In general, grasslands occur in monsoon

Plant Ecology September 13, 2005

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Plant Ecology in a Changing World

Jim Ehleringer, University of Utahhttp://plantecology.net

Grassland and SteppeBiomes

• climate and global distribution

• North American grasslands and steppe

• mammals and grasslands

• global changes in distribution

Grassland and steppe biomes


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Grassland and steppe regions are characterized

as regions where the ratio P / PET is near 1

In general, grasslands occur in monsoon

or aseasonal (summer-wet) climate regions


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Global distribution of grassland biomes

Grasslands occupy approximately 24 % of the land surface

Grasslands by region

• prairies of North America

• pampas of South America

• velds of southern Africa

• grasslands of Sahel in northern Africa

• puszta of eastern Europe

• steppes of Russia

• grasslands of Mongolia and central Asia

• grasslands of Australia


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Steppe is a term incorporating both grasslands and

shrublands, and usually refers to high elevation sites

fire


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Grasslands of North America

• tall-grass prairie (C3 and C4)

• mixed-grass prairie (C3 and C4)

• short-grass prairie (C3 and C4)

• desert grassland (C3 and C4)

(Chihuahuan)

• California grassland (C3)

• Palouse prairie (C3)

Grassland climates

• typically continental climates

• typically hot summers and cold winters

• precipitation is generally 500 to 1,000 mm per year

• during growing season, precipitation approximately

equals evapotranspiration

• soil water typically inadequate for tree growth,

especially establishment


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Climate diagrams for grasslands

Most grasses have fibrous roots penetrating down to 1 to 2 meters depth


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Dominant life forms

• predominantly perennial grasses

• herbaceous dicots

• shrubs and trees restricted to depressions

• shrubs and trees at wetter end of the distribution (parklands)

• cool season grasses tend to be C3

• warm season grasses tend to be C4

C4 grasses predominate in warm habitats


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Grasslands of California, Washington,

and the Intermountain West are all

dominated by C3 plants

• primarily winter rain

• at present, dominated

by invasive species

• California and Washington

grasslands have largely

been converted to crops


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150 mm 300 mm 500 mm

Artemisia tridentata 14% 14% 0%

Agropyron spicatum 45% 35% 51%

Poa secunda 49% 29% 28%

Festuca idahoensis 0% 24% 38%

The relative abundances of grasses and

shrubs in the Great Basin steppe is a

function of precipitation

Creation of the steppe region

in western North America

is geologically recent Cascade


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During the Miocene, the area between British

Columbia and Utah was occupied by forests.

Uplift of Cascade Mountains created a rain

shadow with a dry region between Cascades

and Rockies.

Fire has played a role in determining the

relative abundances of shrubs versus grasses.

Shrub steppe in Utah

Atriplex Coleogyne

Juniperus Atriplex - Sarcobatus


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Burr Desert near Hanksville

Atriplex and Gutierrezia

dominate on both saline

and non-saline soils.


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Artemisia tridentata: big basin sage

Grasslands are heavily impacted by grazing.


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Thure Cerling

In arid regions, grazing

has a negative impact

on productivity.

In mesic regions, grazing has a

positive impact on productivity.


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Since 1700, grasslands, savannas, and forests

have decreased by approximately 15 %

source: Ramankutty and Foley, 1999

Grassland recovery following disturbance can be slow


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What is left of the

original grasslands

of North America?

……very little

ColoradoKansas

South Dakota


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source: Ramankutty and Foley, 1999

Agricultural modification of the land surface was

already a significant part of the landscape in 1700

Human Use of Resources

• extensive clearing of forests, approximately

8 million km2 net loss

• annual consumption of water exceeds

volume of Lake Huron

• since 1850:

- need for grain and water has tripled

- fossil-fuel use increased four-fold


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QuickTime™ and aGIF decompressor

are needed to see this picture.

Source: Ramankutty & Foley, 1999

Global Cropland Distributions




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In general, grasslands occur in monsoon