Forest Atlas Of The United States

Ecological Divisions of North America

The earth’s surface can be broken into progressively uniform units based upon climate, plants, and soil; some classification systems even include human management. The Forest Service’s system of classifying the landscape starts at the broadest scale with four domains defined by precipitation and temperature: polar, humid, dry, and humid tropical. Divisions further subdivide domains, and are identified by more detailed separation in precipitation amount and pattern, along with temperature. These classifications are useful tools for understanding ecological variation across the landscape.

Managers, scientists, and others involved with ecological issues use a wide variety of classification systems. In biology, plants and animals are classified to better identify and understand their similarities and differences. Similarly, landscape ecologists divide the earth’s surface using progressively smaller and more uniform units to facilitate resource management across the landscape. These units — domains, divisions, provinces, sections — are important to forests because similar landscapes support similar types of forests.

Landscapes are generally classified based upon climate, vegetation, and soil. A few management-oriented classifications include agricultural crops and other human factors. The importance of each factor varies among units, but climate is the starting point for dividing the landscape in this system. Temperature and available moisture, and the timing of each, influence both plant communities and soil development. And while a site’s climate is strongly related to latitude and its location on the continent (near the ocean or deep in the interior), mountains have a profound influence and create their own ecosystems.

When applied at broader scales, landscape classification systems can be used to put ecosystems into an international context. These systems also facilitate management of environmental issues occurring over large areas or transcending political and agency boundaries.

As landscape units get smaller, they also become more homogenous. For the sake of simplicity, this map focuses upon ecological divisions, a midpoint in the Forest Service’s landscape classification system.
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Rainforest
Rainforest
Rainforest Rainforest region The Rainforest ecological division is characterized by high levels of precipitation in a tropical climate.
Marine
Marine
Marine Marine region Situated on the Pacific coast, the Marine ecological division receives abundant rainfall from maritime air masses and has a rather narrow range of temperatures because it borders on the ocean.
Temperate steppe
Temperate steppe
Temperate steppe Temperate steppe region Temperate steppes are areas with a semiarid continental climatic regime in which, despite summer rainfall, evaporation usually exceeds precipitation.
Prairie
Prairie
Prairie Prairie region Prairie ecosystem vegetation is dominated by tall grasses, and woody vegetation is nearly absent.
Hot continental
Hot continental
Hot continental Hot continental region The Hot Continental ecological division has hot summers and cool winters. The vegetation is deciduous forest, dominated by tall broadleaf trees that provide a continuous dense canopy in summer.
Warm continental
Warm continental
Warm continental Warm continental region Needleleaf and mixed needleleaf — deciduous forests dominate the Warm Continental ecological division that grow throughout the colder northern parts of the humid continental climate zone.
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The tremendous variation across ecological divisons in the United States is a result of different amounts of precipitation and energy from the sun. These highly diverse landscapes support plant communities uniquely adapted to live there.

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Icecap Tundra Subarctic Warm Continental Hot Continental Subtropical Marine Prairie Mediterranean Tropical/Subtropical Steppe Tropical/Subtropical Desert Temperate Steppe Temperate Desert Savanna Rainforest Mountains with altitudinal zonation

ECOLOGICAL DIVISIONS OF CANADA, THE UNITED STATES, AND MEXICO. A strong temperature gradient from north to south is evident in this map with tundra and subarctic divisions in the north transitioning to subtropical, rainforest, and desert divisions in the south. A precipitation gradient from the East Coast across the Great Plains to the Rocky Mountains is also visible. A thin strip of marine ecosystems hug the Pacific coast, and the wet winters and dry summers of California are captured by the Mediterranean division. Mountains across the continent (noted by hash marks on the map) create large variations in climate and vegetation over short distances.

AREA OF FOREST LAND (million acres) AREA OF FOREST LAND (million acres) 0 20 40 60 80 100 120 140 160 ECOLOGICAL DIVISION Warm Continental Savanna Temperate Desert Temperate Steppe Tropical/Subtropical Desert Tropical/Subtropical Steppe Mediterranean Prairie Marine Subtropical Hot Continental
ECOLOGICAL DIVISION NAME MountainTOTAL Protected Unproductive Productive

AREA OF FOREST LAND IN THE CONTERMINOUS US BY ECOLOGICAL DIVISION. Forests are not distributed evenly across all ecological divisions of the conterminous US. Forests are the common natural vegetation in the subtropics, but trees are generally absent in the temperate steppe. Forests in western divisions are found largely in the mountains. Additionally, most productive forest land area is found in the eastern and southern US. It is no surprise that the subtropics are known as the nation’s “wood basket”.

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