Not sure this will answer your question but the subtree is the base DN. For example:
CN=Users,DC=example,DC=com
If you don't know your baseDN you can find it by doing the following from your AD server:
Type the command: dsquery user -name <known username>
The result will look something like: "CN=John.Smith,CN=Users,DC=MyDomain,DC=com"
Based on the above results your base DN would be CN=Users,DC=MyDomain,DC=com
-Rob