You can click on the profile pic of any member you'd like to privately email. Then you'll see :"send a treemail". It's private and helpful if you're arranging plant trades or sales, or just want to say thanks for your acorn tips.
Also, if the picture of the envelope at the top of the screen is red, then that means someone sent you a tree mail. Just click on the envelope to read it.
BTW -- love the picture you included with your question.