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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 type and the Climax community can be mapped once the climate is known.
The three major axes of the barycentric subdivisions are:
Further indicators incorporated into the system are:
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.
The coldest regions have not much evapotranspiration nor precipitation as there is not enough heat to Evaporation much water, hence . 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.
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 staple 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.
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