It's a two-step process. First click the tree owner's name and go to their profile, and from the profile there should be a link to allow you to send a message.
If there is no link to contact the person, it's probably because there is an option when one sets up one's profile to decline to allow others to initiate messages - that may be what this person has done. If there is no "contact" link on the member profile, try putting a comment on the profile you are viewing, requesting that they contact you through member connect. They should (if the system is working, which it does not always do) get a notice that there is a comment, which they can view and decide whether to contact you.