can anyone tell me where i can obtain tree survey database software for a PDA?

What PDA are you using? What type of inventory? What data will you collect and how many trees? What software will you use on the upload computer? These need to be answered before you jump into a software decision.

There are many database systems available for the handheld PDAs. You can set up your own inventory system with these to record the field data. The biggest issue is connecting to a suitable database on the host computer. There are some PDA databases that can connect directly with databases on the host. Most will at least export the data into text files or other formats that can then be imported into the host database.