It depends on what culture or what country or part of the world really. For say, I know vikings or Norse Mythology have this one. I think the tree for them symbolized the universe itself. The cycle of life and how energy goes up the roots going to the branches. Trees in many culture symbolized life. Like a tree, life forms start with a little leaf, a plant and until it grows. Before it matures, it will be challenge by many storms, winds and calamities. Trees were believed to have some mystical properties on them too. Some trees' wood or bark were said to have some mystical properties depending on what tree. Tree connects heavily directly to earth itself. Do you ever watch the movie Avatar?? Trees were ancient and was one of the first and foremost creation It can be the reason why we value it so much. Trees represent life, the earth, nature, stability and deep connection to our universe.