Wood (mu) is one of the Wu Xing, the five phases or agents of Chinese cosmology, not static elements but dynamic modes through which all change is understood. Wood is the phase of growth, expansion, and upward movement, associated in the classical correspondences with the direction east, the colour green, the season of spring, the emotion of anger, and the sour taste. In the generating cycle (sheng) Wood produces Fire, feeding the flame; in the controlling cycle (ke) Wood restrains Earth, its roots binding the soil. These cycles of mutual generation and control order the relations among the five phases and underlie their use in Chinese medicine, the almanac, feng shui, and fate calculation. These are the conventions of the Chinese cosmological tradition, in which the five phases map onto directions, seasons, organs, tastes, and much else in a comprehensive scheme of correspondence.
The Wu Xing framework is usually traced to the correlative cosmology associated with Zou Yan (c. 305 to 240 BCE) and the Warring States naturalist school, and it was systematised further under the Han dynasty, where it became a general grammar for linking the body, the state and the calendar. Wood sits in this scheme as the phase of the generating cycle (Sheng): Wood feeds Fire, which is why it is placed before Fire in the standard sequence Wood, Fire, Earth, Metal, Water, Wood. It also has a place in the controlling cycle (Ke), where Wood parts Earth, as a root splits soil, while Metal in turn controls Wood, as an axe fells a tree. In classical Chinese medicine, set out in the Huangdi Neijing (the Yellow Emperor's Inner Classic), Wood is paired with the liver and gallbladder, the sour taste, the colour green or blue-green, the east, and spring.
Sources Science and Civilisation in China, Volume 2: History of Scientific ThoughtJoseph Needham with Huangdi Neijing (The Yellow Emperor's Inner Classic)Traditionally attributed to Huangdi; actual authorship unknown