Tree

A tree was a wooden plant which was often rooted into the ground. Numerous tree species existed throughout the universe.

Biology
On Earth, trees were made of wood and had leaves. 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 also claimed some form of sentience, though this was not perceived by the majority of humans. 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)

Uses
Over the festive period, Christmas trees would be decorated with lights and baubles. (TV: The Runaway Bride)

History
Whilst on Earth in the 1820s, the Rani mined a section of forest with mines that turned humans 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)

Types of trees on Earth
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Trees on Gallifrey
A species of tree existed on Gallifrey with silver leaves. These reflected the morning sunlight, making it look like the forests were on fire. (TV: "A Desperate Venture", Gridlock) There were also green forests, similar to those on Earth. (PROSE: Cat's Cradle: Time's Crucible)

Biology
The forests of Eden were dense even in the small section captured by Tryst's Continuous Event Transmuter. (TV: Nightmare of Eden)

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)

Uses
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 trees growing on a nearby planet were used as fuel by the populace of Androzani Major. (TV: The Doctor, the Widow and the Wardrobe)

The ability of trees to exchange gases made them useful in deep space. Treeborgs were used as oxygen factories on spaceships such as the Byzantium, offering the crew unlimited supplies of oxygen. (TV: Flesh and Stone)

History
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)