Grassland and steppe biomes

Transcription

1 Grassland and Steppe Biomes Plant Ecology in a Changing World Jim Ehleringer, University of Utah Grassland and steppe biomes climate and global distribution North American grasslands and steppe mammals and grasslands global changes in distribution 1

2 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 2

3 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 3

4 Steppe is a term incorporating both grasslands and shrublands, and usually refers to high elevation sites fire 4

5 Grasslands of North America tall-grass prairie (C 3 and C 4 ) mixed-grass prairie (C 3 and C 4 ) short-grass prairie (C 3 and C 4 ) desert grassland (C 3 and C 4 ) (Chihuahuan) California grassland (C 3 ) Palouse prairie (C 3 ) 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 5

6 Climate diagrams for grasslands Most grasses have fibrous roots penetrating down to 1 to 2 meters depth 6

7 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 C 3 warm season grasses tend to be C 4 C 4 grasses predominate in warm habitats 7

8 Grasslands of California, Washington, and the Intermountain West are all dominated by C 3 plants primarily winter rain at present, dominated by invasive species California and Washington grasslands have largely been converted to crops 8

9 The relative abundances of grasses and shrubs in the Great Basin steppe is a function of precipitation 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% Cascade Creation of the steppe region in western North America is geologically recent 9

10 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. Atriplex Shrub steppe in Utah Coleogyne Juniperus Atriplex - Sarcobatus 10

11 Plant Ecology September 13, 2005 Burr Desert near Hanksville Atriplex and Gutierrezia dominate on both saline and non-saline soils. 11

12 Artemisia tridentata: big basin sage Grasslands are heavily impacted by grazing. 12

13 Thure Cerling In mesic regions, grazing has a positive impact on productivity. In arid regions, grazing has a negative impact on productivity. 13

14 Since 1700, grasslands, savannas, and forests have decreased by approximately 15 % source: Ramankutty and Foley, 1999 Grassland recovery following disturbance can be slow 14

15 What is left of the original grasslands of North America? South Dakota very little Colorado Kansas 15

16 Agricultural modification of the land surface was already a significant part of the landscape in 1700 source: Ramankutty and Foley, 1999 Human Use of Resources extensive clearing of forests, approximately 8 million km 2 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 16

17 Global Cropland Distributions QuickTime and a GIF decompressor are needed to see this picture. Source: Ramankutty & Foley, 1999 Global Cropland Distributions Source: Ramankutty & Foley,

18 Global Cropland Distributions Source: Ramankutty & Foley, 1999 Global Cropland Distributions Source: Ramankutty & Foley,

19 Global Cropland Distributions Source: Ramankutty & Foley,