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
Whitman notes the tree’s solitude, and considers how it is aging alone. The theme of aging alone is repeated once more throughout the poem, the idea that this is what sets the tree apart from Whitman. That he lives his whole life for the company of friends. The noted moss is most likely  Spanish moss , commonly found on older, more mature live-oaks. 
 
 The curly moss itself might be a direct parallel to Whitman’s signature white bushy beard. 

I Saw in Louisiana a Live-Oak Growing