Trees can symbolize many things. Some of the most common are family, healing and strength.
The symbolism of trees is as vast and varied as the species that inhabit the earth. The most revered species in earth-based Celtic spirituality is the Oak. This tree is synonymous with power, honor, wisdom and longevity. The word "Druid" has its roots in the word "duir" which is the Celtic term for oak. They believe this tree embodies a strong positive spiritual and magical power which gives it dominion over all the other varieties. Physical attributes of the Oak are usually associated with male qualities and are often aligned with various Thunder Gods, such as Zeus and Jupiter.
I came across an interesting site about the symbolism of trees called "The Unfading Tree by Big Green Oak". On this site is a test to see which tree you are. I am a Maple. What type of tree are you? You can read about the symbolism of various types of trees here also. Root System: Tree Symbolism