You can use other tree types. Some 3rd party trees will show up as obsolete though and be replaced by SpeedTree trees. (That's more of an issue with importing a TS2009 or older route though, not something you'd need to worry about when creating a route. Old routes may actually have a mix of SpeedTree trees and older trees for this reason.)
Note that you may need to switch to compatibility mode, at least one of the old methods used for making trees looks really bad in native mode.