Rejecting the temperate latitude bias of Merriam's life zones, L. R. Holdridge devised a life zone classification in 1947 more appropriate to the complexities of tropical vegetation. Holdridge's zones are delineated according to what he came to call "biotemperature". Biotemperature refers to all temperatures above freezing, with all temperatures below freezing adjusted to 0° C. The assumption was that, from the perspective of plant physiology, there is no real difference between 0° C and temperat ures less than zero: plants are dormant. The life zones are thus defined first according to a climatic variable--degrees mean annual biotemperature (and not according to degrees latitude or meters of elevation).
The broad climatically defined life zones are further subdivided into associations on the basis of local environmental conditions and actual vegetation cover or land use. Holdridge was able to define boundaries between these major vegetation units accordi ng to 1) logarithmic increases in mean annual biotemperature; 2) logarithmic increases in total annual precipitation, and 3) the ratio of mean annual potential evapotranspiration to mean total annual precipitation.
The Holdridge Life Zone Model, although conceived as three-dimensional by its originator, is usually shown as a two-dimensional set of hexagons arranged in a triangular frame (see figure 1).
Within each hexagon, there are 6 smaller triangles representing variations in vegetation in areas where local environments are transitional to the core zone (see figure 2).
Including these local variants, over 100 different vegetation associations are represented in the model.
The Holdridge system was intended to be applicable to the entire globe. However, it is primarily used by investigators in the New World tropics.
| Biogeography Home Page | Back to Life Zones and Altitudinal Zonation Page |
Created by S. Woodward, July 1996.