The Trees of Cheem are a species of sapient wood-based humanoids descendant from the tropical rainforests of planet Earth, over five billion years in the future. According to the Doctor, the Trees are an influential financial power with roots all over the galaxy. By the time of Sol expanding into a Red Giant and swallowing the inner planets, a small delegation of Trees was present to observe the final moments of their ancestors' former homeworld the Earth.
Members of this species possess hidden lianas (vines) which they consider it a taboo to show in public. When exposed to extremely high temperatures, their bodies are prone to the danger of spontaneous combustion.
They are also a species with the distinction of having bore witness to the Last Great Time War between the Time Lords and the Daleks