Tree Runeys are brown and resemble peanuts. They eat grass runeys and are eaten by rock runeys. Tree Runeys prefer to live in areas like the mountain pass and the Church district. Tree Runeys are the second most important type of runey in the game. They're required to access the second dungeon in Rune Factory Frontier.
Tree Runeys are easy to multiply because they reproduce in large numbers. However if the Tree runeys are decreasing in number, simply take a lot of Rock or Water runeys and place them into an area. PLace a couple of Grass runeys you can spare there. Once the Grass Runey population is up, make sure there aren't any rock runeys in the area and place Tree runeys there. Wait a few days. After you've had enough Tree Runeys, add Rock and Water runeys to the area.