Holdridge’s work aimed to correlate world plant formations with simple climatic data. The system embraces all major environmental factors in three hierarchical tiers.
Level I - The Life Zone. This is determined by specific quantitative ranges of long-term average annual precipitation, mean annual biotemperature and potential evapotranspiration ratio. These are modified for montane systems.
Level II - The "Association". This is an area of land which, under undisturbed conditions, supports a distinctive natural community adapted through evolution to a specific narrow range of atmospheric and edaphic conditions. No Association can occur in more than one Life Zone.
Level III - The successional stage or cover type, which takes into account that the community may not be in its climax state, either through natural causes or through human intervention.
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