I saw in Louisiana a live-oak growing,
All alone stood it and the moss hung down from the branches,
Without any companion it grew there uttering joyous leaves of dark green,
And its look, rude, unbending, lusty, made me think of myself,
But I wonder'd how it could utter joyous leaves standing alone there
without its friend near, for I knew I could not,
Whitman emphasizes its solitude again, as the tree’s lack of a companion strikes him as a significant misfortune. He is especially surprised that it is able to function without anyone else to support it. 
 
 The oak is  uttering joyous leaves of dark green,  which now refers to the oak as if it were a poet (or at least something that can  utter ). This furthers the comparison between Whitman and the live-oak and starts to personify the tree. 
 He repeats this line continuously, elevating and enforcing the fact that the tree has no one else to support it.
I Saw in Louisiana a Live-Oak Growing