Tree

A tree was a wooden 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. Their age could be determined 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 humans. Maebh Arden was able to communicate with a global forest which grew overnight to protect the Earth from 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: Cat's Cradle: Time's Crucible)

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 of trees on Earth
On Earth, trees could be used for decoration. Over the festive period, Christmas trees 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 an airbag against meteors 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 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 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. Treeborgs were used as oxygen factories on spaceships such as the Byzantium, offering the crew unlimited supplies of oxygen. (TV: Flesh and Stone)

History of trees on Earth
Whilst on Earth in the 19th century, 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)

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)

Other references
Kal, of the Tribe of Gum in 100,000 BC, mistook the TARDIS' police box shape for a tree. (TV: "The Cave of Skulls")