Tokiwa no Mori to Tsugi no Hito

In the not-too-distant future, humanity has evolved to the point where they will eventually turn into trees. Mika, an orphan who cannot become a tree, lives a life of isolation from the villagers, and her only "family" is about to turn into a tree, leaving her behind.
Mika's fate and that of Earth are dramatically altered when she meets Kinoemaru, a monk who takes on the role of childcare for her...

(Source: Hero's Inc, translated)