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,
Live-oak trees are quite  common in Louisiana,  in fact, some of the oldiest Live-oak trees in the world are located there. 
 It sets up Whitman’s initial observation, merely spotting the tree, which is later evolved into an acceptance of its stark loneliness. This line also introduces the idea of aging/growing which will be developed in the following lines.
I Saw in Louisiana a Live-Oak Growing