@Siddhesh: Thank you very much for the answer.
But it seems it's quite inconvenient to restore public folder items in Exchange 2013. If I need to audit and view the items in a public folder backup, I have to create a separate test environment, recreate the folder list (a very hard task to do so...), and then restore the whole public folder to a live db?
It there any way we can at least do the same thing in Exchange 2010: restore a public folder database as a live db, and disable the replication of that database, so that the content in that "restored" db won't affect other production dbs. Then we can access the item-level contents in the db with MAPI. This solution avoids the need to recreate the folders first and a separate Exchange environment.