Tree

A tree was a wooden plant which was often rooted into the ground. An area of multiple trees was called a forest.

Biology
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, a tree had four times the life expectancy of a human being. (TV: The Mark of the Rani) As they grew, trees could mangle fences and topple structures, as they did Nelson's Column. (TV: In the Forest of the Night)

While the majority of trees were rooted into the ground, some were able to move and claimed sentience. (TV: The End of the World)

As a natural resource
Trees could be harvested as a natural resource. Androzani trees were used as fuel by the populace of Androzani Major. (TV: The Doctor, the Widow and the Wardrobe)

As decoration and comfort
At Christmas, some trees were decorated with lights and baubles. (TV: The Runaway Bride) Town planners on Tivoli used trees to provide shade for invading armies. (TV: The God Complex)

As gas exchange
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 oxygen in deep space. (TV: Flesh and Stone)

History of trees on Earth
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)

In 2016, 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)

Types of trees on Earth
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History of trees on other planets
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)

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.

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)

By 5,000,000,000 many trees that had originated on Earth had achieved sentience and humanoid form. (TV: The End of the World)

The Sixth Doctor and Frobisher encountered a talking tree while in Astrolabus' realm. (COMIC: Once Upon a Time Lord)

Miscellaneous trees
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