Tree A tree was a wood en plant which was often rooted into the ground . An area of multiple trees was often referred to as a forest . Numerous tree species existed throughout the universe . Biology . Throughout the universe , trees were commonly made of wood and had leaves . Trees would often be rooted into the ground . However , more evolved trees could move and some even claimed sentience . ( TV : " The End of the World " ) Biology of trees on Earth . Earth trees released oxygen into the atmosphere . You could tell their age by the number of rings across the insides of their trunks . ( TV : " In the Forest of the Night " ) According to the Rani , an Earth tree had four times the life expectancy of a human being . ( TV : " The Mark of the Rani " ) As they grew , these trees could mangle fences and topple structures , as they did Nelson ' s Column in the 2010s . ( TV : " In the Forest of the Night " ) Earth trees had also some form of sentience , though this was not perceived by the majority of human s . Maebh Arden was able to communicate with a global forest which grew overnight to protect the Earth fro a solar flare . ( TV : " In the Forest of the Night " ) Biology of trees on Gallifrey . The leaves on Gallifrey ' s trees were bright silver . ( TV : " A Desperate Venture " , " Gridlock " ) At least two species of tree existed on Gallifrey : Cadonwood and Ulanda trees . ( AUDIO : " Neverland " , PROSE : " Blind Fury " ) These reflected the morning sunlight , making it look like the forests were on fire . ( TV : " Gridlock " ) There were also forests made up of trees with green leaves , similar to those on Earth . ( PROSE : " " ) Elsewhere in the universe . Vislor Turlough once described the trees on Trion as being three times the size of Earth trees , with plate - like thick mauve and purple leaves growing in spirals up blood - red trunks . ( AUDIO : " Loups - Garoux " ) Calderon Beta was home to a four hundred foot tall tree " growing out of a clifftop on the north side of a mountain in the middle of the sea " . ( HOMEVID : " First Night " ) Kromon had towering trees . ( AUDIO : " The Creed of the Kromon " ) Tiro was home to trees which had metal bodies and could move slowly . ( PROSE : " Terror on Tiro " ) Uses . Uses of trees on Earth . On Earth , trees could be used for decoration . Over the festive period , Christmas tree s would be decorated with lights and baubles . ( TV : " The Runaway Bride " ) The more sentient trees of Earth could act as a natural defence . Together they could release oxygen into the atmosphere to act as airbag against meteor s or solar flares , such as during the Tunguska Event . ( TV : " In the Forest of the Night " ) Elsewhere in the universe . Throughout the universe , trees were used for decoration and comfort . On Trenzalore , the town of Christmas decorated evergreen trees with lights . ( TV : " The Time of the Doctor " ) Town planners on Tivoli used trees to provide shade for invading armies . ( TV : " The God Complex " ) Trees could be used as a natural resource . Androzani tree s growing on a nearby planet were used as fuel by the populace of Androzani Major . ( TV : " The Doctor , the Widow and the Wardrobe " ) " The company " harvested trees from Kromon as a natural resource . ( AUDIO : " The Creed of the Kromon " ) The ability of trees to exchange gases made them useful in deep space . Treeborg s were used as oxygen factories on spaceships such as the " Byzantium " , offering the crew unlimited supplies of oxygen . ( TV : " Flesh and Stone " ) History . History of trees on Earth . Whilst on Earth in the 1820s , the Rani mined a section of forest with mines that turned human s into trees . Following their transformation the former - humans retained some level of consciousness . ( TV : " The Mark of the Rani " ) Around the 2010s , a global forest grew overnight to protect the Earth from a solar flare . The forest was able to communicate with Maebh Arden . ( TV : " In the Forest of the Night " ) By 5 , 000 , 000 , 000 many trees that had originated on Earth had achieved sentience and humanoid form . The Forest of Cheem considered themselves direct descendants of the Amazon Rainforest . ( TV : " The End of the World " ) Elsewhere in the universe . The Sixth Doctor and Frobisher encountered a talking tree while in Astrolabus ' realm . ( COMIC : " Once Upon a Time Lord " ) In the year 5345 , the Eleventh Doctor encountered a group of Androzani trees who were attempting to flee their doomed homeworld . ( TV : " The Doctor , the Widow and the Wardrobe " )