Holdridge life zones

Holdridge life zone classification scheme. Although conceived as three-dimensional by its originator, it is usually shown as a two-dimensional array of hexagons in a triangular frame.

The Holdridge life zones system is a global bioclimatic scheme for the classification of land areas. It was first published by Leslie Holdridge in 1947, and updated in 1967. It is a relatively simple system based on few empirical data, giving objective mapping criteria. A basic assumption of the system is that both soil and climax vegetation can be mapped once climate is known.

Scheme

While it was first designed for tropical and subtropical areas, the system applies globally. The system has been shown to fit tropical vegetation zones, Mediterranean zones, and boreal zones, but is less applicable to cold oceanic or cold arid climates where moisture becomes the determining factor. The system has found a major use in assessing the possible changes in natural vegetation patterns due to global warming.

The three axes of the barycentric subdivisions are:

• precipitation (annual, logarithmic)
• biotemperature (mean annual, logarithmic)
• potential evapotranspiration ratio (PET) to mean total annual precipitation.

Further indicators incorporated into the system are:

• humidity provinces
• latitudinal regions
• altitudinal belts

Biotemperature is based on the growing season length and temperature. It is measured as the mean of all temperatures above freezing, with all temperatures below freezing and above 30 °C adjusted to 0 °C, as plants are dormant at these temperatures. Holdridge's system uses biotemperature first, rather than the temperate latitude bias of Merriam's life zones, and does not primarily consider elevation. The system is considered more appropriate to the complexities of tropical vegetation than Merriam's system.

Scientific Relationship between the 3 axes and 3 indicators

Potential evapotranspiration (PET) is the amount of water that would be evaporated and transpired if there were sufficient water available. High temperatures result in higher PET. Evapotranspiration (ET) is the sum of evaporation and plant transpiration from the Earth's land surface to atmosphere. Evapotranspiration can never be greater than PET. The ratio, Precipitation/PET, is the aridity index (AI), with an AI<0.2 indicating arid/hyperarid, and AI>0.5 indicating dry.

         Coldest
          /\
         /  \
    PET - -- - Rain

Coldest regions have not much evapotranspiration nor precipitation, hence polar deserts. In the warmer regions, there are deserts with maximum PET but low rainfall that make the soil even drier, and rain forests with low PET and maximum rainfall causing river systems to drain excess water into oceans.

Classes

The classes defined within the system, as used by the International Institute for Applied Systems Analysis (IIASA), are:

1. Polar desert
2. Subpolar dry tundra
3. Subpolar moist tundra
4. Subpolar wet tundra
5. Subpolar rain tundra
6. Boreal desert
7. Boreal dry scrub
8. Boreal moist forest
9. Boreal wet forest
10. Boreal rain forest
11. Cool temperate desert
12. Cool temperate desert scrub
13. Cool temperate steppe
14. Cool temperate moist forest
15. Cool temperate wet forest
16. Cool temperate rain forest
17. Warm temperate desert
18. Warm temperate desert scrub
19. Warm temperate thorn scrub
20. Warm temperate dry forest
21. Warm temperate moist forest
22. Warm temperate wet forest
23. Warm temperate rain forest
24. Subtropical desert
25. Subtropical desert scrub
26. Subtropical thorn woodland
27. Subtropical dry forest
28. Subtropical moist forest
29. Subtropical wet forest
30. Subtropical rain forest
31. Tropical desert
32. Tropical desert scrub
33. Tropical thorn woodland
34. Tropical very dry forest
35. Tropical dry forest
36. Tropical moist forest
37. Tropical wet forest
38. Tropical rain forest

See also

• Andrew Delmar Hopkins
• Biome
• Ecoregion
• Holdridge life zones in Guatemala
• Köppen climate classification
• Life zone
• Trewartha climate classification