Each tree species carries strength and healing, though obviously longer standing tree with deep roots and spreading branches have greater repositories of energy. Each kind of tree also has its own unique powers. These you may associate with the kind of nature essence dwelling in that species or more abstractly with the qualities filtered through different kinds of wood.
Hold a crystal pendulum close to different trees and you will feel in your hands and body, amplified by the crystal, the differing tree strengths. For example, a redwood may make you feel confident and a willow, dreamy and intuitive. Note these feelings in your nature journal along with any images or words that come into your mind.
There are variations in intensity even with different trees within the same species. Take time to explore these energies and to visit forests (children love them) to get yourself attuned.
An arboretum or botanical garden is a good place to start if you are not familiar with trees. Buy a small tree book to carry with you so you can identify trees wherever you go, even in cities.