The Lite version is fully functional for many tasks but has specific constraints compared to the Pro edition: Getting Started with Forest Pack Lite - itoosoft
If the official 8 trees aren't enough, you are not out of luck. The 3D community is vast, and many artists share "Forest Pack compatible" assets for free. However, be cautious: Most of these are (which tell Forest Pack how to scatter) rather than the actual 3D models. forest pack lite library free download
Instead, understand the power of the legitimate version. The Lite version is fully functional for many