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Lecture 11: Temperate Grasslands (reading Archibold Ch. 7)

Specificdefinition of

grasslandscontrolswhere theyare

mapped;temperate

vs.subtropical

climates;savannasand desertsas othervegetationtypes/ZBs

we willdiscuss

later.!

Allgrasslands!

Only temperate

grasslands!

I. Definition: A biome or vegetation type dominated by

herbaceous species of the Poaceae (grass family); largely moisture controlled (i.e., seasonal drought),with insufficient soil recharge for most shrubs and

trees; this seasonality of climate favors perennialgrasses, with temperature a secondary influence. !

Includes prairie of North America and steppe of Eurasia,pampas of Argentina and grassveld of southern

Africa's high plateaus, as well as smaller grassland

areas of western North America, New Zealand, andsoutheast Australia. !

South Dakota Prairie

Gradients from tall perennial grasses to shorter

bunch grasses to annuals are seen as moisturedeclines, as well as shifts according to latitudebetween warm season and cool season species.Other monocots, leafy dicots and small shrubs

often present in associations. !

Big Bluestem

(Andropogon gerardi)

Little Bluestem!(Andropogon

scoparius)

tallmid

short

Blue Grama(Bouteloua gracilis)!

II. Geographic Distribution - !A. Northern Hemisphere - two major formations:!

1. North American formation -!a. interior lowlands of continent (central N.

America) from southern Canada "prairiestates" into U.S.A. and northern Mexico

across more than 30 degrees of latitude.

Intergrades with largely deciduoushardwood forest to east and montane

coniferous forest to west. Floristicschange along gradients and floristic

diversity increases in lower latitudes. !b. Grasslands of Pacific NW and California

are different but similar to each other (with

California having more native annual

species). Many of the formations arebelieved to be young (Holocene in age). !

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Grassland map fromField Guide to theCommon WesternGrasshoppers (2009)by Robert E. Pfadt(adopted from a mapby Dodd, 1982)!

2. Eurasian formation - oriented largely west-eastacross the continent, including the forest-steppe

(tall grass association) from eastern Europe intoeastern Siberia, where small groves of trees are

interspersed with the grasses. The tuft-grass

steppe is to the south in drier conditions. The

sagebrush-grass steppe is further south in Eurasiaextending discontinuously from the Black Sea intoChina and around the Gobi Desert. Mountainsteppe is found at slightly higher elevations with a

mixture of taller grasses and many herbs.!Eastern Steppe in China!

B. Southern Hemisphere:!1. The pampas of eastern Argentina, Uruguay and southeast Brazil are

the largest SH grassland. Tall and medium bunch grasses with small

patches of trees are common. Shorter grass communities are found onmore alkaline soils with a change in species to the more arid west. Desert

communities are found to the west and subtropical formations to the north.2. In South Africa, dry, cold-temperate veld grasslands are found at

1200-1500 m elevations on plateaus, with many genera not commonelsewhere. To the east as elevations increase, moist cool-temperategrasslands are found. !

3. In New Zealand, both short- and tall-tussock grasslands are found on the

South Island to the lee of the Southern Alps from sea-level to 1500 m asl,with taller bunch grasses at the higher elevations. Most of the zone above

1200 m asl is believed to previously have been forests that were destroyedby repeated fire and heavy grazing. The North Island has a smaller areaof grassland on the central volcanic plateau, but grasslands here are

believed to be largely an early seral stage subsequently replaced by heathand forest. !

4. Austral ia has small patches of grassland scattered throughout thetemperate woodlands in cooler sites. Grass species vary from tall to short

and from warm-season to cool season.!

South African Veld Argentina Pampas

New Zealand GrasslandAustralia Grassland

Archibolds map

Grasslands in South

America are bothNeotropical andPaleotropical inorigin, existing in

subtropical climates

on unusual (old)

soils, and intemperate climates

further south and inthe mountains.!

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III. Environmental Controls!A. Climate:! Primarily Zonobiome VII [Arid-temperate with a cold winter

(continental)], with the degree of aridity varying considerably andleading to the definition of four sub-zonobiomes. !

Temperate grasslands are also found in ZB IV in California (winterrain and summer drought, arido-humid) and in an orobiome withinZB II (Tropical with summer rains, humido-arid) in South Africa.!

Precipitation and water balance: seasonal drought and a prolonged soilmoisture deficit are common, with vegetation diversity across regionsreflecting gradients in precipitation and evapotranspiration. Littlesurplus moisture is present at any time of the year. Precipitation is alsoquite variable annually. !

Temperature: climates are quite variable in terms of temperature,especially in North America with interior continental climates. SouthernHemisphere grasslands have less extreme seasonality. !

Topography is a key influence on climate. !

Zonobiome VII

and others!

CLIMATE GRADIENTS: For the North American prairie, to the lee ofthe Great Rocky Mountain Cordillera, precipitation varies from350-1200 mm (14 - 48 in.) increasing to the east and south. ! In Eurasia, seasonal temperature changes set up pressuregradients with a continental high pressure cell over the central Asiancontinent blocking cyclonic storms. As low pressure develops in thesummer, cyclonic precipitation is received. Precipitation varies from200-600 mm (8 - 24 in.), with a large annual temperature rangetypical of continental interiors.! Climatic conditions are less severe in the Southern Hemispheredue to lower latitude and more maritime locations of the grasslands.

In the Pampas of Argentina and Brazil, precipitation increases to theeast from 450 to 1200 mm (18 - 48 in.). Evaporation demands arehigh. In South Africa, high pressure systems also develop over thecontinent and influence precipitation. Precipitation increases fromwest to east with increasing elevations (450-1000 mm or 18-40 in.).About 70% of the rainfall occurs in the summer. In New Zealand,prolonged droughts can occur, with precipitation varying from350-750 mm (14-30 in.).!

B. Soils - Mollisols are common, with thick dark brown to black Ahorizons rich in organics. High root densities occur throughout thesoil profile, as high organic matter promotes a "crumbly" (crumbs togranular to small subangular blocky) structure. Soils have a neutral toslightly acidic pH in the near surface and are slightly alkaline at depthwith leaching and the accumulation of carbonates(illuviation). Withlower amounts of moisture in semi-arid regions, carbonates mayaccumulate in the soil at depth and form aridisols. !In the Pampas of South America, a variety of soil types are found,including shallow mollisols, andisols, aridisols, and vertisols. ! Most of the South African grassland soils are alfisols, with some

oxisols, as well as vertisols and aridisols with varying parent material. In New Zealand, most of the grassland soils are inceptisols andandisols due to volcanic activity and the young age of many surfaces. In California, mollisols are most common, but aridisols andinceptisols are also found. ! It is also very apparent in grasslands that soils along withtopography set up moisture gradients and result in a mosaic ofcommunities (e.g., a toposequence).!

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C. Fire - Adaptation of grasses to drought also helps

make them resistant to fire. Fires are frequent (10-25

year return interval), but of low intensity in fine fuels.Many tree and shrub species are also eliminated by fire,

with an increase in or persistence of grasses under highfire frequencies (e.g., fire subclimax).! These relatively rapid and low temperature fires havelittle impact on soil organic material, microbes, burrowingmammals, or buried seed banks. ! Rhizomatous grass species are also more resistant tofire than bunch grasses, with less dead matter

concentrated at the grass meristem base. Very frequent

fire however will eventually eliminate most grass species(and everything but annual weeds). ! Fire is also important in maintaining the productivity of

many grasslands, resulting in combustion of litter anddead stems, with increases in light and nutrients.!

In addition to wildland grass fires, fire is also used as a

management tool. Why?!

D. Biotic factors - Grazing is a major influence on

grassland composition, as grass species vary in their

tolerance to it as a function of their growth form andproductivity. Evolution of many mammals, and also

some birds, in the late Tertiary (Neogene) and

Pleistocene parallels the development of grasslands.! Below ground feeding by invertebrates mostly on plant

sap, whereas above ground feeding invertebrates feedmainly on tissues (like grasshoppers).! Small mammals are widely distributed, and many are

obligate burrowers.! Large native grazing animals have been ext irpatedfrom many grasslands, but include deer, elk, bison, etc. !

IV. Floristics: !A. Origins - diverse, deriving from a series of ancestral

floras largely in the late Tertiary (Neogene), primarily(Miocene 10-12 Ma), with cooling and drying trends

in the Pliocene 5 - 6 Ma, as well as during the

Quaternary. (A few S.H. genera older.) Axelrodbelieves that many of our North American grassland

species are Holocene in age (postglacial). In NorthAmerica, Pacific Northwest (Palouse prairie) andCentral Valley grasslands of California are different

than the prairie of the Great Plains/central lowlands.!B. Expansion of grasslands in the Miocene was

accompanied by changes of mammalian faunas,with grazers like the horse, camels, and antelopes

evolving high-crowned teeth in which growth of theteeth replaces the worn surfaces (with the high

silica content of grasses). Speed and endurance

evolved as well to escape predators.!

C. Dispersal of grass seeds aids widespread

distributions of many species - The hairs and hookson leaves and fruits of grasses have aided theirdispersal by birds and mammals, hence the widespread

distribution of many species. All of the continents havesimilar genera in the family Poaceae, but different

species are dominant in different places. ! Some interesting biogeographic patterns are seen onislands, such as the for the South Temperate Oceanic

Islands (e.g. Macquarie Id), suggesting vicarious

species pairs (two distinct but closely related species) orseries (more than two species) for the genus Poa. !

Spiny burgrass (Cenchrus incertus & C. longispinus) -

flowering spikes have 10-40 small burrs.Java Sparrow!

D. Important Native Taxa - (species vary within and betweencontinents - similar genera) - The Grass Family (Poaceae) ranksfifth globally in terms of number of species, third in number ofgenera, is the most geographically widespread, and is the mostdominant as a vegetation component globally. Its "hay-day" isin the temperate grasslands. ! Important genera include Bouteloua(grama), Hilaria(galleta),Andropogon(bluestem), Panicum(switch grass), Stipa(needle-grass), Agropyron(wheat grass), Buchloe(buffalo grass), Aristida(awn) in the Great Plains; Agropyron(wheat grass), Elymus(rye),

Festuca(fescue), and Koeleria(June grass) in the Palouse; Stipaand Poain the Central Valley; Stipa, Agropyron, Poa, BromusandCarexin the Eurasian forest-steppe, Festuca, Koeleria, Poa, andStipain the tuft-grass steppe, and Stipaand Festucain thesagebrush-grass steppe; Stipa, Panicum, Papsalum, andBothriochloain the Pampas; Themeda, Eragrostis, Aristida, andDigitariain South Africa; Festuca, Poa, Agropyron, and Chionochloain New Zealand; and Themeda, Stipa, Poaand DanthoniainAustralia.! Members of the sedge family Cyperaceae are also commonassociates, such as Carex.!

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V. Physiognomy/Autecology:!A. Vegetation types and dominant taxa - In grassland

formations, it is common for relative diversity values forgrasses to be low (20-30% of species present), but for

dominance (cover) values to be high (80-90%). Formationsmay be further described as tall- to mixed- to short-grass

prairie or as a bunch-grass association (less continuous grasscover). It is also common for only two or three species todominate the association.!

Nebraska, Indian Grassand Big Bluestem

B. Life/growth forms -Perennial grasses aredominant species(cryptophytes)in grasslands,but several growth habits exist: tillering, non-creeping grasses(also commonly called bunchor tufted grasses) constantlyproduce new leaves at the tipof the stem and additionalshoots arise in the leaf axilsand produce leafy shoots[species include Stipa comata,(needle and thread) Koelariacristata(June grass), andBouteloua gracilis (bluegrama)]; tillering is suppressedby auxin in the expanding leafsbut promoted with grazing aslong as the short stemprojecting above the ground isnot damaged.!

Calamagrostis nutkaensis

(Pacific Reed Grass)

sod-forming grasses have secondary lateralshoots that arise from vegetative buds in theleave axils or in the nodes of older stems, withthese secondary shoots growing as rhizomesbelow the soil surface; the terminal growingpoint of the rhizome eventually emerges fromthe ground and begins to form leaves;Agropyron smithii(western wheat grass) andPanicum virgatum(switchgrass) arerhizatomous. A few grasses have theirsecondary lateral shoots grow across thesurface as stolons instead of under it asrhizomes, developing roots and shoots atnodes; Buchloe dactyloides(buffalo grass) andAgrostis stolonifera(creeping bentgrass)exhibit this behavior.!

Festuca rubra (red fescue)!

Root systems vary greatly andinclude those of!

1. short-grass species - rootsto 30-90 cm depth;!

2. tall-grass species - rootsto 150-180 cm depth!

3. tillering bunch grasses -dense, fibrous rootsradiating in all directionsfrom stem base;tremendous variation indepth, radius of rootsystem, and rootdiameters betweenspecies.!

Grasses invest in!carbohydrates in their !roots and leaves; helps !them resist both grazing !and drought.!

Stipa!

Koeleria

!An

drop

ogon

!Sporobolus!

C. C3 vs. C4 metabolism (different pathways of carbon fixation);a

species uses one or the other mechanism:!- C3: normal photosynthesis; greater photosynthetic ratesand productivity at lower temperatures; !-C4: these grasses are more dominant in drier and warmerclimates due to their efficient use of water; better able to

handle heat & high light intensities for photosynthesis; not

tolerant of low temperatures; !- Global warming implications? C3 grasses take up CO2more rapidly -- may have an advantage and dominate-- butC4 grasses do better at warmer temperatures --- so, the

response may depend where you are? For Australia, theprediction is that C4 grasses would dominate,outcompeting the C3 grasses.

C-4 Pathway Some plants have developed a preliminary step to the Calvin Cycle (which isalso referred to as a C-3 pathway), this preamble step is known as C-4. While most C-fixationbegins with RuBP, C-4 begins with a new molecule, phosphoenolpyruvate (PEP), a 3-C

chemical that is converted into oxaloacetic acid (OAA, a 4-C chemical) when carbon dioxide iscombined with PEP. The OAA is converted to Malic Acid and then transported from themesophyll cell into the bundle-sheath cell, where OAA is broken down into PEP plus carbondioxide. The carbon dioxide then enters the Calvin Cycle, with PEP returning to the mesophyll

cell. The resulting sugars are now adjacent to the leaf veins and can readily be transported

throughout the plant.

C-4 photosynthsis involves the separation of carbon fixation and carbohydrate

systhesis in space and time. Image from Purves et al., Life: The Science ofBiology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WHFreeman (www.whfreeman.com), used with permission

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D. Microbial and animal associates - ! high numbers of microbes in rooting zone well supplied withdead organic matter; bacteria and fungi important; rapiddecomposition;! high diversity and abundance of soil invertebrates, including

nematodes, microarthropods, millipeds, larvae of some largerinsects, etc.;! above-ground consumers include (1) many invertebrates(such as grasshoppers, beetles and weevils) that feed onyoung leaves and stems typically of a single species, (2) smallherbivorous mammals and birds that feed on young plant partsand seeds, and (3) large grazing mammals. Many of the birdsare omnivorous, feeding on seeds and insects. Many speciesof small mammals, including pocket mice, voles and groundsquirrels. Many of these species are obligate burrowers. ! large herbivores include deer, elk, antelope, bison, guanaco,(and yes, cattle and sheep).! humans - human impacts are many and significant, hencethe loss of many of our native grasslands via over-grazing,agricultural conversion, suppression of fire, and general landuse change. Important restoration efforts are in progress.!

Greater Rhea Maned Wolf(Chrysocyon brachyurus)

Vicuna!Guanaco!

Pampas ofSouth

America!

Mitchell Grasslandsof Australia - heavy,cracking clay soils!

Speckled Brown Snake!

SpencersMonitor!

Singing Bushlark!

Julia Creek Dunnart(Sminthopsis douglasi)

Long-tailed planigale

(Planigale ingrami)

Przewalski's HorseCorsac Fox!

Mongolian SaigaAntelope!

Mongolian

Gerbil!

MongolianGazelle!

Asian Steppe!

VII. Human Impacts:!!A. Grazing: developed in grassland systems; largemammals (especially cattle and sheep) transported into newregions with expansion of human settlement; overgrazing.!!B. Agricultural conversion: grassland soils are oftenrich and easily tilled; accomodate annual crop plants, especially

grasses; with irrigation, other crops can be cultivated;

grassland conversion/loss.!!C. Fire supression: can change species compositionand abundances; may allow woody species establishment;decreased productivity. !!D. Non-native species: often take advantage of alteredgrazing, agricultural and fire regimes; may convert nativegrassland to almost all non-natives; loss of ecological functions.!E. Global warming: more frequent droughts in mid-latitudes, shifting plant communities, increased area likely.!