That's Lonely Angel rhetoric, forest logic, and it doesn't apply to the individual tree. You can't expect anybody to understand that, in the moment of their tragedy. And in this moment, Adelaide striving and fighting for her entire planet and hoping desperately to live -- measuring her own pain against the threat of a world's extinction -- he feels closer to her than he has to anybody since Donna. He can see life as a tree, for just one second.
A second is all it takes. He appears suddenly with them, tossing tools and barking orders, and helps them save their lives. Adelaide protests, afraid he'll die with them, but the Doctor just laughs. After all, Sigma and Carmen told him he would be dead soon, and that "he" will knock four times, and that means he's not dying here now. Andy knocks at the door, three times, and before he can knock a fourth the Doctor has electrocuted him. He directs Mia and Yuri to use heat against them, smiling. Adelaide's confused: Didn't he just say they had to die? For the future? For the human race?
A terrible light takes him over, behind the eyes; his jaw works madly. "There are laws, there are Laws of Time," he admits. "Once upon a time there were people in charge of those laws, but they died. They all died. Do you know who that leaves? Me! It's taken me all these years to realize the Laws of Time are mine. And they will obey me!" The crew, what's left of them, are shoved about as the environmentals crash. His helmet's crushed, but he's not beaten yet. "We're not just fighting the Flood. We're fighting Time itself. And I'm gonna win!"
Maggie screams at the ice, and it begins to crack. The Doctor buzzes with excitement, sounding more like Doctor-Donna than ever: "Thinka thinka thinka think! What have we got? Not enough oxygen! Protein-packs? Useless! Glacier, glacier mints, minty, Monty, molto bene, bunny, Bonny, bish-bash-bosh! Baaaaaaah!" (He sees her, for a moment, just a glimpse before his hearts can't take it anymore: "Binary binary binary binary...") He sends the stupid robot out to the TARDIS with a key in its claw, and sonics the thing away even as Maggie screams. But Adelaide's already set the bomb to blow, and the robot crawls away, pulling and pushing at his body. Water floods the Flood, and they scream in harmony. As the robot opens the TARDIS and Adelaide stares at the mad Doctor, the bomb ticks down. He is manic, and frightening, and proud. There are two seconds left when the TARDIS appears. Bowie Base One is destroyed in a mushroom cloud, and the red planet is quiet. We're left a long time to wonder.