Life zone
A life zone is a biogeographic region defined by characteristic assemblages of plant and animal species that thrive under specific climatic conditions, particularly temperature and precipitation, often varying with latitude and elevation.[1] The concept was pioneered by American biologist C. Hart Merriam in 1889 to map ecological communities across North America, observing that shifts in biota with increasing elevation parallel those with increasing latitude at constant elevation.[2] Merriam's system divided North America into life zones including the Arctic (or Alpine), Hudsonian, Canadian, Transition, Upper Sonoran, and Lower Sonoran—based on mean annual temperature, with zones corresponding to varying elevation changes in mountainous regions (typically 1,000–3,000 feet or more, depending on latitude) or equivalent latitudinal bands of about 200–400 miles.[3] For instance, the Transition Zone, common in mid-elevations, features mixed coniferous forests and species like Douglas fir, while the Lower Sonoran Zone supports desert shrubs and cacti adapted to arid heat.[4] This framework emphasized temperature as the primary driver of species distribution, influencing early wildlife management and agricultural zoning.[2] In the mid-20th century, ecologist L.R. Holdridge expanded the life zone concept into a global bioclimatic classification system, incorporating biotemperature (effective temperature excluding frost periods), annual precipitation, and the ratio of potential evapotranspiration to precipitation to delineate up to 120 distinct zones worldwide.[5] Holdridge's triangular life zone diagram uses logarithmic scales to plot these factors, enabling predictions of vegetation formations from basic climate data and highlighting moisture's role alongside heat in shaping ecosystems, such as tropical wet forests or subtropical dry woodlands.[5] This approach has been applied in land-use planning, conservation, and climate modeling, though both Merriam's and Holdridge's systems have evolved with modern biome classifications that account for additional variables like soil and disturbance.[6]Concept and History
Definition and Principles
A life zone is defined as a geographic region characterized by similar climatic conditions that support distinct assemblages of plant and animal species, forming recognizable biotic communities.[7] These zones arise from the interplay of environmental factors, primarily temperature and precipitation, which determine the physiological limits of species distribution and community composition.[8] The key principles underlying life zones center on the strong correlation between climate variables and biotic patterns, where temperature acts as the dominant factor controlling species ranges by influencing metabolic processes, growth seasons, and survival thresholds.[7] Precipitation further modulates these patterns by affecting moisture availability, soil conditions, and habitat suitability, often creating transitions between wetter and drier zones.[5] Altitudinal and latitudinal gradients drive this zonation, as elevation mimics latitudinal changes in climate—cooler temperatures at higher altitudes parallel those at higher latitudes—leading to predictable shifts in community structure along these environmental continua.[9] The concept was formalized by C. Hart Merriam in his 1898 work on North American distributions.[7] Life zones differ from biomes, which represent larger-scale, globally recurring ecosystems defined by dominant vegetation physiognomy and broad climatic regimes across continents, whereas life zones operate on a finer, more localized scale tied specifically to climatic isotherms and precipitation gradients.[10] In contrast to ecoregions, which encompass broader landscapes incorporating geological, hydrological, and historical factors alongside climate to define areas of unique biodiversity, life zones emphasize bioclimatic homogeneity without extensive edaphic or topographic influences.[11] At their core, life zones reflect basic ecological rationale where continuous environmental gradients—such as decreasing temperature with increasing elevation or latitude—impose selective pressures that result in discrete zonation patterns, observable in mountainous terrains and across continental spans, thereby structuring biodiversity and ecosystem functions.[12] This gradient-driven organization highlights how abiotic factors filter species assemblages, promoting adaptations that align communities with prevailing climatic regimes.[13]Historical Development
The concept of life zones originated from early 19th-century observations by naturalists exploring environmental gradients. Alexander von Humboldt, during his expeditions in South America in the 1800s, documented distinct altitudinal changes in vegetation along the Andes, such as shifts from tropical forests at lower elevations to alpine tundra at higher altitudes, highlighting how temperature and elevation influence plant distributions.[14] These findings, illustrated in his 1807 Tableau Physique, provided an early empirical basis for understanding biotic zonation tied to climatic factors.[15] In the United States, the life zone idea gained formal structure through the work of biologist C. Hart Merriam in the late 19th century. Merriam's foundational field studies began with a 1889 biological survey of the San Francisco Peaks in Arizona, where he noted sharp transitions in flora and fauna corresponding to elevation-driven temperature differences.[16] He extended these investigations to the Cascade Range, observing similar patterns of biotic communities aligned with thermal variations across latitudes and altitudes.[7] These expeditions laid the groundwork for a systematic classification of North American ecosystems. Merriam's key contribution came in 1898 with the publication of Life Zones and Crop Zones of the United States, a U.S. Department of Agriculture bulletin that defined life zones as regions of comparable climate supporting similar plant and animal assemblages.[7] In this work and subsequent refinements that year, Merriam emphasized temperature gradients—particularly annual means and extremes—as the dominant factor controlling zonal boundaries, proposing that zones migrate latitudinally with isothermal lines.[7] The mid-20th century saw expansion of the concept beyond temperature alone. In 1947, ecologist Leslie R. Holdridge published "Determination of World Plant Formations from Simple Climatic Data" in Science, introducing a global bioclimatic scheme that integrated biotemperature, annual precipitation, and potential evapotranspiration to classify life zones on a worldwide scale.[17] This triangular diagram-based system addressed limitations in regional models by accounting for moisture regimes alongside heat. Post-1947 developments included regional adaptations of Holdridge's framework to better fit local conditions. In Central Europe, 20th-century applications recalibrated the system using Carpathian climate data to map vegetation shifts and assess historical climatic influences on zonal distributions.[18] Similarly, refinements for the Mediterranean incorporated seasonal precipitation variability, as noted in Holdridge's later analyses of southern European biomes, enabling more precise delineations of dry and subtropical zones.[5]Merriam's Life Zone System
Methodology and Temperature Focus
C. Hart Merriam's methodology for classifying life zones centered on the principle that temperature serves as the primary climatic factor controlling the geographic distribution of terrestrial animals and plants, with zones delineated by distinct annual temperature ranges during the growth and reproduction period. He established a minimum physiological threshold of 43°F (6°C), below which biological activity ceases, and calculated the total effective temperature—or "sum of heat"—by accumulating daily mean temperatures above this threshold from the onset of spring to the close of fall, typically spanning about 200 days. Southward zone boundaries were further defined by the mean temperature of the six hottest consecutive weeks, reflecting the limiting influence of summer heat on boreal species. This approach equated latitudinal and altitudinal gradients, employing a rule of thumb that a change of 1° latitude corresponds to a 400-foot change in elevation in terms of temperature effect, allowing for parallel zoning schemes across plains and mountains.[19][20][21] Specific temperature thresholds marked the boundaries of Merriam's zones; for instance, the Arctic-Alpine Zone corresponded to regions where the mean temperature of the six hottest weeks falls below 50°F (10°C), while the Transition Zone—representing the warmest boreal region—was delimited by a total heat sum of at least 10,000°F (5,500°C) but with summer means not exceeding 71.6°F (22°C). These metrics derived from simplified annual temperature profiles, where the Arctic-Alpine Zone often aligns with mean annual temperatures below 0°F in extreme high-elevation or northern locales, and the Transition Zone with means roughly between 48°F and 64°F, emphasizing the zone's role as a temperate bridge between colder and warmer biotas. Merriam's framework thus prioritized mean annual and seasonal temperature data to create isothermal maps of life zones, avoiding more complex variables to focus on thermal controls.[19][20][21] The foundational data for this methodology stemmed from extensive U.S. biological surveys conducted in the 1890s, detailed in his 1898 USDA Bulletin No. 10, particularly those targeting the diverse elevational gradients of western North American mountains, such as the San Francisco Peaks in Arizona and the Sierra Nevada. These field expeditions, supported by the U.S. Department of Agriculture and the U.S. Weather Bureau, collected meteorological records and biotic inventories to correlate species distributions with temperature contours, enabling the initial mapping of life zones across the continent. However, the system's scope remained primarily applicable to North America, as it was calibrated using regional climate patterns and overlooked precipitation's influence on vegetation, rendering it less effective in arid or humid extralimital contexts.[19][21][20]Classification of Zones
Merriam's life zone system delineates seven primary zones across North America, ordered from warmer to cooler climates and corresponding to latitudinal or elevational gradients. These zones are primarily distinguished by temperature regimes, with each supporting unique assemblages of plants and animals adapted to specific thermal conditions, precipitation patterns, and topography. Boundaries between zones often occur as ecotones, transitional areas where species from adjacent zones intermingle, facilitating gradual shifts in community composition rather than abrupt changes.[7] The following table summarizes the key zones, their approximate annual mean temperature ranges (derived from observational data in representative regions like the southwestern U.S.), typical elevational equivalents in mountainous areas, and characteristic biota. Temperature ranges reflect Merriam's emphasis on the mean temperature during the warmest six weeks, but annual means provide contextual scale; overlaps occur due to variations in latitude, elevation, and local microclimates.[7][3]| Zone | Approx. Annual Mean Temperature (°F) | Elevational Equivalent (ft, in Southwest U.S.) | Characteristic Vegetation | Characteristic Fauna |
|---|---|---|---|---|
| Tropical | >74 | Near sea level to 1,000 | Royal palm, mango | Jaguar, caracara eagle |
| Lower Sonoran (Lower Austral) | 60–80 | Sea level–4,000 | Creosote bush, saguaro cactus, mesquite | Mockingbird, cotton rat, roadrunner |
| Upper Sonoran (Upper Austral) | 50–65 | 4,000–7,000 | Piñon pine, juniper, sagebrush, oak brush | Opossum, burrowing owl, mule deer |
| Transition | 40–50 | 7,000–8,000 | Ponderosa pine, Douglas fir, oak | Bobwhite quail, bluebird, elk |
| Canadian | 32–45 | 8,000–10,000 | Spruce, fir, aspen, wild berries | Lynx, porcupine, moose |
| Hudsonian | 27–32 | 10,000–11,500 | Engelmann spruce, alpine fir, subalpine meadows | Wolverine, marmot, ptarmigan |
| Arctic-Alpine | <27 | >11,500 (above treeline) | Lichens, mosses, dwarf willow, arctic poppy | Snow bunting, polar bear, arctic fox |