Evolution And Origin Of The Central Grassland Of North .
Journal of the Torrey Botanical Society 133(4), 2006, pp. 626–647Evolution and origin of the Central Grassland of NorthAmerica: climate, fire, and mammalian grazers1Roger C. Anderson2Behavior, Ecology, Evolution and Systematics Section, 4120 Department of Biological Sciences, IllinoisState University, Normal, Illinois 61790ANDERSON, R.C. (Behavior, Ecology, Evolution and Systematics Section, 4120 Department of BiologicalSciences, Illinois State University, Normal, IL 61790). Evolution and origin of the Central Grassland ofNorth America: climate, fire, and mammalian grazers. J. Torrey Bot. Soc. 133: 626–647. 2006.—Grasslandsare a widespread vegetation type that once comprised 42% of the plant cover on earth’s surface. Featurescommonly shared among grasslands are climates with periodic droughts, landscapes that are level to gentlyrolling, high abundances of grazing animals, and frequent fires. World-wide expansion of grasslandsoccurred 8 to 6 MaBP and was associated with increasing abundance of grasses using the C4 photosyntheticpathway, a decline in woodlands, and coevolution of mammals adapted to grazing and open habitats.Beginning with Transeau’s seminal paper on the prairie peninsula in 1935, North American ecologistsdebated the relative importance of fire and climate in determining the distribution of grasslands. In the1960’s, a major research interest was the response of prairies to fire, especially the productivity of burned andunburned grasslands. Understanding mechanisms for increased productivity on burned prairies began in thelate 1960’s and continued into the middle 1980’s. During the past 20 to 25 years, grassland research hasfocused on the coevolution of grasses and mammalian grazers and fire-grazing interactions that affecthabitat heterogeneity and diversity across trophic levels. While this paper does not follow a chronologicaldevelopment of our understanding of grasslands, all of these major research interests are considered.Key words: bison, C4 grasses, Central Grassland, fire, grasslands, keystone species, mammalian grazers,prairie peninsula.General Features of Grasslands. DISTRIBUSTATUS. Grasslands occurred on allcontinents, comprised almost 42% of theworld’s plant cover, and once covered approximately 46 million km2 of the earth’s surface.Grasslands contain few trees or shrubs, aredominated by grasses (members of the familyPoaceae), and have a mixture of non-graminoid herbaceous species called forbs. Plantfamilies most abundant as forbs are thesunflower (Asteraceae) and pea (Fabaceae)families (Curtis 1971, Risser et al. 1981).Extensive grasslands have been greatly alteredby human activity especially those associatedwith their conversion to agricultural landscapes for growing crops or grazing livestock.Nevertheless, a recent estimate is that about 40percent of the global land surface is grassland(excluding Greenland and Antarctica) (WhiteTION AND1 A substantial portion of this manuscript isreproduced with permission from a book on fuelmanagement being published by the USDA ForestService, entitled ‘‘Cumulative Watershed Effects ofFuel Management in the Eastern United States.’’2 I thank M. Rebecca Anderson for reviewing andcommenting on this manuscript. E-mail: rcander@ilstu.eduReceived for publication November 13, 2006, andin revised form December 5, 2006.et al. 2000). This seemingly high estimate forextant grasslands, however, results from including not only ‘‘non-woody grasslands’’ butalso savannas, woodlands, shrublands, andtundra in the definition of grasslands. Thisestimate of existing grassland is potentiallymisleading, because most ecologists would notinclude woodlands, shrublands, and tundra ina definition of grasslands. In addition, whilethis source recognizes that temperate grasslands have experienced heavy conversion toagriculture, it states that at least five percent ofgrasslands world-wide are ‘‘strongly to extremely denuded.’’ This relatively low percentage possibly results from the inclusion of nontraditional landscapes in the definition of‘‘grasslands.’’ Applying a more widely useddefinition of grasslands, many of the world’stemperate grassland ecosystems have beenessentially destroyed by human activities. Forthe United States, Noss et al. 1995) listsgrasslands as being critically endangered (i.e.have declined by more than 98%). Forexample, 99% of the tallgrass prairie lyingeast and north of the Missouri River has beencompletely destroyed (Chapman et al. 1990).In Illinois, of the 8,502,024 ha of originalprairie (60% of the state), only 931 ha or 0.01percent of high quality remnant prairie re-626
2006]ANDERSON: EVOLUTION AND ORIGIN OF GRASSLANDSmains (Robertson et al. 1997). In this review, Isummarize the results of more than fifty yearsof research studies of grassland ecosystemswith a focus on the Central Grassland ofNorth America. While these studies wereconducted in a highly fragmented landscape,some on prairies that are ten hectares or less insize, they provide an overview of the structureand function of the historic grassland ecosystem, which none of the researchers experienced, and provide insight into how theymight be restored.One of the most marvelous sights of mywhole life, unsurpassed in my travels innearly all parts of the world, was that of theprairie in spring. Unfading are my memories of that waving rippling sea of lavender,when the wild sweet william, a species ofphlox two or three feet in height was in fullflower. It stretched away in the distancefarther than the eye could reach As thesea of phlox faded it was succeeded byanother marvelous flower bed of nature’splanting, and this in turn by others untilmid-summer was reached (Herre 1940).CLIMATE. No single climate characterizesgrasslands and they occur in areas of the earththat receive as little as 200 mm of precipitationannually to areas that receive 1300 mm annually, and in areas where mean annual temperatures vary from 0–30uC (Sauer 1950, Risser etal. 1981, Oesterheld et al. 1999). Grasslands arenot necessarily treeless and they are transitional to savannas that are characterized by higherdensities of drought-tolerant, fire-resistanttrees than grasslands. The ratio of trees/grassincreases as precipitation increases (Curtis1971, Anderson and Bowles 1999, Oesterheldet al. 1999) and in landscapes receiving morethan 650 mm of precipitation there is a trendfor increasing cover of woody species with‘‘long-term fire exclusion’’ (Sankaran et al.2004). In areas of low precipitation, grasslandsgrade into desert communities. Commonfeatures found among grasslands include:climates with periodic droughts, frequent fires,occurrence on landscapes that are level togently rolling, and an abundance of grazinganimals (Saur 1950, Risser et al. 1981, Anderson 1982, Anderson 1990).DROUGHT, FIRE, AND GRAZING ANIMALS.Grassland plants evolved under the influence627of periodic droughts, frequent burning, andgrazing animals and are adapted to all three(Gleason 1922, Anderson 1990). This adaptation for grasses is manifested in their ability todie down to underground organs and onlyexpose dead tops above ground (Gleason1922). Grasses can escape drought by havinggrowing tips beneath soil that are not exposedto desiccation. Prairie fires have a narrowflame width and move relatively rapidly and,because the soil is a good insulator, there islittle penetration of heat into the soil beyonda few mm below the surface (Anderson 1982).Consequently, the growing points of prairieplants below the ground surface are protectedfrom the heat of the fire and also from grazing.Grazers can remove aboveground tissues, butnew shoots can emerge from belowgroundonce the grazing pressure is removed (Taintonand Mentis 1984).The adaptation of grasses to fire, drought,and grazing animals may represent a preadaptation of grasses to one or more of thesefactors; however, grasses and herbivores likelyco-evolved based on other features of grasses.The post-Miocene expansion of grasslandsand savannas worldwide was associated withthe adaptive radiation of large mammalsadapted to grazing (Stebbins 1981, Anderson1982, 1990; Axelrod 1985, McNaughton 1993,Oesterheld et al. 1999). Adaptive responses ofgrasses to herbivores that reflect a coevolutionary relationship between grazers and grassesincludes the presence of silica in epidermalcells of grasses, perennating organs belowground level, and aboveground production inexcess of that which decomposes in a singleyear (Stebbins 1981, Anderson 1982, 1990).The widespread expansion of grassland isassociated with the appearance of the C4photosynthetic pathway. The C4 photosynthetic pathway provides an advantage over themore common C3 pathway because it provideshigher quantum yields for carbon dioxideuptake under conditions of high irradianceand temperature. The C4 photosynthesis isalso favored over C3 photosynthesis when theconcentration of atmospheric carbon dioxideis below 500 ppmV (Cerling et al. 1997,Ehleringer et al. 1997, 2002). During theMesozoic, carbon dioxide concentrations werethought to be greater than 1000 ppmV.However, in the early Miocene or lateOligocene (Kellogg 1999), perhaps 20–25MaBP, decline in atmospheric carbon dioxide
628JOURNAL OF THE TORREY BOTANICAL SOCIETYfavored evolution of C4 plants in moisttropical and subtropical regions (Ehleringeret al. 1997). This photosynthetic pathway isfound in less than 2% of all flowering plantsbut approximately one-half of the 10,000species of grasses and sedges use this pathway.While C4 plants are a small percentage offlowering plants, they contribute 25% of totalglobal productivity, largely due to monocotsin grasslands (Eherlinger et al. 2002).Accelerated development of C4 grasslandsworld-wide occurred during the MiocenePliocene transition (8-6 MaBP) when aridityincreased world-wide associated with theexpansion of the Antarctic Ice Sheet andatmospheric carbon dioxide was below500 ppmV. During this period of time, thearea occupied by forest and woodlands declined and there was an explosive evolution ofgrasses and forbs (Cerling et al. 1997, Ehleringer et al. 1997, 2002). However, Keeley andRundel (2005) posit that the conversion offorest to C4 grassland four to seven MaBPwas not directly due to a decline in atmospheric carbon dioxide or increased aridity,but rather to a climate change that encouragedfire. Under the new climatic conditions a warmmoist growing season resulted in high biomassproduction that was converted into combustible fuels by a pronounced dry season. Thismonsoon climate likely would be accompaniedby frequent lightning strikes at the end of thedry season. In the Keeley-Rundel (2005)model, fire would have been a primary driver,as it is today, in the conversion of forest tograsslands and the maintenance of grasslands.Expansion of open grassland and savannahabitats was associated with increased fossilized silica bodies in the epidermis of grasses,which provide protection against grazing.Concomitantly, there was an increase inmammalian fossils with high-crowned teeth(hypsodonty) adapted to grazing (Stebbins1981, Axelrod 1985) and evolution of animalswith more cursorial (running) and saltational(jumping) body forms.The North American Grassland. Grasslandsof North America constitute a diverse assemblage of vegetation types that occur undera wide range of climatic conditions andcovered about 15% of the continent (Fig. 1).These grasslands are referred to as prairies,a French word meaning meadow or field,which was used by early French explorers to[VOL. 133describe the extensive grasslands of NorthAmerica (Curtis 1971, Risser et al. 1981).Along a north-south gradient, grasslandsextended from desert grasslands of southwestern United States and northern and centralMexico to mixedgrass prairies of the CanadianProvinces of Alberta, Saskatchewan, andManitoba (Risser et al. 1981). Across thisgradient mean annual temperatures vary from2.8uC at Regina, Canada in the northernmixedgrass prairie to 22.6uC in Monterrey,Mexico at the edge of Chihuahuan Desertgrasslands. From south to north along theeastern edge of grasslands lying east of theRocky Mountains, precipitation varies fromabout 250 mm in southeast Texas to 750–1000 mm in Indiana (Risser et al. 1981).Central Grassland of North America.GEOGRAPHIC VARIATION. This papers focuseson the Central Grassland of North American,which was a large triangular shaped grasslandwhose base extended from the Canadianprovinces of Alberta and Saskatchewan southward along the eastern foothills of the RockyMountains and then to southeastern Texas(Outlined in Figure 1). The point of thetriangle extended well into the Midwest insouthwestern Wisconsin, Illinois, and westernIndiana, with scattered outliers in Michigan,Ohio, and Kentucky. This area includes thegrasslands of the twelve Great Plains states,and those grasslands described above lyingeast of the Mississippi River. Precipitationincreases from west to east in this grasslandfrom 260–1200 mm and across a north-southgradient annual temperature ranges from 3–22uC (Sala et al. 1988).Ecologists traditionally have divided thegrassland into three sectors based on heightof the native grasses, which is a function ofannual precipitation: a western shortgrassprairie (260–375 mm precipitation), the eastern tallgrass prairie or ‘‘True Prairie,’’ (625–1200 mm precipitation), and between the twothe mid- or mixedgrass prairie (375–625 mmprecipitation) (Fig. 1). Shortgrass prairie occupies an area dominated by grasses that are0.3–0.5 m tall, which includes buffalo grass(Buchloe dactyloides), blue grama (Boutelouagracilis), and side oats (B. curtipendula) andhairy (B. hirsuta) grama grasses. Big bluestem(Andropogon gerardii), Indian grass (Sorghastrum nutans), switchgrass (Panicum virgatum),and little bluestem (Schzachyrium scoparium)
2006]ANDERSON: EVOLUTION AND ORIGIN OF GRASSLANDS629FIG. 1. The distribution of the major grasslands of North America and the air masses that influence theclimate of the Central Grassland. The location of the Central Grassland is outlined. Adapted from Risser etal. 1981 and Anderson 1990. Reprinted from Anderson 1990 with permission of the University ofOklahoma Press.are dominant species in the tallgrass prairieand reach heights of 1.8–2.4 m. The mixedgrass or midgrass prairie is dominated byspecies that are 0.8–1.2 m tall and includeslittle bluestem, western wheatgrass (Pascopyrum smithii), and green needle grass (Nassella viridula). In the mixedgrass prairie,tallgrass prairie species occur in depressedareas that are moister than upland sitesresulting in a mixture of tall- and midgrassprairie species, which gives the midgrassprairie region its alternate designation ofmixedgrass prairie. Across the Central Grassland, species composition and abundancevaries continuously and there are no sharpdivisions between these arbitrarily designatedgrassland regions.VARIATION WITHIN A GEOGRAPHICAL REGION.Within each of the major regions of theCentral Grassland there are different types ofprairie as a function of soil, aspect, slopeposition and other factors. A primary factorcausing these varied vegetation patterns isavailability of soil moisture as a function ofvariation in soils and topographic features(Curtis 1971, Nelson and Anderson 1983,Umbanhowar 1992, Corbett and Anderson2006). For example, there are approximately930 hectares of high quality remnant prairie in
630JOURNAL OF THE TORREY BOTANICAL SOCIETY[VOL. 133Table 1. Leading species in six community types for species with mean (6 SE) quadrat frequency at least2.0%. Letters after species names indicate its modal community in Wisconsin (Curtis 1971), PD5 dry prairie,PDM 5 dry mesic prairie, PM 5 mesic prairie, PWM 5 wet mesic prairie, PW 5 wet prairie, DUN 5 dune,OB 5 oak barren, CG 5 cedar glade, SB 5 sand barren, FN 5 fen, SS 5 southern sedge meadow, ND 5Northern Dry Forest (Modified from Corbett and Anderson 2001, 2006).Community type speciesSchizachyrium scoparium PDOptunia humifusa CGAmbrosia psilostachya SBCalamovilfa longifolia DUNPanicum oligosanthes PDMTephrosia virginiana OBBouteloua hirsuta CGStipa spartea PDMEuphorbia corollata OBEchinacea pallida PMLithospermum caroliniense SBKoeleria cristata SBCallirhoe triangulata PDMBouteloua curtipendula PDSorghastrum nutans PDMDalea purpurea DPEuphorbia corollata OBSolidago nemoralis DPPsoralea tenuifloraAster azreus DMPAmorpha canescens DPSporobolus heterolepis DPRosa carolinianaAster ericoides PDMAndropogon gerardii PMFragaria virginiana NDCarex sp.Solidago gigantea PWPycnanthemum virginianumPWMCalamagrostis canadensis FNSpartina pectinata PWCarex stricta SSHelianthus grossesserrratusPWMSolidago riddellii FNDry ll prairie1.5 17.3 6 2.9 15.41.82.22.01.7 2.4 6 1.61.71.51.0 4.4 6 2.45.4 6 2.03.1 6 2.02.7 6 2.22.5 6 2.02.3 6 1.69.14.52.2 6 2.2 4.54.13.62.82.52.3Illinois representing diverse habitat typesdiffering in topography and substrate. Dryprairies includes hill and bluff prairies thatoften occupy west or southwest facing slopesoverlooking rivers with loess or glacial driftderived soils (Evers 1955). Dry prairies alsooccur on deep sand deposits or on dolomitic orgravel substrates with shallow stony soils.Additionally, there are wet-mesic to wetprairies on loess-derived, till-derived, or dolomite-containing substrates (Table 1). Historically, the most common prairie types weremesic and wet prairies covering as much as55% of the state (Fehrenbacher et al. 1968),although most of these prairies have beenconverted to agricultural or urban uses.6 0.7Gravel/drydolomiteMesic/drymesic11.6 6 1.43.6 6 0.4Wet/wetdolomite4.5 6 1.03.6 6 0.43.4 6 0.6666666660.80.70.40.40.50.40.50.43.8 6 0.84.4 6 0.44.6 6 83.53.22.22.766661.50.82.11.12.7 6 0.9CLIMATE OF THE CENTRAL GRASSLAND. MajorAir Mass Systems. The climate of the CentralGrassland is influenced by three primary airmass systems (Fig. 1): Polar, Gulf, and Mountain Pacific (Borchert 1950, Bryson and Hare1974, Risser et al. 1981). The Polar Air Massinfluence is reflected in part by the increasedsnow cover and decreasing temperatures fromsouth to north within the Central Grassland(Risser et al. 1981) and the resultant northsouth variations in vegetation patterns (Risseret al. 1982, Kebart and Anderson 1987,Diamond and Smeins 1988). Gulf and Mountain Pacific Air Masses are most important indetermining the east-west variation in theCentral Grassland. The Gulf Air Mass origi-
2006]ANDERSON: EVOLUTION AND ORIGIN OF GRASSLANDSnates in the Gulf of Mexico. As the Gulf Massmoves northward into the eastern sector of theCentral Grassland it brings humid air andoften is associated with precipitation when itencounters cooler air or generates moisture forconvectional storms. The Mountain PacificAir Mass arrives on the west coast as a humidair mass. However, as it progresses eastwardthe air mass passes over several westernmountain ranges (Coastal, Sierra, and Rockymountains). As the air mass rises, it coolsadiabatically, and gives up much of itsmoisture as orographic precipitation. The airmass is compressed by an increasing volume ofatmosphere as it descends to lower elevationson the east side of the Rocky Mountains,causing the air mass to become warmer andmore arid as it spills out into the Great Plains.Thus, the Central Grassland occurs in the rainshadow of the western mountains.From west to east in the Central Grassland,the frequency of the Pacific Air Massdecreases and the frequency of the Gulf AirMass increases. Associated with the change inair mass frequency, mean annual precipitation,periodic droughts, and periods of low humidity during summer decrease from west to eastin the grassland (Transeau 1935, Borchert1950, Bryson and Hare 1974, Risser et al.1981). This west-east climatic variation causesthe changes in vegetation from the foothills ofthe Rocky Mountains to the MidwesternUnited S
Evolution and origin of the Central Grassland of North America: climate, fire, and mammalian grazers1 . In this review, I summarize the results of more than fifty years of research studies of grassland ecosystems with a focus on the Central Grassland of North America. While these studies were conducted in a highly fragmented landscape,
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Le genou de Lucy. Odile Jacob. 1999. Coppens Y. Pré-textes. L’homme préhistorique en morceaux. Eds Odile Jacob. 2011. Costentin J., Delaveau P. Café, thé, chocolat, les bons effets sur le cerveau et pour le corps. Editions Odile Jacob. 2010. Crawford M., Marsh D. The driving force : food in human evolution and the future.
Le genou de Lucy. Odile Jacob. 1999. Coppens Y. Pré-textes. L’homme préhistorique en morceaux. Eds Odile Jacob. 2011. Costentin J., Delaveau P. Café, thé, chocolat, les bons effets sur le cerveau et pour le corps. Editions Odile Jacob. 2010. 3 Crawford M., Marsh D. The driving force : food in human evolution and the future.
