Annual average of air temperature at the station.
Average total annual precipitation.
Tropical moist forest
24
°C1414
mm/year0.7072
dimensionlessLeslie Holdridge (1947, 1967) argued that a region's natural vegetation is broadly set by its climate. Instead of describing climate with many variables, he reduces it to three axes and uses them to place any point on Earth into one of about 38 life zones, from tropical desert to tundra and perpetual snow. Each zone predicts the dominant vegetation physiognomy (forest, thorn woodland, steppe, tundra…), which makes it a classic tool in ecology and biogeography.
Plants do not grow at just any temperature: below 0 °C water freezes, and above about 30 °C vegetative growth no longer increases. So Holdridge does not use the ordinary mean temperature but the biotemperature: the annual average of temperature with every value outside the useful 0–30 °C range clipped to 0 °C.
This calculator uses the single-input simplification: Tbio= the mean annual temperature clipped to 0–30 °C. If you already have a biotemperature computed from monthly means, you can enter it directly.
Holdridge combines biotemperature, annual precipitation and the potential evapotranspiration ratio. Potential evapotranspiration (PET) is the water the climate could evaporate; in this model it is estimated directly from the biotemperature:
Potential evapotranspiration
PET ratio (aridity)
The PET ratio measures aridity: if potential evaporation greatly exceeds rainfall (large RPET) the climate is arid; if rainfall dominates (small RPET) it is humid. This axis defines the humidity provinces.
The axes grow in powers of 2(1.5, 3, 6, 12, 24 °C; 125, 250, 500, 1000 mm…) because the biological response to climate is multiplicative: going from 250 to 500 mm of rain shifts the biome as much as going from 1000 to 2000 mm. The logarithmic scale spaces the biomes in uniform steps.
Because the three axes are tied together by the RPET formula, there are really only two degrees of freedom: biotemperature and precipitation alone fix a point. That is what turns the chart into a triangle (a ternary diagram). From top (cold) to bottom (warm) the latitudinal regions and their equivalent altitudinal belts change (basal, premontane, montane, subalpine, alpine, nival); from left (wet) to right (dry) the humidity provinces change. The marker shows where the climate you entered falls.
From driest to wettest: superarid, perarid, arid, semiarid, subhumid, humid, perhumid and superhumid. Each step corresponds to halving the PET ratio.
The province measures the balance, not absolute rainfall. So a very cold climate with little rain can still fall in a humid province: its evapotranspiration is so low that the precipitation is enough to balance it (a polar desert is "dry" by its scant rainfall, yet "superhumid" by its balance).
The model describes the potential vegetation set by climate: it ignores soil, seasonality, wind, microclimates and human impact. The simplified biotemperature (a single annual figure) can overestimate growth in climates with cold winters. It is a first approximation — very useful, but not a map of the actual vegetation.
Holdridge, L. R. (1967). Life Zone Ecology. Tropical Science Center, San José, Costa Rica.