FusionDex

Treeun live in large tree nests shared by others of their kind. As they grow older, they seek out partners with positive electrical charges. When a pair finally connects, sparks burst from their bodies as their currents balance.

At a meeting of Pokémon academics, it was announced that simultaneous exposure to electricity from a Plusle and Treeun will promote circulation and boost vitality. It makes its nest in a giant tree in the forest.