Which life-zone system classifies areas by biotemperature, precipitation, and evapotranspiration?

The life-zone system that classifies areas by biotemperature, precipitation, and evapotranspiration is the Holdridge life zones system. Leslie Holdridge designed it as a global bioclimatic framework linking climate measurements to potential natural vegetation.

Its three principal axes are annual precipitation, mean annual biotemperature, and the ratio of potential evapotranspiration to precipitation. Biotemperature adjusts ordinary temperature to emphasize conditions relevant to plant growth: temperatures below freezing and above 30°C are treated as biologically inactive for the basic calculation.

Holdridge first published the system in 1947 and updated it in 1967. Although originally intended especially for tropical and subtropical regions, it has been applied globally. Its diagram is often shown as a triangular arrangement of zones, although the underlying scheme is three-dimensional.

The system is not simply an elevation map. It can include latitudinal regions, altitudinal belts, and humidity provinces, but climate variables determine the life-zone placement. This distinguishes it from Köppen, which classifies climate types directly rather than focusing primarily on potential vegetation.