The title of the Forest Service documentary "Green Fire" refers to a passage in "A Sand County Almanac" in which Leopold talks about killing a wolf.
The incident happened in the early 1900s, when he was fresh at his job as a forester in the American Southwest. He and some companions were out exploring when they chanced upon a wolf and killed it.
In those days, foresters never passed up a chance to kill a predator. Ranchers feared them because they killed livestock; hunters figured the fewer wolves, the more game.
As Leopold's life unfolded and he studied the interplay of natural systems as a professor at the University of Wisconsin, he often thought back on that day of the wolf.
"We reached the old wolf in time to watch a fierce green fire dying in her eyes," he wrote later. "I realized then, and have known ever since, that there was something new to me in those eyes — something known only to her and to the mountain.
"I was young then, and full of trigger-itch; I thought that because fewer wolves meant more deer that no wolves would mean hunters' paradise. But after seeing the green fire die, I sensed that neither the wolf nor the mountain agreed with such a view."