The diversity of forest types in North America is extensive and comprises a cross section of forest types from the northern boreal to the subtropical regions. A thorough and detailed description of all forest types found in North America is not the intent of this encyclopedia. Rather, a generalized description of the broad categories of types across the North American continent is the objective. North American forest types are found from the boreal regions of Canada at the northern limit of tree growth to the southern most regions of Mexico where tropical and subtropical forests predominate. Not only is there a great expanse of latitude across this region, there is extreme variation in altitude, soils, precipitation, and other site characteristics all of which combine to produce a wide diversity of forest types.
A forest type is defined by a common assemblage of species. The distribution of individual species is influenced by the same site characteristics described above. Each species responds uniquely to these factors, creating a mosaic on the landscape. Two bordering forest types rarely have abrupt margins, but often transition gradually in areas known as “ecotones.” Two adjacent forest types may even share many of the same species, making them difficult to distinguish. The classification system presented in this article will focus on the dominant tree species found within each forest type.
The distribution of forest types in North America is primarily influenced by latitude. Boreal forests are found at the highest latitudes, extensively in Canada and Alaska, and are in the majority comprised of conifers or evergreens, such as spruce (Picea spp.), firs (Abies spp.), and pines (Pinus spp.). Temperate forests are at the mid-latitudes in North America and consist of pure stands and mixtures of conifers and deciduous species (non-evergreen species). Examples of these include oaks (Quercus spp.), maples (Acer spp.), ashes (Fraxinus spp.), and pines. Although there are tropical forest types within the southern portions of North America, we believe the forest types of Central America merit their own article and we will not discuss them here.
The North American continent has had a long history of settlement and exploitation, with many areas being settled long before the Europeans arrived. The Europeans have drastically impacted the landscape and forests in particular and these impacts have continued to the present. Original forests have been exploited and much of the landscape has been converted to agriculture and other forms of development. However, there remains considerable area in forest cover with many parts of Canada and the United States supporting natural forests similar to the original types. The extent of changes that have taken place in North America vary widely across the continent with some areas much more impacted by human settlement and exploitation than others. In this article, we will describe the forest types as they exist today and not attempt to describe the original forest cover prior to European settlement.
Despite the impact of human exploitation and contrary to popular belief, plantations comprise less than 2% of the forests of North America with over 98% of the forest cover being of natural origin. According to the Food and Agriculture Organization of the United Nations, Canada has 33.6% of its total land area in forests, which is comprised of 26% temperate and 76% boreal forest types. The United States has 33.1 % of its land area in forests, which is comprised of 37% subtropical, 48% temperate, and 15% boreal forest types. Mexico has 33.7% of its total land area in forest with subtropical forests comprising 30% and the remainder being tropical.
The following broad classification of forest types is based on the distribution of “natural forest types,” created when the North American continent was subdivided by seas, mountain ranges, deserts, grasslands or glaciers. Within these broad types are many subtypes that are adapted to specific sites, which are in turn comprised of many species. We will attempt to highlight some of the major types and species in our discussion. We encourage readers to research specific forest subtypes of interest within each general forest type in order to learn more about these forests.
Figure 1 shows the forest types of North America, per Young and Giese (2003). We describe these broad areas in the text in this article.
Figure 1. Forest Types of North America (Young and Giese 2003).
The boreal forest is truly transcontinental in extent, stretching from the Atlantic Ocean in New England west throughout Canada and including much of interior Alaska. It is not only a forest type, but also considered a global biome as it is found circumpolar in the Northern Hemisphere. The region is characterized by short growing seasons (typically less than 3 months) and a cold, snowy climate. As latitude increases, species diversity decreases in this ecosystem. Most of this northern region has a history of glaciers that carved out lakes and bogs during retreat. Bogs (or muskegs) are often colonized by sphagnum moss which accumulates organic matter over time filling in the depressions and making it suitable for tree growth.
Black spruce (Picea mariana) and eastern larch or tamarack, (Larix laricina) are two coniferous species that can tolerate these organic, saturated soils. Black spruce is typified by its slender form, retention of low branches and slow growth on these sites. Eastern larch is one of the few deciduous conifers and its open, pyramidal crown allows shrubs to grow underneath. A suite of alder species (Alnus spp.) can be found on wet sites, commonly around lakes and along waterways, where they have access to abundant sunlight.
On drier sites, white spruce (Picea glauca) often out-competes black spruce as it is more tolerant of shade and can take advantage of higher nutrient availability. It often succeeds paper birch (Betula papyrifera) and quaking aspen (Populus tremuloides), two early successional hardwood trees that frequently colonize sites after a fire or clearcut harvesting. East of the Rocky Mountains, balsam fir (Abies balsamifera) is another short-lived, shade tolerant species that can be found in pure stands or mixed with white spruce.
Fire is an important force in shaping the boreal forest. Two pine species especially adapted to fire are jack pine (Pinus banksiana) east of the Rocky Mountains and lodgepole pine (Pinus contorta) to the west. Both pines can grow on sandy, poor soils where they often become established in even-aged stands. They grow in dense stands and retain dead, lower branches which creates “ladder fuels” that promote crown fires. Both pines are adapted to fire by having serotinous cones which are opened by heat following stand-replacing fires. The seeds are released onto the burnt soil, rich in nutrients and germinate to form the next generation of pine trees.
Boreal forests, especially white and black spruce, represent the northernmost extent of tree growth before giving way to arctic tundra. The tree line is not a distinct demarcation, but is often composed of small pockets of trees in protected valleys. Gallery forests of trees and shrubs like willow (Salix spp.) and alders (Alnus spp.), form along rivers and streams and stretch north into the tundra. Trees in the far north are smaller in stature, often beaten by wind and snow into a prostrate “krummholz” form. Research has shown that some trees can tolerate living north of the current treeline, they simply have not been able to colonize those areas since the glacial retreat.
Rocky Mountain Complex
Rising to over 14000 feet above the Great Plains and Great Basin Desert is the continental divide of the Rocky Mountains. These mountains stretch from northern New Mexico into Alberta and British Columbia and into Alaska to Mt. McKinley, the highest point in North America at 20320 feet. The distribution of forests in the Rocky Mountains is controlled mainly by two factors: elevation and latitude. As elevation increases, there is a decrease in average annual temperature and generally an increase in annual precipitation. Increasing latitude has a similar effect on temperature and also causes the elevation range of forest types to decrease. For example, the southern Rockies have a timberline around 11500 feet whereas in the northern Rockies near the Canadian border, timberline is above 6000 feet and only 2000 feet in Alaska. The majority of the Rocky Mountain forests have not experienced heavy logging due to their rugged geology and inaccessibility. The major disturbance agents in these forests are fire, avalanches, windthrow and insect and disease outbreaks such as those currently occurring with bark beetles and western spruce budworm. There are generally six major forest types in the Rocky Mountains that change as elevation increases, as well as riparian forests that form along rivers and streams:
- Pinyon-Juniper Woodland
- Ponderosa Pine Forest
- Aspen Grove
- Lodgepole Pine Forest
- Spruce-Fir Forest
- Subalpine Forest
- Riparian Forest
In general, the arid slopes that receive slightly more precipitation than prairies and shrublands are occupied by pinyon-juniper woodlands. These areas, though not found in the northern Rocky Mountains, are much more common on western slopes at 5000-7000 feet throughout Utah, Arizona, and New Mexico. They are dominated by two-needle pinyon pine and an array of junipers: oneseed (Juniperus monosperma), Rocky Mountain (J. scopulorum), Utah (J. osteosperma) or alligator (J. deppeana) depending on location. These trees are short, typically growing less that 20 feet tall, and do not form a continuous canopy allowing for grasses to grow between. Other small trees sometimes found in this ecosystem are ponderosa pine (Pinus ponderosa var. scopulorum), Gambel oak (Quercus gambelli) and mountain mahogany (Cercocarpus montanus). Rabbitbush (Ericameria nauseosus), sagebrush (Artemisia spp.), prickly pear cactus (Opuntia spp.) and the exotic tumbleweed or Russian thistle (Salsola tragus) are common in the interspaces between trees. There is evidence that this ecosystem was formerly maintained at low tree densities by fire, but due to fire suppression has increased density and invaded some grasslands.
Ponderosa Pine Forest
In the transition zone between dry and moist areas are found ponderosa pine forests. These are historically park-like stands of trees with bunchgrasses common between clumps of large trees up to 150-180 feet tall. As with pinyon-juniper forests, fire suppression has led to increased stem density and subsequently to crown fires and insect outbreaks. This forest is named for the Rocky Mountain variety of ponderosa pine, a drought-tolerant pine that is widely dispersed throughout western North America. It is prevalent from 4,000-8,000 feet in elevation in the Rockies. At lower elevations, it occurs with a Rocky Mountain juniper and Gambel oak understory, and at higher elevations alongside quaking aspen, lodgepole pine and inland variety of Douglas-fir (Pseudotsuga menziesii var. glauca) This forest type is also dominant in the Black Hills, east of the Rocky Mountains, along with disjunct populations of white spruce, lodgepole pine and bur oak (Quercus macrocarpa).
Quaking aspen is an early successional hardwood tree that often forms vast stands with each stem being genetically identical (clonal) that arise from root sprouts following fires and other disturbances. It predominates on moist, protected sites from 5,600 feet elevation to over 11,500 feet in the southern Rockies. Aspen reproduces mainly from root suckering but also spreads widely by seed. Large, clonal stands of over 100 acres have been documented, making it one of the largest single organisms on earth. Aspen stands turn a brilliant yellow in the fall making for spectacular scenery in the mountains. Aspen is usually replaced during natural succession with spruce, fir and other late successional species.
Lodgepole Pine Forest
Lodgepole pine is another early successional species that can form pure stands following fire. A young forest of lodgepole pine grows dense, straight and quickly but as it ages, growth slows. In the Rocky Mountains, the species can also be found in mixed stands with spruce, fir, Douglas fir and ponderosa pine between 8,500-10,000 feet. Lodgepole pine forests and aspen groves often occur in close proximity with aspen on fine textured soils, rich in calcium rather than coarse soils on granitic bedrock.
Beginning above 9000 feet in the sourthern Rocky Mountains and at lower elevations to the north are spruce-fir forests. These forests grow in cool, moist environments with 28-40 inches of precipitation per year, mainly falling as snow. The heavy snow favors the conical or pyramidal growth form of Engelmann spruce (Picea engelmanii) and subalpine fir (Abies lasiocarpa), which is replaced by corkbark fir (A. lasiocarpa var. arizonica) in the south. Other species occurring sporadically in the spruce-fir community between 6,000-11,000 feet in the Southern Rockies include a variety of white fir (Abies concolor var. concolor) and blue spruce (Picea pungens), mainly along streams and wet meadows. In the Northern Rockies, western white pine (Pinus monticola) and western larch (Larix occidentalis) occur in this forest community. In the far north the dominant species are white spruce and balsam fir.
The dry, windswept areas below tree line in the Rocky Mountains create conditions for the twisted, gnarled trees of the subalpine forests. This rocky area does not typically have continuous canopy, but rather scattered patches of trees rarely greater than 30 feet tall. Long-lived (up to 2,000 years) Rocky Mountain bristlecone pine (Pinus aristata) and limber pine (P. flexilis) are most common in the southern Rocky Mountains giving way to whitebark pine (P. albicaulis), subalpine larch (Larix lyallii) and mountain hemlock (Tsuga mertensiana) in the north. Flag trees that retain dead branches on the windward side of the crown for protection and krummholz, shrub-like trees are found surviving at timberline below the alpine tundra.
In contrast to the conifer-dominated forest types of the Rocky Mountains, riparian forests in the region are principally composed of hardwood trees with thickets of willow (Salix spp.), alder (Alnus spp.) and various vines. The drainages east of the Rockies are dominated by plains cottonwood (Populus deltoides ssp. molinifera), a subspecies of eastern cottonwood. The western slopes have Fremont cottonwood (P. fremontii) and narrowleaf cottonwood (P. angustifolia), with black cottonwood (P. trichocarpa) in the Northern Rockies , as well as netleaf hackberry (Celtis reticulata). At low elevations are found boxelder (Acer negundo), peachleaf willow (Salix amygdaloides), green ash (Fraxinus pennsylvanica), and two exotic invasive species: Russian olive (Elaeagnus angustifolia) and salt cedar (Tamarix spp.). Higher elevation, balsam poplar (P. balsamifera), quaking aspen , blue spruce (Picea pungens) and white fir (Abies concolor) are common.
Pacific Coastal Complex
The Pacific Coast of North America is very young and active in geological terms. As part of the “Ring of Fire,” earthquake and volcanic activity is common, and glaciers dramatically shaped the landscape in recent history. Despite the upheaval, the largest conifers on the planet reside in this region supported by the ample winter rainfall of a maritime climate, young volcanic soils, and a low frequency disturbance regime. Mountains like the Coast Range and Cascades, with peaks reaching 14000 feet, also have a profound impact upon the distribution of forests in the region. Not only do forest types change as elevation increases, but due to the rainshadow effect, annual precipitation is often less than 10 inches east of the mountains. Compared to annual precipitation totals often exceeding 100 inches along the coast, this creates a dramatic divergence of species and life history strategies within relatively close proximity. Land use history has also had a significant impact upon much of this region. The timber industry was extremely active from the late 1800s to the early 1980s, often clearcutting forests to allow young, vigorous trees to reestablish the sites. In recent decades, harvesting has slowed and there has been a political push to preserve the remaining areas of uncut forest. There are four general forest types common to the mountains of Oregon, Washington, and British Columbia, and three forest types that dominate the coastal fogbelt from southern Alaska south to central California:
- Northwest Oak-Pine Forest
- Northwest Riparian Forest
- Douglas-fir Forest
- Subalpine Forest
- Spruce-Hemlock Forest
- Redwood Forest
- Closed-Cone Pine Forest
Northwest Oak-Pine Forest
Found on dry, gravelly sites is the open, park-like oak-pine forest. It has a wide but sporadic range from southern Vancouver Island throughout the Willamette Valley of Oregon, then south where it merges into the oak-pine woodland of California. In a region known for its conifers, these forests are associated with the hardwoods Oregon white oak (Quercus garryana) and Pacific madrone (Arbutus menziesii). Both species can reach 50 feet in height, but can also be found in a shrub form. Pine trees common to the Northwest oak-pine forest include knobcone pine (Pinus attenuata), a short form of lodgepole pine known as shore pine near the coast, and the coastal variety of ponderosa pine (P. ponderosa var. ponderosa) found mainly to the east of the Cascade Mountains. It is believed that this forest type is increasing in density due to fire suppression.
Northwest Riparian Forest
Large rivers fed by mountain snowmelt and glacial runoff empty into the Pacific Ocean from southern Alaska to northern California. Black cottonwood (Populus trichocarpa) is found along this entire latitudinal gradient and even into the rainshadow regions of western Montana. This fast growing, shade intolerant tree can reach up to 175 feet in height and 10 feet in diameter. Rarely found 100 miles from the ocean, red alder (Alnus rubra) is another broadleaved species known for its wide latitudinal extent and rapid growth. It is also a nitrogen fixing species and locally important for timber. In the southern Cascades it is replaced by white alder (A. rhombifolia) and at high elevations by Sitka alder (A. viridis ssp. sinuata). The only ash native to the Pacific Northwest, Oregon ash (Fraxinus latifolia) is found along riverbanks and in canyons from Washington to southern California. Almost 30 species of willow are found in the region, including the thicket-forming pioneer Scouler willow (Salix scouleriana) and the larger Pacific willow (S. lucida ssp. lasiandra) which can reach heights of 60 feet.
The dominant forest type of inland Oregon and Washington is the Douglas-fir forest. It is named for the coastal variety of Douglas-fir (Pseudotsuga menziesii var. menziesii) which is found on both sides of the Cascades, though with distinct associates. On the western slopes, coast Douglas-fir reaches its best growth and can form nearly pure stands, especially in former clearcuts where it shades out pioneer species red alder and bigleaf maple (Acer macrophyllum). It can also be found growing with western redcedar (Thuja plicata), the shade tolerant western hemlock (Tsuga heterophylla), noble fir (Abies procera), and silver fir (A. amabilis) which becomes co-dominant in the northern Cascades. On drier sites it is associated with ponderosa pine, incense-cedar (Calocedrus decurrens), sugar pine (Pinus lambertiana), grand fir (A. grandis), white fir, western white pine, and western larch.
In the high elevations of the Cascade Mountains, snowfall often totals 500 inches annually. Combined with wind exposure, thin soils, and a shorter growing season few species can tolerate growing near a timberline which decreases from 9,500 feet on Mt. Shasta in California to 5,500 feet in southern British Columbia and 1,500 feet in southern Alaska. The species mix is similar to that of the northern Rocky Mountains, including the conical crowned subalpine fir, whitebark pine, Engelmann spruce, and subalpine larch, with mountain hemlock mainly confined to the western slopes. A species unique to the region is yellow-cedar or Alaska-cedar (Chamaecyparis nootkatensis) a slow growing tree that can live over 1,000 years.
High levels of precipitation and the presence of coastal fog during drier months allows for the growth of giant conifers along the edge of the Pacific Ocean. The spruce-hemlock forest is named for Sitka spruce (Picea sitchensis) and western hemlock, which are the dominant species along the coast from southern Alaska to southern Oregon. The range of Sitka spruce is limited to within 30 miles of the coastline, although it does grow at treeline in Alaska. It is an important timber species that can reach heights over 200 feet and diameters in excess of 10 feet. Equally impressive in stature is western redcedar, a shade tolerant tree with decay resistant wood used by indigenous populations to make rope, baskets, canoes, and planks for construction. The coastal variety of Douglas-fir attains its greatest heights as part of this coastal forest type, reaching up to 300 feet. Limited to southwestern Oregon, Port-Orford-cedar (Chamaecyparis lawsoniana) is another tree that can reach over 200 feet in height and 10 feet in diameter. Pacific yew (Taxus brevifolia) is an understory tree sporadically dispersed throughout the extent of this forest type. Another shade tolerant tree, Pacific dogwood (Cornus nuttallii) is found from southern British Columbia to California. On poorly drained soils, the twisted forms of shore pine and yellow-cedar are able to gain a foothold in this forest.
South of the spruce-hemlock forest is the home of the tallest tree on the planet, the redwood (Sequoia sempervirens). Reliant on the coastal fogbelt, it can reach 379 feet in height and survive over 2,000 years. It is a fire-adapted species, with thick bark and seedlings that prefer burned soils. It has surprisingly shallow roots and is subject to windthrow. It is associated with western hemlock, Douglas-fir, Sitka spruce, bigleaf maple, tanoak (Lithocarpus densiflorus), and California laurel or Oregon-myrtle (Umbellularia californica) at the northern extent of its range. In the southern part of its range, in central California, it is often restricted to river bottoms. Redwood forests were heavily exploited for timber, and now only 12% of the nearly 2-million acres it covered remains as old growth.
Closed-Cone Pine Forest
The Pacific Coast of California has many endemic conifer species with very limited ranges. They are sometimes called closed-cone pine forests because many of the indicative pine trees have serotinous cones and are descended from Mason pine, a species that went extinct several million years ago. North of San Francisco Bay these forests are typically comprised of shore pine, Bishop pine (Pinus muricata), and Sargent’s cypress (Cupressus sargentii). To the south, Monterey pine (P. radiata), Monterey cypress (C. macrocarpa), and Gowen cypress (C. goveniana) are found. Monterey pine can reach 100 feet in height, although is usually much smaller in its native habitat, and is now grown in short rotation timber plantations throughout the southern hemisphere, especially in Chile and new Zealand. Southern California is home to Coulter pine (P. coulteri) and Torrey pine (P. torreyana). In addition, the introduced species blue gum eucalyptus (Eucalyptus globulus) from Tasmania has become naturalized and is now a common sight along the hills of coastal California.
Sierra Nevada Complex
Standing above the agricultural Central Valley to the west and the Great Basin and Mojave Desert to the east are the rugged Sierra Nevada Mountains. Approximately 400 miles in extent and an average of 70 miles in width, they present a continuous wall moving south from the end of the Cascades into southern California. Like the Cascades, they are geologically young mountains that receive ample snowfall and create a strong rainshadow to the east. Generally, this means that forest types occur at a higher elevation on the eastern slope of the mountains due to lower precipitation and higher temperatures. A similar effect occurs when moving from the northern end of the range to the south. Much of the forest in the Sierra Nevadas is protected within national parks and preserves. Moving from low elevation to high, the six typical forest types of the Sierra Nevadan Complex are:
- Oak-Pine Woodland
- Sagebrush-Pinyon Forest
- Mid-Elevation Pine Forest
- Giant Sequoia Grove
- Montane Fir Forest
- Subalpine Forest
South of the northwest oak-pine forest and east of the closed-cone pine forest described above begins the oak-pine woodland that stretches throughout western California. Mainly confined to sheltered valleys, it is the most common forest type in the coast range of California and the foothills west of the Sierra Nevadas to 5,000 feet in elevation. On drier hills the trees are dispersed, allowing for the grassy interspaces of a savanna. This community is dominated by an array of oak species, including: Oregon white oak, interior live oak (Quercus wislizeni), coast live oak (Q. agrifolia), blue oak (Q. douglasii), the isolated Engelmann oak (Q. engelmannii) of southwestern California, and valley oak (Q. lobata) which is the largest of all growing to heights over 100 feet and diameters up to 7 feet. Associated species include Pacific madrone, California laurel, and incense-cedar in the north, as well as California buckeye (Aesculus californica), California walnut (Juglans californica), and gray or digger pine (Pinus sabineana) to the south.
Restricted to the dry, eastern slopes of the Sierra Nevadas between 5,000-7,000 feet is the sagebrush-pinyon forest type. Similar to the pinyon-juniper woodland of the Rocky Mountains, here the pinyon species is singleaf pinyon (Pinus monophylla) and the only juniper found is California juniper (Juniperus californica) strictly in the southern portion of the range. Singleleaf pinyon pine is a shrubby tree, rarely reaching taller than 25 feet. Big sagebrush (Artemisia tridentata) is the most common sagebrush associated with this forest type. Seeds of big sagebrush germinate best following fire and many of the other understory species are also fire-adapted.
Mid-Elevation Pine Forest
As elevation increases, there is enough precipitation to support larger trees in the mid-elevation pine forest. On the western slope of the Sierra Nevadas, the coastal variety of ponderosa pine is prevalent between 2,400-6,000 feet in elevation. Associated species include the coastal variety of white fir (Abies concolor var. lowiana), California black oak (Quercus kelloggii), incense-cedar, sugar pine, and Douglas-fir to the north. On the eastern slope, the mid-elevation pine forest occurs from 7,000-8,000 feet and is predominantly composed of Jeffrey pine (Pinus jeffreyi). Jeffrey pine is closely related to the ponderosa pine it resembles, though it is more tolerant of drought and cold temperatures. It mixes with similar species as ponderosa pine, but usually has a less developed understory.
Giant Sequoia Grove
The range of giant sequoia (Sequoiadendron giganteum) is limited to 35,000 acres split amongst 75 groves on moist, unglaciated soils. This ancient species was widespread millions of years ago, but is now solely found on the western slopes of the Sierra Nevada Mountains between 5,000-7,000 feet. It is called giant sequoia because it can reach diameters of 30 feet and heights over 250 feet. It grows in park-like stands with grasses and many sequoia seedlings in the interspaces. It is adapted to frequent fire with serotinous cones and bark 1-2 feet thick. It usually dominates the groves, but can be found with sugar pine, white fir, incense-cedar, and California black oak. Sugar pine is a slender tree that often reaches 200 feet in height and is known for its extremely large cones that grow up to 20 inches in length. Some cutting of giant sequoias occurred from the 1880s-1920s, but the wood often shattered during logging and was not useful for making lumber. Today they are well protected and widely cultivated as ornamental or natural stands throughout the world.
Montane Fir Forest
The montane fir forest is mainly associated with California red fir (Abies magnifica) which occurs in almost pure stands from 6,000-8,200 feet on the western slopes of the Sierra Nevadas, and 8,000-9,000 feet on eastern slopes. California red fir grows 60-120 feet tall on mesic sites where it forms dense stands with a sparse understory. Western white pine is found above 7,500 feet on both slopes of the Sierra Nevadas, as well as lodgepole pine which occupies lakesides and recently burned areas. White fir grows strictly on the western slopes, usually at lower elevations and on slightly drier sites, where it commonly has a well-developed understory. In the northern Sierra Nevadas, coast Douglas-fir dominates montane fir forests on cool, moist slopes from 4,000-5,000 feet in elevation. Pacific yew, tanoak, and California nutmeg (Torreya californica) are three associates of these Douglas-fir stands.
Above the montane fir forest is the rugged, open subalpine forest where the growing season is typically less than 9 weeks. These forests are occupied by mountain hemlock, western juniper (Juniperus occidentalis), and suite of pines: whitebark, lodgepole, limber, western white, foxtail (Pinus balfouriana), and the long-lived Great Basin bristlecone pine (P. longaeva). The twisted form of the bristlecone pine rarely tops 40 feet, but has been documented with ages in excess of 4,600 years making it the oldest living tree on the planet. It is only found in the mountains east of the Sierra Nevadas, but the similar foxtail pine grows on both slopes from 6,000-11,500 feet. Western juniper is another twisted, spreading tree of the granite boulder fields of the subalpine forest with trunks that can reach 15 feet in diameter and survive over 2,000 years.
Southwest Dry Forest Complex
Stretching from southern California to western Texas, and south into central Mexico is a land of low precipitation and vast deserts: the Great Basin, Mojave, Sonoran, and Chihuahuan. This region is dominated by drought tolerant cacti, shrubs, and grasses, but where mountains rise and rivers flow there are often forests. Precipitation and exposure are the key to the distribution of forests on mountains, with higher elevations and northern slopes allowing for greater growth. Along intermittent rivers, natural springs, spring snowmelt, and summer monsoon rains are keys to the establishment of forests. In this region there are four general forest types, two defined by rivers and two by mountains:
- Canyon and Riparian Forest
- Lower Rio Grande Forest
- Sky Island Forest
- Madrean Foothill Forest
Canyon and Riparian Forest
In an arid landscape, gallery forests mainly consisting of hardwood trees follow the intermittent waterways of rivers and canyon bottoms. Deep taproots are important to water loving species like California sycamore (Platanus racemosa) and California walnut, which give way to Arizona sycamore (P. wrightii) and Arizona walnut (Juglans major) to the east. Throughout the region are found: the conifer Arizona cypress (Cupressus arizonica), Fremont cottonwood (Populus fremontii), narrowleaf cottonwood (P. angustifolia), bigtooth or canyon maple (Acer grandidentatum), boxelder, New Mexico locust (Robinia neomexicana) as well as various species of ash (Fraxinus spp.), willow (Salix spp.), alder (Alnus spp.) and oak (Quercus spp.). On drier sites, especially arroyos which are often dry on the surface except during flash floods, these species may be replaced by shrubby trees like honey mesquite (Prosopis glandulosa), catclaw acacia (Acacia greggii), blue paloverde (Parkinsonia florida), yellow paloverde (P. microphylla), and desert ironwood (Olneya tesota).
Lower Rio Grande Forest
Southern Texas has a species mix unique from areas to the east and the west, although only 5% of the Lower Rio Grande Valley remains in its natural condition. The region is semiarid, receiving 18-25 inches of annual precipitation and trees rarely exceed 75 feet in height even along the rivers. The Rio Grande River is home to Montezuma baldcypress (Taxodium mucronatum), cedar elm (Ulmus crassifolia), sandpaper tree or anacua (Ehretia anacua), Jerusalem thorn or retama (Parkinsonia aculeata), and the endangered sabal palm (Sabal texana). In the drier upland forests, including the Edwards Plateau, Ashe juniper or Mexican cedar (Juniperus ashei) is found principally on limestone soils. Characteristic oak species include Texas live oak (Quercus fusiformis), Lacey oak (Q. laceyi), Texas red oak (Q. texana), bastard oak (Q. sinuata), and post oak (Q. stellata). Some of the region’s many leguminous species are Texas ebony (Ebenopsis ebano), honey mesquite, catclaw acacia, sweet acacia or huisache (Acacia farnesiana), guajillo (A. berlandieri), and great leadtree or tepeguaje (Leucaena pulverulenta), a fast-growing species that is a pioneer on disturbed sites.
Sky Island Forest
The mountains of southeastern Arizona and northern Mexico, like the Santa Catalina and Chiricahua ranges, often support pine forests above 6500 feet where a cooler, moister climate prevails. These forests are called sky islands because of their geographical separation from each other. While many of the species found are present in southern Rocky Mountain forests, there is a strong influence of species common to Mexico, such as Chihuahua pine (Pinus leiophylla), Apache pine (P. engelmannii), southwestern white pine (P. strobiformis), and Arizona pine (P. arizonica) which is closely related to ponderosa pine. At higher elevations, 8,000-11,500 feet, the forest is typically composed of inland Douglas-fir, corkbark fir, Engelmann spruce, limber pine, lodgepole pine, and quaking aspen.
In northern Mexico, between 4,500-8,000 feet are the Madrean foothill forests. In the Sierra Madre Occidental of the west coast, the dry foothills are home to the widespread Mexican pinyon (Pinus cembroides), Arizona pine, Chihuahua pine, and alligator juniper, Some of the 112 oak species within the Mexican Plateau common to this region include: Arizona white oak (Quercus arizonica), silverleaf oak (Q. hypoleucoides), and Mexican blue oak (Q. oblongifolia).
Above 8,000 feet occurs Apache pine, Mexican white pine (Pinus ayacahuite), inland Douglas-fir, and ponderosa pine, with Arizona cypress typical on sheltered sites. In the Sierra Madre Oriental foothills to the east, Mexican pinyon remains common, along with weeping juniper (Juniperus flaccida), Texas madrone (Arbutus xalapensis), gray oak (Q. grisea), and Mohr oak (Q. mohriana). In addition, Nelson pine (P. nelsonii), Mexican weeping pine (P. patula), Pince pine (P. pinceana) and Gregg pine (P. greggii) are endemic to this region. In the southern portions of the Sierra Madres, tropical deciduous species reach the northern extent of their distribution.
Eastern Deciduous Forest
The relatively moderate climate of the eastern one-third of the United States supports one of the most diverse aggregations of forests in the temperate regions of the world. The region supports hundreds of species of trees and associated shrubs and understory plants. Most of the trees are hardwoods (deciduous) but there are also several conifers that are important locally. Forests with the greatest biodiversity and species richness in North America are found within this forest type. There is great variation in soils and sites that results in an incredible number of species combinations with hardwoods often intermingling with conifers. Land use history has also been significant in influencing species distributions and dominance. Much of this land has been cleared for agriculture and second, third and even fourth growth forests have re-colonized many sites. The effect of selective timber harvesting, widespread grazing and elimination of natural fires have also had major influences on tree distribution and dominance. Exotic plant species, insects and diseases are having major effects on forest composition. For example, in the early 20th century, the chestnut blight (Endothea parisitica) essentially eliminated the dominant American chestnut (Castanea dentata) from this forest and caused a major change in composition. The hemlock wooly adelgid, (Adeleges tsugae) is threatening to eliminate eastern hemlock (Tsuga canadensis) from its range in the Appalachian mountains and the emerald ash borer (Agrilus planipennis) is eliminating several species of ash (Fraxinus spp.) in many areas of the eastern deciduous forest. The eastern deciduous forest stretches from Florida west to central Texas, north to Minnesota and east to Maine. Within this area, there are at least eight general subtypes including:
- Northeastern Coniferous Forest
- Northern Hardwood Forest
- Midland Hardwood Forest
- Ozark-Piedmont Forest
- Mixed Mesophytic Forest
- Appalachian Forest
- Bottomland Hardwood Forest
- Southeastern Pine Forest
Northeastern Coniferous Forest
Species comprising the northeastern coniferous forest are relegated to site extremes where hardwoods are less competitive. Such sites include swamps and bogs where balsam fir and black spruce are dominant to drier upland sites where jack pine, red pine (P. resinosa) and white pine (P. strobus) make up the majority of stems. Other conifers such as white and red spruce (Picea rubens) occupy intermediate to better quality sites in combination with northern hardwoods such as sugar maple (Acer saccharum), American beech (Fagus grandifolia), and yellow birch (Betula allegheniensis). In many forests within this type, the conifers have been selectively removed and now the stands are degraded and have heavy components of low-value hardwoods. Spruce will typically be found in these stands in the understory since they are tolerant of shade and can grow beneath a hardwood overstory. Within this type, red pine and jack pine typically occupy dry sandy sites and are perpetuated by fires. White pine is found on the better drained sites either in pure stands or in mixtures with red pine or northern hardwoods. Other associated species in this type include quaking aspen, bigtooth aspen (Populus grandidentata), and the conifers; northern white cedar (Thuja occidentalis) and eastern hemlock.
Northern Hardwood Forest
The northern hardwood forest intergrades with the northeastern coniferous forest on mesic, better quality sites in the northeastern United States and Lake States. The type often has substantial components of eastern hemlock, white pine and spruce especially on moderate quality sites or where moisture becomes limiting. This forest is characterized by mixtures of sugar maple, red maple (Acer rubrum), American beech and yellow birch. The occurrence of this type is not continuous and there are disjunct stands in the north on protected sites such as occur on Cape Breton Island in Nova Scotia. The northern hardwood forest extends into the southern Appalachian Mountains at the higher elevations on cool, moist sites where it transitions from the mixed mesophytic forest from lower elevations. Throughout the distribution of this type are many associated species ranging from more boreal species in the north to mesic hardwoods to the south such as basswood (Tilia americana), yellow poplar, northern red oak (Quercus rubra), black cherry (Prunus serotina) and white ash (Fraxinus americana). On the very dry sites, associated species include white oak (Quercus alba), hickories (Carya spp.), chestnut oak (Quercus prinus), scarlet oak (Quercus coccinea) and pitch pine (Pinus rigida). On disturbed sites, pioneer species such as quaking aspen, paper birch (Betula papyrifera), sweet birch (Betula lenta), pin cherry (Prunus pensylvanica) and striped maple (Acer pensylvanicum) are common associates. There are also variations in species composition within this type between the northeastern United States and the Lake States. Red spruce is not a component in the Lake States and there is a decrease in the percentage of American beech, eastern hemlock and an increase in the percentage of basswood, paper birch and quaking aspen into western Minnesota.
Midland Hardwood Forest
The midland hardwood forest is the western-most forest in the eastern United States and provides the transition from open grasslands to the eastern deciduous forest. It covers a zone from southern Minnesota, Wisconsin and Michigan south across Indiana, Illinois, Arkansas, Oklahoma and central Texas. It is characterized by a patchwork of closed forest and open grasslands, low precipitation and high evaporation. This forest type is composed primarily of oak and hickory species, typical of the mixed mesophytic forest.
Common oaks include: red, white, post (Quercus stellata), black (Q. velutina), scarlet and chestnut. Dry site species such as bur oak are found in savannah-type settings along with elm, hackberry (Celtis occidentalis), cottonwood (Populus deltoides), beech, sweetgum (Liquidambar styraciflua) and pin oak (Q. palustris) on the wetter sites. Species diversity increases with increased moisture to the east and north with these areas often supporting many species including numerous oaks, hickories, elms, ashes, maples and black walnut (Juglans nigra). The original forest in this type was much more extensive and has been cleared for agriculture since the soils are excellent and well-suited for agriculture. There are very few coniferous species in the type, the exception being eastern redcedar (Juniperus virginiana) which is found primarily on limestone soils, or the widely planted exotic Scotch pine (Pinus sylvesris).
This forest extends from southern New Jersey to east Texas on the “Piedmont Plateau”. This is an “oak-pine” type with many species of the mixed mesophytic type present along with several species of Southern pines including shortleaf (Pinus echinata), longleaf (P. palustris), pitch (P. rigida) and Virginia (P. virginiana) on the drier sites and loblolly (P. taeda) on the more mesic sites. Oaks and hickories are the predominate hardwood species including scarlet oak, southern red oak, white oak, northern red oak, pignut hickory (Carya glabra), shagbark hickory (C. ovata) along with sweetgum and red maple. Much of this area has been farmed or pastured over the past 200+ years and has suffered severe soil erosion resulting in reduced site productivity. Many stands are of “old-field origin and the site quality is low due to past high-grade harvesting. Nutrient deficiencies and droughty soils along with high average temperatures all contribute to the reduced productivity of these sites. A considerable portion of this type is currently being converted to residential and commercial development, especially in the “I-85 Corridor” from Birmingham to Atlanta though the Piedmont Crescent in North and South Carolina up to Washington, D.C.
Mixed Mesophytic Forest
The mixed mesophytic forest is the richest forest in eastern North America with literally hundreds of species of trees and woody shrubs growing in various combinations depending on elevation, aspect, slope and soils. The most diverse stands are found on “cove sites,” or protected valleys or coves on the north and east facing slopes. The development of these forests results from a unique set of environmental conditions consisting of high precipitation, low evaporation, moderate temperatures and fertile soils derived from nutrient-rich sedimentary rocks. A sample of the tree species that are found in this forest include: yellow poplar, sugar maple, basswood, American beech, American chestnut (shrub-form), yellow buckeye (Aesculus flava), white ash, cucumber-tree (Magnolia acuminata), northern red oak, butternut (Juglans cinerea), black walnut, chestnut oak, mockernut hickory (Carya alba), shagbark hickory, pignut hickory, American elm (Ulmus americana), Kentucky yellowwood (Cladrastis kentukea), black cherry and many more. Forest Types adjacent to the Mixed Mesophytic Forest in all directions have fewer species and are less diverse owing to the less-favorable environmental conditions. The only conifers of any importance in this forest type are shortleaf pine and eastern redcedar.
This forest can be classified as a subdivision of the mixed mesophytic forest and covers the lower elevations of the Appalachian Mountains and extends northward into New England. This forest has fewer species than the forests of the Appalachian Mountains at higher elevations but locally on the better sites, it can approach these forests in complexity and species numbers. Originally, this forest was dominated by American chestnut, but since the blight eliminated this species, the predominant genus is oak (Quercus). This lower elevation forest is characterized by relatively poor, droughty soils and lower precipitation compared to the higher elevation Appalachian Forest. This is a hardwood-dominated forest but there are two conifers that are locally important: eastern white pine and hemlock.
Bottomland Hardwood Forest
The bottomland hardwood forest can rival the mixed mesophytic forest in diversity in some floodplain areas. This forest is found along the riparian or floodplain areas of the major river systems in the southeastern United States. The soils are alluvial and have been deposited over many years. Much of the newest soil material has come from eroded upland areas following destructive farming practices over the past 300 years. Sites range from permanently flooded to seasonally flooded. Moisture is not a limiting factor so growth, productivity and biodiversity are high, making these areas very productive for wood products, wildlife and ecological services. Characteristic species in the bottomland hardwood forest are wet site hardwoods such as swamp chestnut oak (Quercus michauxii), overcup oak (Q. lyrata), green ash (Fraxinus pennsylvanica), sweetgum, black willow (Salix nigra), swamp white oak (Q. bicolor), water hickory (Carya aquatica), swamp blackgum (Nyssa sylvatica), tupelo gum (Nyssa aquatica), red maple and the deciduous conifer, baldcypress (Taxodium distichum). The only other conifers that are of any importance are loblolly pine, pond pine (Pinus serotina), spruce pine (Pinus glabra) and Atlantic white cedar (Chamaecyparis thyoides).
Southeastern Pine Forest
This forest extends across over 200 million acres in a wide swath of the southern United States from Virginia to east Texas and north to Arkansas, the Highland Rim, and the Piedmont areas of the Carolinas. It encompasses the eastern and southern coastal plains, the upland Piedmont areas and the Ozark and Ouachita Mountains in Arkansas. The major pine species in order of importance: loblolly, slash (Pinus elliottii), shortleaf, longleaf and Virginia. Loblolly pine is the most extensively planted species on all sites with slash and longleaf planted primarily on the Coastal Plain and in Florida. Shortleaf pine becomes more abundant in the upper Piedmont, Highland Rim, and mountains in Arkansas. Much of the current southern pine forest has resulted from early harvesting and most importantly, the abandonment of depleted agriculture lands beginning in the mid-nineteenth century and continuing into the early twentieth century. All the southern pines are naturally fire dependent. Over the past century, fire has been greatly reduced across the region, which has had major negative effects on the southeastern pine forest. Even with significant prescribed burning in managed pine forests, the occurrence of fire has declined with a concurrent increase in fuel loads and invasion by hardwoods. After being severely exploited over the past 250 years for timber and naval stores (resin, pitch, and tar), longleaf pine is being increasingly planted on the Coastal Plain and some Piedmont sites as an alternative to loblolly and shortleaf pine.
North America is home to a variety of forest types, ranging in extent from the transcontinental boreal forest to the remnant giant sequoia groves in California. Insects, diseases, and human settlement have all had a significant influence on the distribution of forests in the past and will continue to do so in the future. The distribution and composition of forest types are constantly in flux as they face a changing physical and social climate. This flux is likely to increase in response to climate change, which may accelerate or exacerbate shifts in the range of forest species. Furthermore, these shifts be accelerated even more by exceptional insect and disease attacks or extreme storm events.
It is important to note that a “forest type” is a human construct designed to simplify nature’s complexity for our own understanding. Such classification systems can be applied at a global scale, describing biomes, to a fine scale, dividing a stand of trees based on understory plant diversity. In this article, we have described forest types at a scale somewhere in middle, focusing on dominant tree species. We hope we have interested the reader to learn more about specific forest types, as well as the many subtypes and individual species that comprise them.
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Forest Types of the Americas V3
Submitted for Posting 15 September 2010