A tree planted or valued chiefly for its shade from sunlight WordNet 3.0 & Open English WordNet

A shade tree is a large tree whose primary role is to provide shade in the surrounding environment due to its spreading canopy and crown, where it may give shelter from sunlight in the heat of the summer for people who seek recreational needs in urban parks and house yards, and thus, also protecting them from the sun's harmful UV rays and sunburns. Wikipedia