Teach your baby the sign for the word tree in baby sign language with the following description and video.
Hold one arm at waist level with the palm of the hand facing up. Place the elbow of the other hand on the palm. Rotate the arm back and forth in the motion of a tree waving in the wind.
ASL Pro: http://www.aslpro.com/cgi-bin/aslpro/aslpro.cgi
Signing Savvy: http://www.signingsavvy.com/