Is it necessary to touch a tree or lean against it in order to establish a relationship to get healing help?

This is not necessary but if it makes the person feel closer and to have a stronger association, it can add greater power of intention to the exercise. But the trees will know you are thinking about them and focusing on them. They are very much attuned to human consciousness of all kinds and will be perceiving you and will tune to your wavelength, so to speak, when you approach them with your thoughts alone.