From Combine OverWiki, the original Half-Life wiki and Portal wiki
The Tree is a plant-like organism found on the borderworld Xen that attacks living beings that move too close.
- A brownish tree-like creature with no leaves or branches, the Tree somewhat resembles an upturned root and in that regard is similar to what could be an infant Tentacle. Towering at around 13 to 16 ft tall (4 to 5 m), it has a fully mobile "trunk", and a large, hook-like appendage at the top, used for attacking nearby prey.
- Trees are scattered around Xen and can prove a danger to an unwary explorer and other Xen wildlife by violently stabbing at any nearby source of movement.