The trees and their leaves are really the only things that prove that the wind does exist. Without them, we wouldn't really hear or feel the wind like we do in "Who Has Seen the Wind?" So the proof isn't in the pudding this time. It's in the trees.
Lines 3-4: Those "trembling" leaves kind of look like people because of the speaker's use of personification. By imagining them in this slightly more animated way, we really feel the wind in the same way the trees do in the poem as it passes through.
Lines 7-8: The trees also "bow down their heads" (more personification, Shmoopers), which also adds a level of respect or understanding that the trees appear to have with the wind. They allow the wind to pass by without any fussing and fighting.