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Life zone
Life zone
Life zone
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The life zone concept was developed by C. Hart Merriam in 1889 as a means of describing areas with similar plant and animal communities. Merriam observed that the changes in these communities with an increase in latitude at a constant elevation are similar to the changes seen with an increase in elevation at a constant latitude.[1]

Merriam

[edit]

The life zones Merriam identified are most applicable to western North America, being developed on the San Francisco Peaks, Arizona and Cascade Range of the northwestern USA. He tried to develop a system that is applicable across the North American continent, but that system is rarely referred to.

List

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The life zones that Merriam identified, along with characteristic plants, are as follows:

The Canadian and Hudsonian life zones are commonly combined into a Boreal life zone.

Criticism

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This system has been criticized as being too imprecise. For example, the scrub oak chaparral in Arizona shares relatively few plant and animal species with the Great Basin sagebrush desert, yet both are classified as Upper Sonoran. However it is still sometimes referred to by biologists (and anthropologists) working in the western United States. Much more detailed and empirically based classifications of vegetation and life zones now exist for most areas of the world, such as the list of world ecoregions defined by the World Wide Fund for Nature,[2] or the list of North American ecoregions defined by the Commission for Environmental Cooperation.[3]

Holdridge

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Main article: Holdridge life zones
Holdridge life zone classification scheme. Although conceived as three-dimensional by its originator, is usually shown as a two-dimensional array of hexagons in a triangular frame.

In 1947, Leslie Holdridge published a life zone classification using indicators of:

  • mean annual biotemperature (logarithmic)
  • annual precipitation (logarithmic)
  • ratio of annual potential evapotranspiration to mean total annual precipitation.

Biotemperature refers to all temperatures above freezing, with all temperatures below freezing adjusted to 0 °C, as plants are dormant at these temperatures. Holdridge's system uses biotemperature first, rather than the temperate latitude bias of Merriam's life zones, and does not primarily use elevation. The system is considered more appropriate to the complexities of tropical vegetation than Merriam's system.[4]

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Life zone

View on Grokipedia
from Grokipedia
A life zone is a biogeographic region defined by characteristic assemblages of plant and animal that thrive under specific climatic conditions, particularly and , often varying with and . The concept was pioneered by American biologist C. Hart Merriam in 1889 to map ecological communities across , observing that shifts in biota with increasing parallel those with increasing at constant . Merriam's system divided into life zones including the (or Alpine), Hudsonian, Canadian, Transition, Upper Sonoran, and Lower Sonoran—based on mean annual temperature, with zones corresponding to varying changes in mountainous regions (typically 1,000–3,000 feet or more, depending on latitude) or equivalent latitudinal bands of about 200–400 miles. For instance, the Transition Zone, common in mid-s, features mixed coniferous forests and like , while the Lower Sonoran Zone supports desert shrubs and cacti adapted to arid heat. This framework emphasized temperature as the primary driver of , influencing early and agricultural zoning. In the mid-20th century, ecologist L.R. Holdridge expanded the life zone concept into a global bioclimatic classification system, incorporating biotemperature ( excluding frost periods), annual , and the ratio of potential evapotranspiration to to delineate up to 120 distinct zones worldwide. Holdridge's triangular life zone uses logarithmic scales to plot these factors, predictions of formations from basic climate data and highlighting moisture's role alongside heat in shaping ecosystems, such as tropical wet forests or subtropical dry woodlands. This approach has been applied in , conservation, and climate modeling, though both Merriam's and Holdridge's systems have evolved with modern classifications that account for additional variables like and disturbance.

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. These zones arise from the interplay of environmental factors, primarily temperature and precipitation, which determine the physiological limits of species distribution and community composition. The key principles underlying life zones center on the strong between climate variables and biotic patterns, where acts as the dominant factor controlling ranges by influencing metabolic processes, growth seasons, and thresholds. further modulates these patterns by affecting moisture availability, conditions, and suitability, often creating transitions between wetter and drier zones. Altitudinal and latitudinal gradients drive this zonation, as mimics latitudinal changes in —cooler temperatures at higher altitudes parallel those at higher latitudes—leading to predictable shifts in community structure along these environmental continua. The concept was formalized by C. Hart Merriam in his 1898 work on North American distributions. Life zones differ from biomes, which represent larger-scale, globally recurring ecosystems defined by dominant physiognomy and broad climatic regimes across continents, whereas life zones operate on a finer, more localized scale tied specifically to climatic isotherms and gradients. In contrast to ecoregions, which encompass broader landscapes incorporating geological, hydrological, and historical factors alongside to define areas of unique , life zones emphasize bioclimatic homogeneity without extensive edaphic or topographic influences. At their core, life zones reflect basic ecological rationale where continuous environmental gradients—such as decreasing with increasing or latitude—impose selective pressures that result in discrete zonation patterns, observable in mountainous terrains and across continental spans, thereby structuring and functions. This gradient-driven organization highlights how abiotic factors filter assemblages, promoting adaptations that align communities with prevailing climatic regimes.

Historical Development

The concept of life zones originated from early 19th-century observations by naturalists exploring environmental gradients. , during his expeditions in in the 1800s, documented distinct altitudinal changes in vegetation along the , such as shifts from tropical forests at lower elevations to at higher altitudes, highlighting how and influence distributions. These findings, illustrated in his 1807 Tableau Physique, provided an early empirical basis for understanding biotic zonation tied to climatic factors. In the United States, the life zone idea gained formal structure through the work of biologist C. Hart Merriam in the late . Merriam's foundational field studies began with a 1889 biological survey of the in , where he noted sharp transitions in flora and corresponding to elevation-driven temperature differences. He extended these investigations to the , observing similar patterns of biotic communities aligned with thermal variations across latitudes and altitudes. These expeditions laid the groundwork for a systematic 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. 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. 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 , and potential evapotranspiration to classify life zones on a worldwide scale. 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 , 20th-century applications recalibrated the system using Carpathian climate data to map vegetation shifts and assess historical climatic influences on zonal distributions. Similarly, refinements for the Mediterranean incorporated seasonal variability, as noted in Holdridge's later analyses of southern European biomes, enabling more precise delineations of dry and subtropical zones.

Merriam's Life Zone System

Methodology and Temperature Focus

C. Hart Merriam's methodology for classifying life zones centered on the principle that serves as the primary climatic factor controlling the geographic distribution of terrestrial animals and , with zones delineated by distinct annual ranges during the growth and period. He established a minimum physiological threshold of 43°F (6°C), below which ceases, and calculated the total —or "sum of "—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 of the six hottest consecutive weeks, reflecting the limiting influence of summer on boreal species. This approach equated latitudinal and altitudinal gradients, employing a that a change of 1° corresponds to a 400-foot change in in terms of effect, allowing for parallel zoning schemes across plains and mountains. Specific thresholds marked the boundaries of Merriam's zones; for instance, the Arctic-Alpine Zone corresponded to regions where the mean 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 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 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 data to create isothermal maps of life zones, avoiding more complex variables to focus on thermal controls. The foundational data for this methodology stemmed from extensive U.S. biological surveys conducted in the , detailed in his 1898 USDA Bulletin No. 10, particularly those targeting the diverse elevational gradients of western n mountains, such as the in 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 , 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.

Classification of Zones

Merriam's life zone system delineates seven primary zones across , 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 and animals adapted to specific thermal conditions, patterns, and . Boundaries between zones often occur as ecotones, transitional areas where from adjacent zones intermingle, facilitating gradual shifts in community composition rather than abrupt changes. The following table summarizes the key zones, their approximate annual mean ranges (derived from observational in representative regions like the southwestern U.S.), typical elevational equivalents in mountainous areas, and characteristic biota. ranges reflect Merriam's emphasis on the mean during the warmest six weeks, but annual means provide contextual scale; overlaps occur due to variations in , , and local microclimates.
ZoneApprox. Annual Mean (°F)Elevational Equivalent (ft, in Southwest U.S.)Characteristic VegetationCharacteristic Fauna
Tropical>74Near to 1,000Royal palm, , caracara eagle
Lower Sonoran (Lower Austral)60–80–4,000 bush, cactus, mesquite, ,
Upper Sonoran (Upper Austral)50–654,000–7,000Piñon , , , oak brush, ,
Transition40–507,000–8,000Ponderosa , , Bobwhite quail, ,
Canadian32–458,000–10,000, , aspen, wild berries, ,
Hudsonian27–3210,000–11,500Engelmann , alpine , subalpine meadows, , ptarmigan
Arctic-Alpine<27>11,500 (above treeline)Lichens, mosses, dwarf , poppy, ,
In the Lower Sonoran zone, hot desert scrub dominates arid lowlands, where drought-tolerant succulents like cacti thrive alongside small mammals and birds adapted to extreme heat and low water availability. Moving upslope or northward, the Upper Sonoran zone features semi-arid grasslands and woodlands, supporting herbivores like that graze on shrubs and scattered trees. The Transition zone represents montane forests with mixed , providing habitat for forest-dwelling birds and larger ungulates that migrate seasonally. Higher boreal zones, such as Canadian and Hudsonian, host dense coniferous stands suited to cooler, moister conditions, with large mammals like navigating understories rich in berries and fungi. The Arctic-Alpine zone, stark and treeless, sustains low-growing perennials and cold-hardy vertebrates in perpetual harshness. These biota exemplify how controls , with ecotones like forest-meadow interfaces allowing hybrid communities to form.

Applications and Criticisms

Merriam's life zone system found practical applications in early 20th-century U.S. , where it guided the of timber resources and types across national forests, helping to delineate areas for sustainable harvesting based on climatic gradients. The system's temperature-based also supported mapping by the U.S. Biological Survey, which produced life-zone maps of to identify distributions of plant and animal species, informing early federal conservation efforts such as the establishment of forest reserves totaling 148 million acres by 1908. In , the framework was employed by the USDA for crop zoning, as detailed in Merriam's 1898 bulletin, which mapped seven transcontinental zones to recommend suitable crops and reduce experimental failures; for instance, it identified the Transition Zone for wheat varieties like Red Fife and the Lower Austral Zone for and sugar cane, with adaptations in arid regions such as Utah's St. George Valley yielding 123 bales of in 1896–97. Educational uses emerged through these surveys, promoting awareness of bioclimatic patterns in studies. A notable example is its application in elevation studies, where Merriam's 1889 expedition for the U.S. Biological Survey observed zone transitions from Lower Sonoran (cacti at canyon depths) to Canadian (spruce-fir forests at higher rims), equating 1,000 feet of elevation to approximately a 200-mile latitudinal shift in biota. Criticisms of the system center on its overemphasis on temperature, which neglects precipitation variability and leads to lumping dissimilar habitats; for example, the Lower Sonoran Zone encompasses both arid deserts and moister woodlands with starkly different rainfall regimes, obscuring ecological distinctions. The zones are also deemed too broad for fine-scale ecology, failing to account for microclimates influenced by topography or soil, rendering them inadequate for detailed habitat analysis. While influential in early conservation by providing a foundational framework for land-use planning and species distribution mapping, the system has been largely superseded by multi-factor approaches that incorporate humidity, precipitation, and other variables for greater accuracy.

Holdridge Life Zone System

Methodology and Bioclimatic Parameters

The Holdridge life zone system employs a bioclimatic classification framework that integrates three key climatic parameters to delineate global vegetation formations: biotemperature, annual precipitation, and the ratio of potential evapotranspiration (PET) to precipitation. Biotemperature serves as a measure of effective heat for biological activity, calculated as the sum of monthly mean temperatures exceeding 0°C divided by 12, thereby excluding periods of frost that limit plant growth. Annual precipitation represents the total water input in millimeters, encompassing rain, snow, and other forms but excluding dew or fog. The PET ratio, which indicates moisture balance and humidity conditions, is derived by dividing estimated PET by annual precipitation; PET itself is approximated through empirical relations, such as PET ≈ 58.93 × biotemperature (in °C), reflecting potential water loss under non-limiting conditions. This methodological innovation, introduced by Leslie R. Holdridge in his 1947 paper, utilizes a triangular for visualization, where biotemperature is plotted on a logarithmic vertical axis, annual precipitation on a logarithmic horizontal axis (increasing from left to right), and diagonal lines represent contours of constant PET ratio (increasing from right to left). The logarithmic scaling accommodates the wide range of global climatic variability, enabling precise delineation of life zone boundaries without reliance on complex computations. Unlike C. Hart Merriam's earlier temperature-centric approach, Holdridge's multi-variable model accounts for both thermal and hydrologic influences on ecosystems. Designed for terrestrial applications worldwide, the system proves particularly effective in classifying life zones across tropical, subtropical, and temperate regions by linking simple climatic data to patterns and associated biota. Calculations typically draw from long-term monthly averages to ensure robustness against interannual variability, supporting applications in ecological mapping and .

Classification Scheme and Diagram

The Holdridge life zone classification scheme employs a triangular diagram, known as the Holdridge triangle, to delineate ecosystems based on climatic variables. The vertical axis represents biotemperature on a from 0 to 30°C, capturing annual heat availability while excluding frost and extreme heat periods. The horizontal axis denotes annual on a from 0 to 8,000 mm, reflecting moisture input. A diagonal axis incorporates the potential evapotranspiration ratio (PET/), divided into nine intervals ranging from 0.01 (excessively wet) to 100 (arid), which collectively form the boundaries of 37 distinct life zones through hexagonal and triangular subdivisions. Points within the triangle are assigned to specific zones by their intersection with these axes and ratio lines; for instance, regions with high biotemperature (20–30°C), high (>2,000 mm), and low PET ratios (<0.5) fall into the tropical rainforest zone, characterized by multi-story evergreen forests with diverse epiphytes and lianas, while low biotemperature (0–5°C), low (250–500 mm), and moderate PET ratios (1–2) correspond to tundra, featuring sparse lichens, mosses, and permafrost-adapted shrubs. Key zones include the warm temperate dry forest (biotemperature 10–20°C, 500–1,000 mm, PET ratio 2–4), dominated by drought-tolerant oaks and pines with grassy understories, and the subtropical thorn woodland (biotemperature 15–25°C, 250–500 mm, PET ratio 4–8), supporting thorny acacias and succulents in semi-arid conditions. These zones are further subdivided into associations influenced by local factors such as soil type or topography—climatic (zonal vegetation), edaphic (soil-limited), atmospheric (exposure-related), or hydric (water-influenced)—and successional stages from pioneer communities on disturbed sites to climax formations. The diagram's logarithmic scaling ensures equitable representation across global climatic gradients, with transitional areas between hexagons indicating ecotones where zones blend. Refinements to zone boundaries, as applied in global classifications, adjusted ratios and thresholds for better alignment with observed vegetation patterns, such as narrowing arid zone limits to account for montane effects. Zonal biota vary distinctly; for example, the boreal wet forest (biotemperature 5–10°C, precipitation >1,000 mm, PET ratio <1) hosts tall like and in high-rainfall settings, supporting mossy understories and adapted to cool, moist environments.

Global Applications and Limitations

The Holdridge Life Zone System has been widely applied in global mapping efforts to classify terrestrial ecosystems based on bioclimatic parameters. Prominent examples include the 1992 UNEP-WCMC global raster map of derived from climatic data and a 2002 map for the conterminous identifying 38 life zones, with applications extended to regions like for broader ecological analysis. This system supports applications in by predicting potential for , in biodiversity assessments to identify conservation priorities across biomes, and in GIS modeling to integrate climatic layers for . Practical implementations include the classification of montane zones in , where the system delineates transitions from tropical wet forests to premontane rain forests using mean annual biotemperature and data, aiding in mapping. In African savannas, primarily falling within the tropical dry forest life zone, Holdridge's framework has been used to correlate climatic gradients with patterns, such as grass-dominated areas in semi-arid conditions. Furthermore, integration with technologies, like NOAA/AVHRR data, enables predictions of distribution by overlaying Holdridge parameters on raster imagery for large-scale monitoring. Despite its utility, the Holdridge system has notable limitations stemming from its foundational assumptions. It presumes an equilibrium climate based on annual averages, neglecting and transient dynamics that influence response. The model also overlooks properties and availability, which can lead to inaccuracies in zones like humid grasslands, and it simplifies or ignores human impacts such as land-use changes from and . Accurate implementation requires high-resolution climatic data, posing challenges in data-poor regions where station coverage is sparse or unreliable. In the , digital adaptations have enhanced the system's flexibility for dynamic modeling, incorporating time-series projections to simulate life zone shifts without relying solely on static equilibria. These updates, often using GIS and reanalysis datasets, allow for iterative assessments of potential under varying scenarios. As of 2025, the system continues to inform , including refinements for global ecological (2023) and uncertainty assessments in classifications (Elsner et al., 2025).

Comparisons and Modern Uses

Differences Between Major Systems

The Merriam life zone system, developed in 1898, is fundamentally unidimensional, relying primarily on annual temperature ranges to delineate ecological divisions, with a focus on North American regions where elevation drives temperature gradients. In contrast, the Holdridge system, introduced in 1947, employs a three-dimensional approach incorporating biotemperature (annual thermal efficiency excluding extremes below 0°C and above 30°C), annual precipitation, and the ratio of potential evapotranspiration to precipitation, enabling a global classification of up to 120 life zones. This shift from Merriam's temperature-centric model to Holdridge's multifaceted bioclimatic framework addresses limitations in capturing moisture influences on vegetation and fauna. Merriam's emphasis on simplicity facilitates its application in altitudinal studies, particularly for mapping faunal distributions along temperature gradients in mountainous terrains of , though it overlooks quantitative effects. Holdridge's model, by integrating , provides greater comprehensiveness for predicting patterns worldwide, allowing for precise correlations between data and through its diagrammatic representation. These strengths reflect their respective scopes: Merriam's for regional zoological analysis and Holdridge's for broader, data-driven ecological forecasting. Both systems acknowledge interactions between and in shaping life zones, but Holdridge quantifies these through logarithmic scales and the potential evapotranspiration ratio, resulting in finer subdivisions of Merriam's coarser categories—for example, aligning temperature-based zones with provinces to distinguish moist from dry variants within similar thermal regimes. This overlap underscores a shared recognition of climatic controls on biota, yet Holdridge's approach refines Merriam's by incorporating objective metrics derived from field validations in diverse regions. Historically, Holdridge built upon Merriam's foundational work by critiquing its subjectivity—rooted in observed animal distributions rather than measurable climatic variables—and its regional bias, which failed to account for global variations in moisture and . By introducing biotemperature and a triangular scheme, Holdridge expanded the framework into a more universal tool, validated through extensive Latin American studies since the , while retaining as a core axis to bridge with earlier models.

Integration with Broader Ecology

Life zones serve as finer-scale subdivisions within broader biomes, capturing variations in vegetation and ecosystems driven by local climatic gradients. For instance, the Holdridge life zone system delineates sub-units such as boreal dry scrub and polar moist tundra within the overarching tundra biome, allowing for more precise mapping of ecological transitions. Robert H. Whittaker's 1975 classification provides a conceptual bridge between life zones and biomes by employing two-dimensional graphs of annual precipitation and temperature to delineate vegetation formations, mirroring the bioclimatic approach of Holdridge while emphasizing continuum-based biome transitions from desert to rainforest. In ecoregional frameworks, life zones integrate with physiographic features to define hierarchical ecological units. Robert G. Bailey's ecoregions of the , developed in the 1990s and revised in , combine elements of Merriam's and Holdridge's life zones—such as altitudinal bands and regimes—with landform characteristics like and to classify provinces like the Adirondack- Mixed Forest-Coniferous Forest-Alpine . This synthesis enables multi-scale analysis, where life zones nest within ecoregions to account for both climatic and geomorphic influences on patterns. Other classification systems complement life zones by providing abiotic foundations or regional adaptations. The Köppen-Geiger climate classification establishes temperature and precipitation thresholds for global zones, serving as an underlying abiotic template that life zones extend through biotic responses like vegetation structure and species composition. In , altitudinal variants of life zones, such as the planar (lowland forests), montane (beech-conifer belts), and alpine (shrub-herb zones) in the and Carpathians, adapt Holdridge-like principles to local and soils, emphasizing elevational gradients in coniferous and forests. These integrations facilitate synergies in multi-scale , particularly for conservation planning, by nesting life zones within biomes and ecoregions to identify refugia and prioritize interventions. For example, analyses of Holdridge zones within 847 global ecoregions reveal hotspots of stability amid climatic variability, guiding strategies to protect nested ecosystems like boreal forests spanning multiple ecoregional units. Such approaches enhance hierarchical conservation by aligning fine-scale biotic details with broad-scale biogeographic planning.

Impacts of Climate Change

is driving observable shifts in life zones worldwide, with many ecosystems exhibiting upward altitudinal migration as temperatures rise. In mountainous regions of the , such as the Sierra Nevada and , vegetation and species distributions have shifted upward at rates averaging 10-20 meters per since the mid-20th century, reflecting a compression of lower-elevation zones and expansion at higher altitudes. These patterns align with broader global trends where montane life zones are contracting due to warming, as documented in meta-analyses of range shifts. Dynamic applications of the Holdridge life zone system have been integral to climate modeling, particularly in IPCC assessments and related projections. For instance, simulations using Holdridge parameters under IPCC emission scenarios (e.g., RCP4.5 and RCP8.5) indicate that up to 42.6% of global land area could experience significant life zone transitions by 2070, with accelerated changes under high-emission pathways leading to zone compression in the where and gradients are narrow. In , Holdridge-based models project shifts in zones due to increased and altered rainfall patterns. Particular vulnerabilities emerge in boreal and montane life zones, which face disproportionate risks from warming and associated disturbances. Boreal forests, spanning northern high latitudes, are projected to decrease by approximately 284 million hectares by 2080 under moderate emissions as and zones shift poleward or fragment, exacerbating risks and thaw. Montane zones in temperate and tropical mountains are similarly at risk, with 58% of montane forests in the Peruvian potentially impacted by 2070 through elevational compression and . A stark example is the , where portions of the tropical moist life zone are transitioning toward drier savanna-like conditions under combined and pressures. Despite these insights, significant research gaps persist in life zone modeling under . Current Holdridge applications often overlook direct CO2 fertilization effects on and interactions with non-climatic disturbances like and land-use change, necessitating finer-resolution dynamic models that integrate these factors for more accurate vulnerability assessments. Additionally, long-term empirical on zone resilience in underrepresented regions, such as the and poles, remain limited, hindering robust predictions of tipping points. Recent 2025 analyses using on classifications project that 16–19% of terrestrial surface has uncertain trajectories by 2080 under high emissions, highlighting ongoing uncertainties in boreal and shifts.

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