# Holdridge life zones

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 criteria. A basic assumption of the system is that both soil and the climax vegetation can be mapped once the climate is known.

## Scheme

While it was first designed for tropical and subtropical areas, the system now applies globally. The system has been shown to fit not just tropical vegetation zones, but Mediterranean zones, and boreal zones too, but is less applicable to cold oceanic or cold arid climates where moisture becomes the predominant factor. The system has found a major use in assessing the potential changes in natural vegetation patterns due to global warming.
The three major 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 annual temperatures, with all temperatures below freezing and above 30 °C adjusted to 0 °C, as most 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 directly. The system is considered more appropriate for 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 enough water available. Higher temperatures result in higher PET. Evapotranspiration (ET) is the raw 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.
The coldest regions have not much evapotranspiration nor precipitation as there is not enough heat to evaporate much water, 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

All the classes defined within the system, as used by the International Institute for Applied Systems Analysis (IIASA), are:
* Polar desert
* Subpolar dry tundra
* Subpolar moist tundra
* Subpolar wet tundra
* Subpolar rain tundra
* Boreal desert
* Boreal dry scrub
* Boreal moist forest
* Boreal wet forest
* Boreal rain forest
* Cool temperate desert
* Cool temperate desert scrub
* Cool temperate steppe
* Cool temperate moist forest
* Cool temperate wet forest
* Cool temperate rain forest
* Warm temperate desert
* Warm temperate desert scrub
* Warm temperate thorn scrub
* Warm temperate dry forest
* Warm temperate moist forest
* Warm temperate wet forest
* Warm temperate rain forest
* Subtropical desert
* Subtropical desert scrub
* Subtropical thorn woodland
* Subtropical dry forest
* Subtropical moist forest
* Subtropical wet forest
* Subtropical rain forest
* Tropical desert
* Tropical desert scrub
* Tropical thorn woodland
* Tropical very dry forest
* Tropical dry forest
* Tropical moist forest
* Tropical wet forest
* Tropical rain forest

## Climate change

Many areas of the globe are expected to see substantial changes in their Holdridge life zone type as the result of climate change, with more severe change resulting in more remarkable shifts in a geologically rapid time span, leaving less time for humans and biomes to adjust. If species fail to adapt to these changes, they would ultimately go extinct: the scale of future change also determines the extent of extinction risk from climate change.
For humanity, this phenomenon has particularly important implications for agriculture, as shifts in life zones happening in a matter of decades inherently result in unstable weather conditions compared to what that area had experienced throughout human history. Developed regions may be able to adjust to that, but those with fewer resources are less likely to do so.
Some research suggests that under the scenario of continually increasing greenhouse gas emissions, known as SSP5-8.5, the areas responsible for over half of the current crop and livestock output would experience very rapid shift in its Holdridge Life Zones. This includes most of South Asia and the Middle East, as well as parts of sub-Saharan Africa and Central America: unlike the more developed areas facing the same shift, it is suggested they would struggle to adapt due to limited social resilience, and so crop and livestock in those places would leave what the authors have defined as a "safe climatic space". On a global scale, that results in 31% of crop and 34% of livestock production being outside of the safe climatic space.
In contrast, under the low-emissions SSP1-2.6 (a scenario compatible with the less ambitious Paris Agreement goals, 5% and 8% of crop and livestock production would leave that safe climatic space.