Definition of life zone in US English:

life zone

noun

Ecology
  • 1A zone in which life occurs, or in which the greatest number of living organisms may be found; (specifically) that down to a certain depth in the sea.

  • 2Chiefly North American a geographical area or zone and its associated ecosystems or communities, either on a large scale (a biome) or, more usually, on a small scale (as an altitudinal zone).

Origin

Mid 19th century; earliest use found in The Gentleman's Magazine.