Trees lend themselves nicely to a host of metaphors that shed light on human experience.
As Entropie pointed out, they're well grounded. They're content to stay where they are and use only what is given them in order to grow and produce their beautiful raiment. They use light, a metaphor for understanding, and turn it into life giving oxygen and carbohydrates that benefits others. They send their roots deep into the earth to reach water that is unseen, and water is a common metaphor for that which gives life. And have you ever wondered how plants make fruit? They take water, light, and minerals and turn these elements into oranges, etc. I can't do that. How do they make such brilliant colors out of dirt and water? How do they create such wonderful flavors out of mud? That is a great metaphor for developing the unseen potential in things that, at a glance, seem worthless or plain. They provide shade in the heat, and, in death, they provide warmth in the cold...
Trees are cool.