The Holdridge Life Zone model (Holdridge 1947, 1972; Prentice 1990) is based on the concept of the biotemperature and how it interacts with precipitation and evaporative demand to determine potential natural vegetation. It is based primarily on correlations of these factors with the distribution of vegetation in Costa Rica, but has been applied to global vegetation studies, including studies of global change. The average biotemperature (ABT; °C) is calculated in two steps. First, add up the mean monthly temperature (°C) for all months in which the temperature exceeds 0°C. Second divide that total by 12. The formula is as follows:
(1) ABT = (sum(MMT>0°C))/12
The average potential evapotranspiration (APE; mm) is estimated from the average biotemperature by:
(2) APE = 58.93*ABT
Finally, the ratio of annual potential evapotranspiration (APE; mm) to annual precipitation (MAP; mm) must be calculated.
In truth, one only needs two of the following three values (ABT, MAP, APE/MAP) to classify vegetation according to the Holdridge system. The classification is done automatically in ClimVegWin.
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