Shinju no Moribito

There are three worlds - Heaven, Underground and human one. Humans gets sandwiched between two warring factions as devils decide to crush humans and attack Heavens from there. The earthlings are ignorant about the other two worlds, so angels decide to plant sacred trees everywhere in human world and use their divine energy to exorcise evil. An angel guardian is appointed to protect these forests from destruction.

[From vndbreview]