The mistake was mine. I apologize. The trees that had Alexandria Linden as owner were not part of the package I purchased. I discovered that these objects have an 'add' option. If I click on that the trees are copied into my objects folder with me as the owner, after which I can rez them without difficulty. I guess these objects were a contribution of sorts from the Lindens.
Sorry for any confusion and thanks for your efforts to help!
Bongo Fhang