SpeedTree offers a of their Modeler for students and indie artists. While it does not include the full premium library, it does include a starter library of roughly 30 species. You can export these models for learning purposes, but you cannot use them in commercial products without a license.
I understand you're looking for information on and possibly free library downloads. However, I must clarify a few important points before providing a useful essay on the topic. speedtree library free download