Currently there is a lack of applications that both implement and use Android Lollipop's document tree API because of the lack of apps that support document trees. The only way to fix this problem is by fixing the current lack of document trees in cloud service provider apps and file managers.

Document trees allow a user to give an app access to a folder through the DocumentProvider APIs. The app can take ownership of the folder and do operations (create files and directories, read and write files, list files in directories). The operations are handled by the target app; for Solid Explorer this would allow other apps to access storages inside of Solid Explorer without leaking sensitive information about the storages.

Interoperability with fingerprint and password protected storages:

In the case where a storage is password protected, Solid Explorer should spawn the same dialog thats visible in the app when selecting a storage, and the storage should remain unlocked for the app that is accessing it (and only that app, essentially sessions for specific apps).

Eventually, once document trees gain more support, more and more apps will utilize them. Searching through Github issues reveals that a majority of apps that are requested to add support for document trees end up not implementing it because of the lack of cloud storage providers to test their code with.