From MangaHelpers:

Very old trees can have spirits that may take human form, and they sometimes live among and marry humans. The descendants of these unions have some special abilities granted by their spirit blood. In Hoshikawa, there were once many tree spirits, but now only one is left. This spirit, Hoozuki, married a man named Tenma, who left and never returned. She remained, watching over her descendants as they watched over her. Five generations later, her family contains a teenage boy named Kazuho, a boy she has cared for all his life. Now that he is growing up, he realizes that he has feelings for her, but will he ever be able to compete with the memory of Tenma? [tethysdust]