screen of Xfrog 3.5 for example, or with the pdf´s that come with Xfrog products, then you´ll notice something ;-)
All content created and delivered by Xfrog is of course copyrighted by Xfrog. You are not allowed to give away models or textures, that have been created by Xfrog/Greenworks. You are not allowed to take original models, change them and give them away, let it be for free or on a commercial base. And Xfrog of course is interested in protecting its property. I mean, you are also not allowed to take parts of the code of Terragen and redistribute.
However, you can of course create your own textures and models from scratch. They are yours.
By the way, the trees look nice.