Life zone is a term in ecology and biogeography referring to a geographic area defined by characteristic climatic conditions and the associated communities of plants and animals. The concept was developed to describe and classify areas with similar biotic communities, and it exists in two principal formulations: the Merriam life zone system and the Holdridge life zone classification.
Merriam Life Zones
The life zone concept was introduced by American biologist C. Hart Merriam in 1889. Merriam observed that changes in plant and animal communities with increasing latitude at a constant elevation are similar to changes seen with increasing elevation at a constant latitude. His system, developed primarily on the San Francisco Peaks in Arizona and the Cascade Range of the northwestern United States, identified the following life zones (from lowest/hottest to highest/coldest):
- Lower Sonoran – low, hot desert (e.g., creosote bush, Joshua tree)
- Upper Sonoran – desert steppe or chaparral (e.g., sagebrush, scrub oak, pinyon pine, Utah juniper)
- Transition – open woodlands (e.g., ponderosa pine)
- Canadian – fir forest (e.g., Rocky Mountain Douglas fir, quaking aspen)
- Hudsonian – spruce forest (e.g., Engelmann spruce, bristlecone pine)
- Arctic-Alpine – alpine meadows or tundra (e.g., lichen, grass)
The Canadian and Hudsonian zones are often combined into a Boreal life zone. Merriam's system has been criticized as imprecise—for example, the scrub oak chaparral of Arizona and the Great Basin sagebrush desert share few species yet are both classified as Upper Sonoran. Nevertheless, it is still referenced by biologists and anthropologists working in the western United States.
Holdridge Life Zones
In 1947, American botanist and ecologist Leslie Holdridge published a more quantitative life zone classification system. The Holdridge system uses three climatic axes:
- Mean annual biotemperature (logarithmic) – the average of temperatures between 0 °C and 30 °C, with temperatures below 0 °C and above 30 °C adjusted to 0 °C, as plants are dormant at these extremes.
- Annual precipitation (logarithmic)
- Potential evapotranspiration ratio – the ratio of mean annual potential evapotranspiration to mean annual precipitation.
The system is typically represented as a two-dimensional array of hexagons within a triangular frame. It defines approximately 120 distinct life zones globally. Unlike Merriam's system, Holdridge's system uses biotemperature rather than elevation as a primary factor and is considered more applicable to tropical and subtropical vegetation. The Holdridge system has been used extensively for mapping ecosystems, assessing potential changes in vegetation patterns due to global warming, and evaluating land-use potential.
Other Uses
In the context of the Search for Extraterrestrial Intelligence (SETI), "life zone" (also called the habitable zone) refers to the distance from a star where temperatures allow liquid water to exist on an orbiting planet's surface—a prerequisite for life as it is currently understood.
Summary
The term "life zone" is an established concept in ecology and biogeography, referring to biogeographic divisions based on climate and the characteristic life forms they support. The two major systems—Merriam's (based on temperature and elevation) and Holdridge's (based on biotemperature, precipitation, and evapotranspiration)—provide frameworks for classifying terrestrial ecosystems. The concept has also been adopted in astrobiology to describe circumstellar habitable zones.
