Biomes

Joseph Mason

2 Biomes

A biome is a group of terrestrial (land-based) ecosystems with similar climates and vegetation structures. Vegetation structure refers to the relative abundance and spatial patterns of trees, shrubs, and grasses (growth forms of plants). An example of vegetation structure that is distinctive and occurs in at least two global biomes is savanna, with scattered trees and shrubs and an understory formed by grasses (Figure 1).

photo of scattered trees and grass in a tropical savanna in East Africa — Figure 1. Example of a tropical savanna in East Africa. In this case the savanna looks like a grassland with scattered trees, but the term savanna can be applied to a range from the vegetation structure shown here to one with many more trees, though still more widely spaced than in a forest.

Both vegetation structure and the global distribution of biomes are related to climate, although we’ll also look at other factors influencing vegetation structure. Figure 2 is a simple diagram relating vegetation structure to the range from wet to dry climates. Figure 3 illustrates the distribution of biomes with latitude (mostly related to temperature) and from wet to dry climates.

diagram showing how wet climates favor forest, high NPP, and a large pool of carbon in biomass, while drier climates favor savanna/grassland/shrubland or desert, low NPP, and less carbon in living biomass — Figure 2. Relationship between climate (wet to dry) and vegetation structure (forest to desert). Note also that this range in vegetation structure corresponds to a range from high NPP (forest) to low NPP (desert) and a similar range of carbon stored in living biomass.

Triangular diagram showing relationship between major global biomes and two climate variables, temperature and moisture. Temperature is related to distance from the Equator — Figure 3. Diagram illustrating how major global biomes are related to temperature and moisture. Temperature (colder toward the top) is related to distance from the Equator. Moisture is represented by the horizontal direction, wet to the left and more arid to the right. From textbook, Mason, Burt, Mueller, and de Blij (2015) Physical Geography

Figure 4 is a generalized map of showing the distribution of major biomes. “Generalized” mainly means simplified, when applied to a map like this. The boundaries between biomes are simplified, small isolated areas occupied by a biome are not shown, and areas that don’t fit well in one or another of the biomes are also not shown. A good example of the last kind of area is mountain ranges where vegetation is often quite different than surrounding lowlands. As we discuss each biome, you can refer back to this map to see where it occurs.

Map of global biomes — Figure 4. Generalized map of biomes. From textbook, Mason, Burt, Mueller, and de Blij (2015) Physical Geography.

For each biome, we’ll discuss its climatic setting, its vegetation structure, typical soil orders, examples of typical adaptations to environmental conditions in that biome, and the role of fire or other disturbance in that biome.

Tropical Rainforest

The tropical rainforest biome occurs in wet tropical climates, characterized by high temperatures and rain throughout the year, though generally with more rain in some seasons that others. This biome is dominated by forests with large trees, high NPP, and high biodiversity (high numbers of species of plants and animals) (Figure 5). Because of the large trees and the large land area occupied by this biome, much of the total pool of carbon in terrestrial biomass is stored there. Soils in this biome are generally Ultisols or Oxisols, with some Spodosols on sandy parent materials and extensive Histosols in wetlands. Because of the lack of strong seasonal contrasts in temperature or moisture, trees in tropical rainforests are more likely to be evergreen (leaves fall in small numbers through the year so the tree is always green) rather than deciduous (most leaves fall at the same time and there is a season when the tree does not have leaves). Note that “evergreen” as used here does not refer to needle-leaved, cone-bearing trees, as it often does in the US; most tropical evergreen trees are broadleaved.

Two interesting adaptations of plants in this biome are represented by buttress roots and epiphytes (Figures 6). Buttress roots are an adaptation that stabilizes large, tall trees with shallow root systems. Because in many tropical rainforest soils the nutrients available to plants are concentration in a thin A horizon, tropical rainforest trees can have large but shallow root systems. Buttress roots help stabilize shallow-rooted trees during wind storms. Epiphytes are plants that grow on other plants, usually trees. Often they grow from cavities on tree branches that trap some water, organic matter, and dust, forming a kind of soil.

View of a forest canopy with multiple species of trees and a fog-covered mountain in the distance — Figure 5. Canopy of a tropical rainforest, In a single view like this, there are usually several different tree species visible, which is less often the case in temperate region forests. This represents part of the high biodiversity of tropical forests.

photos of 1) large buttresses formed by roots that stabilize a large tree, and 2) a plant growing on a branch of a large tree — Figure 6. Examples of buttress roots and an epiphyte. The epiphyte is growing from a branch (now broken off) where it joined the tree trunk, and location where water and organic matter can be trapped, providing the epiphyte with a source of water and nutrients.

Wildfire and other forms of disturbance not directly related to human activity are rare in tropical rainforests, with one important exception: Hurricanes/typhoons/cyclones (all the same kind of storm, as covered in a later course module) can cause extensive wind disturbance to tropical forests that lie in their paths. Human disturbance is widespread and is a major environmental issue in many parts of the tropics. This disturbance can include logging (Figure 6) or deliberate burning and replacement of the forest by planted crops, including tree crops like oil palms or eucalyptus trees used to make paper.

Figure 7. Logging of tropical rainforest in Borneo. Note that this form of logging not only destroys the forest but can trigger serious erosion during heavy rainfall that is common in the tropics.

Monsoon Rainforest and Dry Tropical Forest

Tropical forests that have distinct dry seasons and generally lower rainfall that tropical rainforests are widespread (all are grouped together on the map in Figure 4). Monsoon rainforest is found in the wetter parts of regions that have monsoon climates. They have a dry season but still receive significant rainfall, overall. There are also dry tropical forests in the rainshadows of mountain ranges (rainshadows are covered later in the course). There are good examples of dry tropical forests in Hawaii, Puerto Rico, and Central America; these have received quite a bit less scientific study than tropical rainforests. Soils of monsoon rainforests and dry tropical forests include Ultisols, Alfisols, and Oxisols, with Andisols in some areas where the soils are formed in volcanic ash. The generalizations about disturbance in tropical rainforest also apply here.

Tropical Savanna

Tropical savanna is a very widely distributed and important biome (see map in Figure 3), and is widely believed to be the original home of the human species. “Savanna” refers to the vegetation structure, with scattered trees and an understory of grasses. There are many local and regional variations in this pattern and in the general appearance of tropical savannas (Figures 8 and 9). There are also areas of grassland with few if any trees within the tropical savanna biome (Figure 10). Tropical savanna occurs mainly in regions with monsoon climates. The monsoon climograph of Ndjamena (Figure 11) that we looked at earlier is from the region of Africa called the Sahel, a kind of transition zone from tropical savanna to desert. Much of the tropical savanna biome has a dry season a little less extreme than Ndjamena, but still very distinct. Soils of tropical savanna are mainly Alfisols and Ultisols, as well as Vertisols in low areas with clay deposits. Mollisols are rare, even in the grasslands within this biome.

photo of small trees with a grass understory — Figure 9. Tropical savanna in a part of Australia with a pronounced monsoon climate

photo of elephants at a water hole with grassland in the background — Figure 10. Elephants at a water hole in a tropical grassland in northern Cameroon. The dark-colored soils here may be Vertisols.

image shows a climograph — Figure 11. Climograph of Ndjamena, Chad, an example of an extreme monsoon climate in the Sahel region

Disturbance by fire and herbivory (animals feeding on plants) are very important in tropical savanna, in fact, by many accounts the biome wouldn’t exist without this frequent disturbance. Fire is or has been common in most if not all tropical savannas, set by people or ignited by lightning, particularly in the dry season when the grasses are more flammable (Figure 12). The deliberate use of fire by Indigenous people for many different purposes is well-documented for some tropical savannas, in northern Australia for example, although tropical savanna existed and experienced fire before the emergence of modern humans. Fire generally favors grasses, maintaining the open vegetation structure of savannas. Grasses can regrow from a growing point that is at or below the soil surface and survives fires, and have a large belowground biomass (Figure 13). Some grass species are especially well-adapted to frequent fires. Trees and shrubs have less of their biomass below ground and growing points along stems and branches and they are much more likely to be killed or have their growth set back considerably by fires.

Herbivory can be by wild animals or domestic livestock. Many grazing animals feed preferentially on grasses, which can then be set back in their growth relative to trees and shrubs that are less palatable to herbivores, so herbivory can potentially favor trees and shrubs over grasses. On the other hand, a variety of wild animals and domestic goats often feed on leaves and stems of trees and shrubs. There is also a widely held view, though still somewhat controversial, that herbivory can stimulate additional growth by grasses, so grass biomass is greater when there is a moderate amount of grazing than when there are no grazing animals present. Overall, the effects of herbivory are more complex and controversial than the effects of fire in tropical savanna. One clear effect of herbivory is that many tropical trees and shrubs have developed adaptations such as thorns that protect them from browsing by animals.

photo shows grasses and trees burning in a tropical savanna — Figure 12. Fire is and probably always has been common in most tropical savannas. In the dry season, grasses provide highly flammable fuel but the fires generally are more destructive to trees and shrubs.

Drawing of grasses and their deep root systems — Figure 13. Drawing of various grass species and their root systems, showing how some have a very large proportion of their total biomass below ground in roots. Also illustrates how grass stems grow from a point at or below the soil surface, allowing them to resprout after fires.

Desert

A look at the map in Figure 3 will show that many of the world’s deserts are located in the subtropics, centered around 30º North or South latitude. At these latitudes, air descends from higher levels in the atmosphere, suppressing rainfall and creating very dry climates (more on this later in the course). Deserts also occur at higher latitudes in the rainshadows of high mountain ranges. The desert biome is generally thought to have low biodiversity, but in fact there is a wide range from desert landscapes dominated by only one or two plant species (Figure 14), to those where many species grow together and species diversity is relatively high (Figure 15). Common soils in the desert biome include Aridisols, Inceptisols, and Entisols.

two photos of desert landscapes, each with many different plant species visible, and relatively high vegetation cover — Figure 15. In the Sonoran Desert of northern Mexico and southern Arizona, gentle slopes on hills and at the foot of mountains have high biodiversity. Note the large number of different plant species visible and the relatively high vegetation cover. Diversity of birds and some other animals is also high in this relatively warm desert.

Desert plants have many adaptations to the low availability of soil moisture because of low precipitation. Some are succulents, able to expand their stems to store water when it is available after rainy periods. Cacti, which are native to deserts in North and South America, use a special form of photosynthesis that allows them to lose less water while taking in carbon dioxide. Shrubs like creosote brush lose their leaves and are dormant for long periods when there is little water in the soil. Finally, there are many desert species that emerge and flower only in years of unusually high rainfall, creating rare “desert blooms” that attract many people to see the landscape covered in flowers for a brief period of time (Figure 16).

flowers blooming in a desert landscape, next to a cactus — Figure 16. Flowers blooming in the Sonoran Desert after a rare winter and early spring with unusually high rainfall

Disturbance by fire is rare in deserts because the sparse vegetation offers only a limited amount of fuel. Desert valleys are disturbed by rare flash floods that erode stream channels and flood low parts of the landscape. Disturbance through grazing by domestic sheep and cattle is common in many deserts.

Mediterranean Biome

The Mediterranean biome includes distinctive ecosystems with many plants adapted to dry conditions during the high sun season, and its distribution corresponds closely to the areas with Mediterranean climates. Many plant species occur only in specific areas of the Mediterranean biome, creating clusters of relatively rare species (areas of the Mediterranean biome in South Africa are a particularly good example-see Figure 17). Plants in the Mediterranean biome are mostly shrubs and small trees, with adaptations to limit water loss such as small leathery leaves (Figure 18). Many Mediterranean plants produce volatile oils that impart a distinctive smell to the landscape. Well-known herbs such as thyme, rosemary, and lavender are good examples; all grow wild in the Mediterranean biome of southern Europe. Soils of the Mediterranean biome include Alfisols, Inceptisols, and Entisols.

Rocky landscape covered with shrubs and small trees — Figure 17. An example of the Mediterranean biome near Cape Town, South Africa. Many of the plant species here are found nowhere else, though they have adaptations to dry summers similar to those of plants in other areas of this biome.

Photo of a shrub with small leathery leaves — Figure 19. Small, leathery leaves are common in shrubs of the Mediterranean biome.

Fire is a common form of disturbance in the Mediterranean biome. The volatile oils that make Mediterranean plants aromatic also make them highly flammable. In southern California, devastating fires occur when strong “Santa Ana” winds blow through the local Mediterranean vegetation, called chaparral. Similar fires occur in the Mediterranean biomes of southern France and Italy. There is some debate about how frequent fires were in the California chaparral before 20th century population growth increased the number of fires set accidentally by people (and their cars, etc.). It appears that fire frequency is increasing with more frequent drought in many areas of this biome.

Temperate Forest

Temperate forest is widespread in the wetter parts of the midlatitudes of the northern hemisphere, and occurs in smaller areas in the southern hemisphere. Climates of Charlotte, North Carolina, Madison, Wisconsin, and Portland, Oregon, all discussed earlier in class, are good examples of the climatic conditions typical of this biome. Temperate forest occupied much of northern and eastern Wisconsin prior to the late 1800s, along with large parts of the Midwestern and eastern United States. Natural vegetation across much of Europe, central China, Korea, and Japan belongs in this biome as well. Some temperate forests are dominated by conifers, others by broadleafed trees, and still others have a mixture of the the two (Figure 20). Soils of the temperate forest include Alfisols, Inceptisols and Ultisols, with smaller areas of other soil orders.

two photos showing conifers and broadleafed deciduous trees. — Figure 20. Temperate forest dominated by conifers in Idaho (left) and by broadleafed deciduous trees in northern Michigan (right).

Temperate forests generally have lower biodiversity than tropical forests. An interesting observation is that, while different species occur in the temperate forests of North America, Europe, and Asia, there are many examples of very similar-looking trees growing in all three. Figures 21 and 22 show an example: two very similar species of “tulip trees,” (Liriodendron sp.), in eastern North America and East Asia.

branch of a tulip poplar — Figure 21. Leaves of a tulip poplar (*Liriodendron tulipifera*) in North Carolina.

small tulip trees in Nanjing, China — Figure 22. A very similar species of *Liriodendron* in China

One of the most important adaptations of broadleafed trees in temperate forest is the seasonal loss of leaves, making them deciduous. It is widely believed that this adaptation first developed in dry tropical forests, because of limited moisture in the dry season. In temperate forests, it serves the purpose of preventing the loss of water in winter, when soils are frozen and roots cannot extract water from them.

Windstorms are a common form of disturbance across the temperate forest biome. In many temperate forests, the ground surface is covered with pits and mounds produced when trees are tipped over and uprooted by the wind. The tree roots pull up soil to form a pit. The soil then falls off or is washed off the roots by rain and forms a mound next to the pit. The importance of fire as a a disturbance varies widely across this biome. Some temperate forests are adapted to frequent fires (Figure 23) while others experience them only every few hundred to a thousand years (Figure 24).

fire burning grass in the understory of a ponderosa pine forest — Figure 23. A ground fire burning grasses and other plants in the understory of a ponderosa pine forest in the western US. Many ponderosa pine forests experienced fire as frequently as every few years before the late 1800s. The fires were low-intensity, however, and didn’t kill mature trees. Instead, they maintained an open forest floor and limited growth of plants that would have competed with the pines. They also reduced the fuel needed to drive more destructive, high-intensity fires. Since fire control began in the early 1900s, the lack of frequent fire has allowed fuel to build up in these forests, setting the stage for highly destructive intense fires in recent decades.

photo of mature deciduous forest — Figure 24. Deciduous broadleafed forest in the Appalachian Mountains of North Carolina. In the past, these forests probably experienced fire only every several hundred to a thousand years.

Many temperate forests in Europe and Asia have a very long history of human disturbance in the form of logging and clearance for agriculture. Areas used in the past for agriculture or settlements have returned to forest in some cases, especially in recent years. In North America Indigenous use of fire and other practices altered forests, but except in river valleys, most were not extensively cleared until European settlement in the 1700s and 1800s. In the 1800s to early 1900s, widespread logging occurred across much of the temperate forest in the Midwest and eastern US. Now, in places like Wisconsin, “second-growth” forests have regrown after logging. In all of these places, the forest that returns after human disturbance is not identical to the one that existed previously. Some tree species and understory plants are more common, while others have disappeared.

a mountainous landscape with forest mixed with fields and pasture. A village of wooden houses is visible. — Figure 25. Northern Sichuan Province, China. The forests here have been inhabited for millennia, with extensive disturbance of forest. Some areas of forest were preserved because of their cultural significance, while others have regrown in recent times.

deciduous forests made up of relatively small trees — Figure 26. Second growth (or third- or fourth-growth?) forests in northern Wisconsin (left) and the Ozarks of Missouri (right). These forests post-date widespread logging and are relatively young. They also differ somewhat in species composition from the forests they replaced.

Temperate Grassland

The temperate grassland biome occurs in drier parts of the midlatitudes, often in the interior of continents, far from ocean moisture sources. The climograph of Williston, North Dakota, discussed earlier, is typical of this climatic setting (Figure 27). Regions that exemplify this biome include the grasslands of the Great Plains in the US and Canada, and the steppes of southern and eastern Ukraine, southern Russia, Kazakhstan, and Mongolia (Figures 28 and 29). The vegetation structure in temperate grassland is dominated by perennial grasses, but scattered shrubs are also present as well as trees along rivers. Soils of temperate grasslands are often Mollisols, although Entisols and Inceptisols occur in more rapidly eroding parts of the landscape, and Alfisols are present in relatively dry and warm grassland. Mollisols of temperate grasslands store large quantities of organic carbon, especially in cooler climates.

graph of average monthly temperature and precipitation at Williston, North Dakota — Figure 27. Climograph for Williston, North Dakota, typical of colder regions where the temperate grassland biome occurs, in both North America and Eurasia

rolling hills covered with yellowish grass — Figure 28. Temperate grassland in the Great Plains, USA (Nebraska Sandhills). The grasses here are a mix of warm-season and cool-season

flat grassland with hills in the distance — Figure 29. Temperate grassland in eastern Inner Mongolia, China (Xilinggol Grassland). The climate is relatively cold here and the grasses are mostly cool season

Temperate grasslands are maintained in part by fire, which is more important toward the wetter edge of the grasslands where they border on temperate forest. As discussed earlier for tropical savanna, grasses are able to survive fire to a much greater extent that trees and shrubs. The common grass species in areas like the Great Plains are also adapted to frequent drought, going dormant until moisture returns. Warm season grasses, which grow and flower in the warmest part of the summer, use a distinctive form of photosynthesis that allows high productivity at high temperatures (corn is a warm season grass with this adaptation, though a domesticated one). Cool season grasses, which grow and flower in early to mid-summer, are better adapted to colder climates such as the northern Great Plains. In the Great Plains, grasses common toward the east are tall and flourish in the higher rainfall there. Short grasses are more abundant toward the west, where it is drier. There are similar variations in grass size and warm season/cool season proportions in other temperate grasslands.

landscape mostly covered with dry grasses — Figure 30. View of Great Plains grassland during a severe drought in 2002. The grasses are dormant. The shrubs and small trees may survive on deeper soil moisture but could have eventually died if the drought persisted.

same area as in Figure 30, but with green, lush grass — Figure 31. The same Great Plains grassland in 2003, after abundant rain in the spring of that year. Cool season grasses are now growing rapidly with adequate soil moisture

Large areas of temperate grassland have been converted to agricultural fields. In the US Great Plains, this is includes most grassland that was not too hilly or steep, cut by gullies, or on sand dunes. Fire is now controlled on many temperate grasslands, which has allowed shrubs and trees to expand (Figures 32 and 33). Controlled burns can sometimes be used to prevent this process, which threatens many grassland remnants.

grassland with many clumps of shrubs — Figure 32. Grassland on sand dunes in Montana. Serviceberry, a common shrub, has replaced grasses in many patches because of the absence of fire

green grasses growing under burned shrubs — Figure 33. Aftermath of a controlled burn in the same area as Figure 32. The fire killed most of the shrubs but grasses have survived and are now growing rapidly in a wet spring.

Boreal Forest and Tundra

The boreal forest and tundra biomes occur in cold climates, mainly at high northern latitudes, but also at high elevations in the lower latitudes. Tundra lacks trees because they cannot survive the cold conditions there, while boreal forest occurs south of the tundra where it is slightly warmer. Tundra can be dominated by either shrubs or sedges. Boreal forest is made up of predominantly coniferous trees. Permafrost, soil that is permanently frozen at some depth though surface horizons thaw in summer, is common under tundra and can also be found under the northernmost boreal forest. However, snow trapped by trees in the boreal forest insulates the ground in winter and can limit the growth of permafrost even if it occurs under tundra not far away. There are extensive wetlands in the boreal forest of Alaska, Canada, and Russia. Soils in boreal forest and tundra include Alfisols, Gelisols (where there is permafrost), and Histosols (in wetlands).

Forest of conifer trees, with a large river in the background — Figure 34. Boreal forest in the valley of the Tanana River, central Alaska

landscape covered with low, green shrubs — Figure 35. Shrub tundra in central Alaska, at relatively high elevation where temperatures are lower than in the Tanana valley.

Fire is relatively common in boreal forest, but rare in tundra, although they could become more frequent in tundra as warm, dry summers become more common. Both boreal forest and tundra have been extensively disturbed by road-building and construction associated with oil and gas extraction. Building on permafrost is hazardous because heat from buildings or pipelines can thaw the permafrost, causing local sinking of the land surface. As the climate in northern regions becomes warmer, permafrost is thawing and soils are becoming drier. Vegetation is also changing; for example, the abundance of shrubs is greatly increasing across large areas of tundra. Forest will also expand northward, though affected by more fires.

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