In my opinion, this poem is an indictment of mankind's failure to care for the earth. Sarcasm is obviously involved, as the poet enumerates the various ways we attempt to kill a tree, only to have it survive.
The act of killing a tree is more than a simple act of taking a knife to it. Why? Because the tree has deep roots, which have absorbed water; its leaves have absorbed sunlight; leaves sprout out from the bark, which looks as if nothing could possibly grow from it.
To actually kill the tree, something much more fierce and intentional must be done. It is necessary to actually pull it up by its roots, separating it from all that has made it grow and become its strong self in the first place. Even so, it will take much time for the uprooted tree to actually decompose and turn to nothingness. It will even make attempts to regrow.
Do you think it is possible that the poet is speaking ONLY about trees, or could he also be speaking about other entities which are deeply rooted? People, perhaps?
This should have gotten you started. Now explore for yourself.