Emergency services officer

An emergency services officer advised the Twelfth Doctor and Clara Oswald to stay back, as he was about to burn the trees that grew overnight in the vicinity of Green Park tube station in London to create a path. Clara asked about the whereabouts of Maebh Arden, but the officer simply repeated "Stay back. We're about to burn." The trees failed to catch fire when the officer used his flamethrower on the trees. The Doctor realised that this meant the trees were smothering the flames by withholding the oxygen in the air. This effectively meant that the forest had "its own in-built fire extinguisher". (TV: In the Forest of the Night)