Why Forests Grasslands and Deserts Grow Where They Do
How Geography and Climate create The World’s Biomes that Shape Human History
Michael Magoon
Five forces of physical geography shape a region's climate, creating the
vegetation patterns that constrain life across the planet.
A major theme of my book series and this Substack column is the importance of geography to human history. Through all of human history, geography has been a key constraint on the types of societies that could evolve.
Geography does not determine outcomes, but geography does highly constrain the choices available to humans in a specific location. It was only with the Industrial Revolution that humans were able to innovate technologies that enabled us to push back on those constraints significantly.
I believe that biomes are the most significant geographical constraint on human history. I have already written a few articles on the topic, so as a refresher, I will say that a biome is a category for a geographical area based upon its dominant vegetation.
You may not be familiar with the concept of biome, but I am that you will recognize some of them:
Tropical Forest biome
Temperate Rain Forest biome
Temperate Grassland biome
Tropical Grassland biome (or Savanna)
Mediterranean biome
Desert biome
Tundra biome, etc.
The
dominant vegetation in any area sets very broad constraints on what
type of plants and animals can evolve there. The dominant vegetation
sustains the local herbivores, which then sustain the local carnivores.
Since humans use a portion of these plants and animals as base materials
for food production, biomes also play a critical role in constraining
the types of human societies that can develop within them.
The concept of biome has recently been systematized by the World
Wildlife Fund (WWF), an organization dedicated to understanding the
diversity of life on planet Earth. The WWF’s system divides the earth
into 14 different biomes containing 867 smaller ecoregions. See below
for their map of biomes.
One example of a biome is the Temperate
Grassland biome. Not surprisingly, grasses dominate this biome, and the
biome is located within the temperate latitudes. The California Central
Valley and Mongolian-Manchurian grassland are ecoregions. Each of these
ecoregions is an instance of the Temperate Grasslands biome. While these
two ecoregions differ from each other, they share far more in common.
Now that we have gotten that out of the way, let’s move onto the main topic of this essay:
So what causes biomes?
At
first glance, the answer seems obvious: climate. A Tropical Forest is
hot and wet, a Desert is dry, a Temperate Forest has moderate
temperatures and adequate rainfall, and Tundra is extremely cold.
But this just pushes the question back one step. Why is one part of the world hot and wet while another is cold or dry?
Ultimately, most of these differences can be traced back to five basic features of physical geography.
As
is almost always the case, there is not just one cause. Biomes have a
long, causal change with many separate causes interacting. As a
simplified model, I argue for the following causal chain:
Level 1 cause = Physical Geography (Latitude + Atmospheric Circulation + Elevation + Oceans/Currents + Topography)
↓
Level 2 cause = Climate (Temperature + Water availability + Seasonality)
↓
Biome
First let’s explore the first level of causes: Physical geography. The first and most important geographical cause is latitude.
Because
the Earth is a sphere, sunlight strikes its surface at different
angles. Near the equator, sunlight strikes the Earth relatively
directly, concentrating solar energy over a small area. As you move
toward the poles, sunlight arrives at an increasingly oblique angle and
spreads the same energy over a larger area. This is the fundamental
reason that the tropics are generally hot, while the polar regions are
cold.
Latitude also affects seasonality. The tilt of the Earth’s
axis means that the amount and angle of sunlight received at higher
latitudes change dramatically over the course of a year. Near the
equator, this seasonal change is comparatively small. At high latitudes,
summers have extremely long days while winters have extremely short
days, or even periods with no sunlight at all.
This produces one
of the most obvious patterns on a biome map. Tropical Forest and
Tropical Seasonal Forest/Savanna cluster around low latitudes. Temperate
Forest and Temperate Grassland dominate much of the middle latitudes.
Boreal Forest and Tundra appear as we approach the poles.
If
latitude were the only thing that mattered, the world’s biomes would
form relatively neat bands running east to west. Obviously, they do not.
One reason is atmospheric circulation.
The
unequal heating produced by latitude sets the atmosphere in motion.
Warm, moist air rises near the equator. As it rises, it cools, causing
water vapor to condense and fall as rain. This helps create the
extremely wet conditions associated with many Tropical Forest regions.
The
now drier air eventually descends around 30 degrees north and south of
the equator. Descending air makes precipitation less likely. Not
coincidentally, many of the world’s great deserts, including the Sahara
and Arabian deserts, lie near these latitudes. Farther toward the poles,
other circulation systems produce another series of wet and dry
climatic zones.
Atmospheric circulation also changes with the
seasons. This can produce regions that receive abundant rain during part
of the year followed by months of drought. The result may be Tropical
Seasonal Forest/Savanna rather than Tropical Forest.
Temperature also changes with elevation.
Anyone
who has climbed a high mountain has experienced this directly. Even in
the tropics, sufficiently high elevations can be surprisingly cold. As
elevation increases, temperatures generally decline and growing seasons
become shorter.
This can produce an interesting effect: traveling a
relatively short distance up a mountain can resemble traveling
thousands of miles toward the poles.
The Andes mountains provide a
dramatic example. A traveler can move from hot tropical lowlands
through increasingly cool mountain environments and eventually reach the
cold, largely treeless environments of the high Andes. Similar patterns
occur in the Himalayas, East African mountains, and Rocky Mountains.
This
is why the WWF map contains a separate Highlands category scattered
across mountain ranges that otherwise lie within very different climatic
regions.
The oceans provide another major influence.
Water
heats and cools much more slowly than land. Large oceans therefore
moderate the temperatures of nearby land. Coastal regions commonly
experience cooler summers and warmer winters tha continental interiors
at similar latitudes. Oceans are also the source of much of the water
that eventually falls over land as precipitation.
This helps
explain why the interiors of large continents can be so different from
their coasts. Central Asia, for example, lies thousands of miles from
major sources of oceanic moisture. Large portions are consequently
dominated by Temperate Grassland and Desert rather than forest.
Ocean currents
complicate the pattern further because they redistribute enormous
quantities of heat around the planet. Warm currents transport tropical
heat toward higher latitudes, while cold currents move colder water
toward the equator. These currents can significantly alter the climate
of nearby land.
Western Europe provides the most famous
example. The North Atlantic transports enormous quantities of heat
toward Europe, contributing to its relatively mild maritime climate.
Cold currents along other continental coasts can have almost the
opposite effect.
Topography
is related to elevation, but its effect is different. Elevation affects
climate primarily because higher land is colder. Topography affects
climate primarily by changing the movement of air and water across the
landscape. Mountains are therefore important not simply because they are
high, but because they form physical barriers.
The most
important example is their effect on precipitation. When moisture-laden
air encounters a mountain range, it is forced upward. As the air rises,
it cools, causing water vapor to condense and fall as rain or snow. By
the time the air crosses the mountains and descends on the opposite
side, much of its moisture may be gone. The result can be a wet windward
side and a much drier leeward side, known as a rain shadow.
The
Andes provide a dramatic example. Moist air coming from the Atlantic
crosses the Amazon Basin and releases enormous amounts of precipitation
along the eastern side of the Andes. The mountains severely restrict how
much of that moisture reaches regions farther west. The Himalayas have a
similar effect, helping produce heavy monsoon precipitation on their
southern slopes while contributing to much drier conditions on the
Tibetan Plateau and in Central Asia.
Topography also affects
drainage. Mountains determine where rivers flow, valleys can concentrate
water, and enclosed basins can prevent water from reaching the ocean.
These effects can create important local differences in water
availability.
The distinction between our two mountain-related
causes is therefore fairly simple: Elevation primarily changes
temperature, while topography primarily redistributes water.
Hang in there with me, we are getting to biomes soon…
The five base geographic and physical causes discussed in the previous do not directly determine which plants grow in a region. Instead, they determine its climate.
For understanding biomes, we can simplify climate down to three particularly important characteristics:
temperature,
water availability, and
seasonality (which is essentially variations in the above)
The first is temperature.
Latitude is its most important global cause. Regions near the equator
receive more concentrated solar energy throughout the year and are
therefore warmer. As we move toward the poles, the same solar energy is
spread over a larger surface, causing average temperatures to decline.
Elevation
modifies this basic pattern. As air rises into the thinner atmosphere
at higher elevations, it expands and cools. A mountain near the equator
can therefore have temperatures more characteristic of regions thousands
of miles farther north or south. This is why tropical mountain ranges
can contain snow and ice despite being surrounded by hot lowlands.
Oceans
and ocean currents modify temperatures further. Because water warms and
cools more slowly than land, oceans moderate the temperatures of nearby
regions. Coastal areas therefore tend to have cooler summers and warmer
winters than continental interiors at the same latitude. Ocean currents
also transport heat. Warm currents can raise temperatures along nearby
coasts, while cold currents can lower them.
The
second climatic condition is water availability. Atmospheric
circulation plays a particularly important role here. Warm air rises
near the equator, cools, and releases large amounts of precipitation.
The
resulting belt of abundant rainfall helps explain the Tropical Forests
of the Amazon, Congo Basin, and Southeast Asia. Farther from the
equator, descending air produces much drier conditions. Many of the
world’s great deserts are consequently concentrated around 20 to 30
degrees north and south latitude.
Oceans provides most of the
moisture that ultimately falls over land. Regions close to an ocean and
exposed to moisture-bearing winds therefore tend to receive more
precipitation than continental interiors. As air masses travel farther
inland, they can gradually lose their moisture. This helps explain the
increasing dryness found across much of the interior of Eurasia.
Topography
can redistribute this moisture dramatically. When moist air encounters a
mountain range, it is forced upward, cools, and releases precipitation.
The windward side can consequently be extremely wet. Once the air
crosses the mountains, however, it descends with much of its moisture
already removed. This creates a rain shadow on the opposite side.
Temperature
also affects water availability. Rainfall alone does not tell us how
much water is available to vegetation. Higher temperatures increase
evaporation and the amount of water lost by plants. A hot region
receiving 20 inches of annual rainfall can therefore be much drier from a
plant’s perspective than a cool region receiving the same amount.
The
third climatic condition is seasonality. Latitude is again important,
but for a different reason. Because the Earth’s axis is tilted, the
amount of sunlight received during the year changes much more
dramatically at higher latitudes. Near the equator, temperatures remain
relatively constant throughout the year. At higher latitudes, warm
summers alternate with cold winters.
Distance from the ocean
affects this temperature seasonality. Oceans absorb heat during warm
periods and release it during cold periods. Coastal regions therefore
tend to experience smaller seasonal temperature changes. Continental
interiors can experience much greater extremes, including very hot
summers and extremely cold winters.
Water availability can be
seasonal as well. Atmospheric circulation shifts north and south during
the year as the region receiving the most intense solar heating moves
between the hemispheres. Some places therefore experience alternating
wet and dry seasons. Monsoon systems provide an especially dramatic
example, bringing heavy summer rainfall to much of South and East Asia
followed by much drier conditions during other parts of the year.
Variations
in the Level 2 causes (temperature, water availability, and
seasonality) gives every location on Earth a particular climatic
combination. One place might be hot, wet, and relatively constant
throughout the year. Another might be equally hot but experience a long
annual drought. A third might receive adequate precipitation throughout
the year but alternate between warm summers and freezing winters. These
differences can all ultimately be traced largely to variations in
latitude, atmospheric circulation, elevation, oceans and ocean currents,
and topography.
Now, we are finally getting to biomes!
Once
physical geography has created variations in temperature, water
availability, and seasonality, the next step in the causal chain is vegetation.
Different combinations of these three climatic conditions favor
different types of plants. Over large areas, this produces the
distinctive vegetation that we classify as biomes.
Temperature
establishes the first broad constraint. Plants need sufficient warmth
to grow, but different types of vegetation are adapted to very different
temperature ranges.
In the tropics, warm temperatures can permit plant growth throughout the year.
At higher latitudes, winter cold interrupts growth and shortens the growing season.
Move
still farther toward the poles, and the growing season eventually
becomes so short and cold that trees can no longer survive.
This
produces a broad progression from Tropical Forest near the equator to
Temperate Forest at middle latitudes, Boreal Forest at higher latitudes,
and finally Tundra near the poles. Elevation can produce a similar
progression over a much shorter distance.
Water
availability creates another broad division. Where temperatures are
warm enough for trees and water is abundant, forests can dominate. As
water becomes less available, maintaining large trees becomes
increasingly difficult. Grasslands and shrublands become more
competitive. Where water becomes extremely scarce, vegetation becomes
sparse and Desert takes over.
This is why regions at similar latitudes can have very different biomes.
Eastern North America, for example, receives enough moisture to support Temperate Forest.
Move
westward into the continental interior, and declining water
availability contributes to the transition into Temperate Grassland.
In still drier parts of western North America, Desert becomes common.
Seasonality
creates another set of differences. Tropical Forest occurs where warm
temperatures and abundant water are available through most or all of the
year. Where temperatures remain warm but rainfall becomes strongly
seasonal, Tropical Seasonal Forest/Savanna becomes more common. Plants
there must survive a pronounced dry season even though total annual
rainfall may be substantial.
Mediterranean provides an especially
clear example of the importance of seasonality. These regions generally
have moderate temperatures and enough annual precipitation to support
substantial vegetation. But most of the rain falls during the cooler
part of the year, followed by a hot, dry summer. This unusual
combination favors drought-resistant trees, shrubs, and grasses rather
than the vegetation characteristic of Temperate Forest.
The three
climatic conditions therefore work together. Temperature determines how
much energy is available for plant growth and how long the growing
season lasts. Water availability determines how much vegetation that
energy can support. Seasonality determines whether favorable conditions
persist throughout the year or are interrupted by periods of drought or
cold.
For measurement purposes, we can use the following metrics:
Temperature is measured by mean annual degrees Celsius.
Water availability is measured by millimeters of annual precipitation.
Seasonality is measured by the number of months per year with adequate temperature and water for plant growth.
Using the metrics that I list above, we can define the approximate climatic combinations that determine each biome:
Tropical Forest biome: >20°C + >2,000 mm + 10-12 growing months
Tropical Seasonal Forest: >20°C + 1,000-2,000 mm + 7-10 growing months
Savanna: >18°C + 500-1,500 mm + 4-7 growing months
Temperate Forest: 5-15°C + 600-1,500 mm + 5-8 growing months
Temperate Grassland: 0-15°C + 250-750 mm + 4-7 growing months
Mediterranean: 10-20°C + 300-900 mm + 5-8 growing months, interrupted by summer drought
Desert: <250 mm + 0-3 growing months
Boreal Forest: -5-5°C + 300-1,000 mm + 3-5 growing months
Tundra: <0°C + 1-3 growing months
Highlands: highly variable because elevation rather than a single climatic range defines the biome
Obvioulys,
this simplified explanation does not capture every local variation.
Hydrology is particularly important for two WWF biomes. Regular
inundation helps create Flooded Savanna, while the combination of
coastal flooding, waterlogged soils, and salt water produces Mangroves.
Fire, grazing, and other disturbances can also affect whether a
particular landscape develops into forest, grassland, or shrubland.
But across most of the Earth’s surface, the basic relationship remains remarkably simple:
Level 1 cause = Physical Geography (Latitude + Atmospheric Circulation + Elevation + Oceans/Currents + Topography)
↓
Level 2 cause = Climate (Temperature + Water availability + Seasonality)
↓
Biome
The
easiest way to understand how this process works is to follow the
causal chain for three very different biomes. In each case, the five
base geographic conditions create a particular climate, and that climate
favors a particular type of vegetation.
The
Mediterranean biome is one of the most distinctive biomes because of
its unusual seasonality. It occurs primarily on the western sides of
continents between about 30 and 45 degrees latitude.
Latitude gives these regions relatively warm temperatures. Atmospheric circulation is particularly important because:
the subtropical high-pressure zone expands toward them during summer, producing dry conditions.
during winter, the circulation shifts and moisture-bearing westerly winds bring precipitation.
nearby oceans moderate temperatures, while ocean currents and local topography further modify the climate.
The
result is a climate with mean annual temperatures commonly around 10 to
20°C and annual precipitation of roughly 300 to 900 mm. More important
than either annual average is the timing. Winters are relatively cool
and wet, while summers are warm and dry.
Plants must therefore
survive a predictable annual drought. Evergreen shrubs and small trees
commonly have small, tough leaves that reduce water loss. Deep roots
allow plants to reach moisture during the dry season. Many plants are
also well adapted to recurrent fire.
Below is a map of the Mediterranean ecoregions (ecoregions are separate instances of a biome)
The Mediterranean Basin between Europe and Africa provides by far the
largest example. Southern Europe, North Africa, and the Levant
experience the characteristic combination of winter rainfall and summer
drought.
Remarkably similar vegetation occurs in California.
The two regions are separated by thousands of miles and contain
different species, yet California’s chaparral resembles Mediterranean shrubland because the underlying climatic pressures are similar.
Central
Chile provides a third example. Its latitude, position on the western
side of South America, Pacific Ocean influence, Humboldt Current, and
the Andes combine to produce another winter-wet, summer-dry climate. The
resulting matorral again resembles Mediterranean vegetation despite evolving independently.
Desert Biome
The
Desert biome provides an especially useful example because very
different combinations of the five base causes can produce the same
result.
The defining climatic problem is extremely low water
availability. Many deserts receive less than 250 mm of precipitation
annually, and hot deserts lose additional water through rapid
evaporation. Vegetation is therefore sparse and dominated by plants
capable of surviving long periods without rainfall.
The
Sahara illustrates one route to creating the Desert biome. Its latitude
places much of it beneath the descending branch of the Hadley circulation.
Descending air inhibits cloud formation and precipitation. Its enormous
continental size also leaves much of the region far from major sources
of atmospheric moisture.
Below is a map of the Desert ecoregions.
The Atacama Desert of South America arrives at a similar result through a different combination of causes. The cold Humboldt Current
contributes to extremely dry coastal conditions, while the Andes
restrict moisture arriving from the east. Atmospheric circulation
reinforces both effects. Parts of the Atacama consequently receive
almost no rainfall despite lying beside the Pacific Ocean.
Central
Asia provides a third route. Much of the region is extremely far from
an ocean, while major mountain ranges intercept moisture before it
reaches the continental interior. The result is an extensive Desert
biome even though these deserts occur at much higher latitudes and
experience much colder winters than the Sahara.
The Sahara,
Atacama, and Central Asian deserts therefore look different and contain
different species. But all three demonstrate the same basic principle.
Different combinations of the five base causes can converge upon
extremely low water availability and thereby produce the Desert biome.
The
Temperate Forest biome demonstrates the opposite situation. Instead of
being defined by extreme water scarcity or an unusual dry season, it
develops where moderate temperatures and relatively abundant water
permit dense tree growth.
These forests generally occur in the middle latitudes.
Mean annual temperatures commonly fall between about 5 and 15°C, with annual precipitation of roughly 600 to 1,500 mm.
Seasonal cold interrupts plant growth during winter, but summers are sufficiently warm and long for large trees to flourish.
Where precipitation remains adequate throughout the growing season, forests can dominate the landscape.
Below
is a map of the Temperate Forest ecoregions (you might notice how
incredibly important human societies in those ecoregions in global
history).
Eastern North America provides one of the world’s great Temperate
Forest ecoregions. Moisture from the Atlantic Ocean and Gulf of Mexico
combines with middle-latitude temperatures to support extensive forests.
Cold winters produce strong seasonality, favoring deciduous trees that
shed their leaves before winter.
Europe developed another large
Temperate Forest region. Its proximity to the Atlantic and the influence
of North Atlantic ocean circulation moderate temperatures and provide
substantial moisture. Western Europe’s winters are therefore
considerably milder than those of many places at comparable latitudes in
continental interiors.
East Asia provides a third example.
Eastern China, Korea, and Japan combine middle-latitude temperatures
with abundant moisture from the Pacific and strong seasonal circulation.
The species are different from those in Europe and North America, but
the basic vegetation structure is similar.
These three regions are
particularly revealing because their forests evolved in substantial
geographic isolation. Oaks, maples, beeches, and many other plant groups
occur in different combinations and species on each continent. Yet
similar climates repeatedly favored tall, dense forests dominated by
broadleaf trees.
The
distribution of the world’s biomes may look extraordinarily
complicated, but most of the pattern can be traced back through a
surprisingly simple chain of causes. Five basic features of physical
geography largely determine variations in temperature, water
availability, and seasonality. Those climatic differences, in turn,
largely determine the dominant vegetation of a region and therefore its
biome.
This is important for far more than understanding why
forests grow in one place and grasslands in another. For most of human
history, people had only limited ability to escape the ecological
constraints of the environments in which they lived. Biomes influenced
which plants and animals surrounded them, which species were available
for domestication, and ultimately what kinds of agriculture were
possible.
That is where we will pick up the story in the next article in this series…
The material for this article largely comes from Susan Woodward’s eight-book series Greenwood Guides to Biomes of the World.
