Change & Persistence Among Prairie Grasses
Story and Photos by Dan Carter
There are many misconceptions about prairies that cloud restoration, reconstruction, and management. Prominent among these is the tallgrass prairie “big four,” a concept that situates big bluestem (Andropogon gerardii), Indiangrass (Sorghastrum nutans), switchgrass (Panicum virgatum), and little bluestem (Schizachyrium scoparium) atop the dominant hierarchy of plants on tallgrass prairie. The “big four” has far-reaching influence on grassland management, scientific study, and seed mix design. It’s Tallgrass prairie after all!
The “big four” are indeed co-dominant in many places where prairie vegetation occurs today. But, except for little bluestem, they were not historically the most dominant grasses on much of the prairie landscape, nor are they most dominant on many of the best remaining old-growth prairies.
John Curtis (1959)1 described the composition of the least disturbed old-growth prairies in Wisconsin. Big bluestem was present on all studied mesic prairies, but porcupine grass (Hesperostipa spartea), Leiberg’s panic grass (Dichanthelium leibergii), and prairie dropseed (Sporobolus heterolepis) were the most frequent grasses. Frequencies from Curtis are the percentage of square meter quadrats a species occurs within for a given community type — basically how likely the species is to be at your feet if you are walking in the prairie. porcupine grass was twice as frequent on mesic prairie as big bluestem! Big bluestem was the fifth most frequent grass on dry prairies behind little bluestem, side-oats grama (Bouteloua curtipendula), long-stalked panic-grass (Dichanthelium perlongum), and prairie dropseed; and third most frequent on dry-mesic prairie behind little bluestem and side oats grama. Only on wet-mesic prairie was big bluestem the most frequent among the grasses. Still, on wet-mesic prairie little bluestem’s frequency was about three quarters that of big bluestem. Prairie cordgrass (Spartina pectinata) and Canada blue-joint grass (Calamagrostis canadensis) were the species most often present (frequency data lacking) on wet prairie.
In Iowa, The only grasses noted by Ada Hayden (1919)2 among the “principal” species of prairie remaining on the gently rolling uplands (mesic) immediately north of Ames, Iowa were porcupine grass and prairie dropseed. Later, Brotherson (1969)3, Kennedy (1969)4, and Glenn-Lewin (1976)5 studied composition on three old growth prairies in northern and western Iowa and found prairie dropseed, Leiberg’s panic grass, and porcupine grass to be the most common on uplands at the respective sites.
In the Red River Valley of NW Minnesota, Dziadyk and Clambey (1980)6 described old growth prairie communities dominated by blue grama (Bouteloua gracilis) and porcupine grass on dry ground, prairie dropseed followed by little bluestem on gentle slopes, little bluestem followed by prairie dropseed on moderately well-drained level areas, and big bluestem and slim-stem reed-grass (Calamagrostis stricta) together on low prairie over poorly drained soils.
Weaver’s and Clements’ (1938)7 concept of “true prairie,” which they extend to a region stretching from Illinois to Nebraska and northwest Minnesota to Oklahoma, is co-dominated by mid grasses—Porcupine grass, prairie dropseed, rough dropseed (Sporobolus compositus), little bluestem, side-oats grama, and needlegrass (Hesperostipa comata, in the west). Weaver worked extensively on prairies in the western part of the tallgrass prairie during the first half of the 20th century, including early study of fire effects at the Agricultural Experiment Station just north of Manhattan, Kansas. There, little bluestem and Junegrass (Koeleria macrantha) were initially the top two grasses (big bluestem was third). Composition shifted toward big bluestem with annual late spring burning but not late fall burning or earlier spring burning8,9. Indeed, late spring burning in the western and southwestern tallgrass prairie region to promote big bluestem for cattle pasture is part of why prairie composition changed there during the 20th century. Weaver and Clements observed these changes occurring and attributed them to the grazing and burning practices of the time, saying that the result was “that their [the mid grasses’] tallgrass competitors, notably Andropogon, gradually moved up the slopes and today appear to be essential members of the prairie relicts” (page 458).
Why did European land use sometimes drive compositional change towards the tall grasses like big bluestem?
Late spring burning favors the growth form of long rhizomatous, warm-season grasses. Their growing points remain below the soil surface until very late spring or early summer, so growth of their active shoots can continue uninterrupted despite damage to aboveground foliage with late spring burns. The growing points of most bunchgrasses (e.g., porcupine grass, prairie dropseed, Leiberg’s panic grass, little bluestem, Junegrass, etc.) rise above the soil surface and become vulnerable to fire shortly after they initiate growth. If these are burned off, the bunchgrasses must activate reserve buds to replace the lost shoots. That alone puts them at a disadvantage, but their reserves of buds tend to be small compared to long-rhizomatous big bluestem and Indiangrass10, so their regenerative capacity is sooner exhausted (meristem-limited) in response to removal of active shoots. The cool-season bunchgrasses are hit especially hard by late spring burning because of their early growth, but even little bluestem, a warm-season species, can be harmed by later spring burns due its difference in growth form. Prairie dropseed, another warm-season grass, is harmed because it initiates growth nearly as early as the cool-season species despite its warm-season physiology. On most upland old growth prairie, late spring burning favors a subset of native grasses that was not historically so abundant.
The effect of fire exclusion on composition can be similar to those of frequent late spring burning. Species with elongating rhizomes are better able to emerge through excessive accumulations of thatch. Hensel (1923)11 observed this 100 years ago in the Kansas Flint Hills. Little bluestem increased with annual early spring burning, but big bluestem replaced little bluestem atop the dominance hierarchy when fire was excluded. Weaver and Rowland (1952)12 also observed this in eastern Nebraska in the absence of burning, haying, or grazing:
“Consequences of the effects of the mulch upon the environment were production of a nearly pure, but somewhat thinner than normal, stand of Andropogon [big bluestem]. The understory of upland prairie had all but disappeared. The usual mid grasses of upland were few or none. Only a few taller forbs remained.”—Page 19
Burning in the presence of excessive litter accumulation, which often occurs on prairies that are occasionally burned (as opposed to frequentyly)kill or weaken little bluestem13 and other bunchgrasses (e.g., needlelegrass)14. Their buds are at or just above the soil surface and vulnerable to increased fire duration when excessive litter has built up. This is not the case for the deeply buried buds along the rhizomes of big bluestem or Indiangrass. Interestingly, excessive litter may interact with fire to affect prairie bunchgrasses and certain invertebrates (skippers: Hesperia ottoe and H. Dakotae)15 in similar ways, with responses contingent on the amount of litter accumulation!
Native bunchgrasses decrease for many of the same reasons in response to confined grazing. Porcupine grass is very palatable and emerges before most other prairie grasses, so it disappears quickly upon pasturage16. The long-rhizomatous prairie grasses also decrease in response to grazing16, but they persist and recover relatively well during rest periods because they have greater reserves of belowground buds available for recovery and their elongating rhizomes help them colonize openings where vegetation has been thinned by disturbance. The position of buds on these long-rhizomatous grasses an inch or two beneath the soil surface also protects their regenerative capacity from mechanical disturbance 10,17. Weaver recognized the importance of rhizomatous habit for recovery from disturbance, but not bud depth or number. Nonetheless, where grazing was too intense and prolonged, most prairie grasses were replaced by long-rhizomatous, cool-season species like Kentucky bluegrass (Poa pratensis), except on the driest sites1,2,16.
The work of Weaver, Curtis, Hayden, and others adds important context to our interpretation of more contemporary studies of prairie. They help us discern between research and management outcomes from altered grasslands that no longer retain old growth composition, and prairies that still do. Porcupine grass, little bluestem, prairie dropseed, side-oats grama, and/or Leiberg’s panic grass are usually among the prominent grass species on the best remaining old growth, upland prairies. All of those species differ from big bluestem in their ecologies in ways that have implications for management. Except side-oats grama, many of those differences stem from growth form, cool-season physiology, or both. Earlier work on composition also highlights the amazing persistence of well-stewarded and less historically exploited old-growth prairies in the face of unprecedented change. Upland old-growth prairies that retain much of their composition have typically experienced:
- fewer periods of excessive litter accumulation.
- fewer late spring burns and more burns between fall and early spring—the more frequent the better9,18,19. True prairie composition was and is an expression of dormant season fire.
- minimal fenced grazing. Free-roaming deer, elk, bison, and their predators/hunters are separate issues.
- less fragmentation19, but consider that small, less exploited prairies that are well-stewarded retain more of their historical botanical composition than landscape grasslands in the western tallgrass region. Little prairies are more vulnerable to neglect, which argues for their protection and care.
While the confluence of these conditions is tragically rare, the persistence of what remains is reason to keep hope. True prairie in the Midwest has been home to members of east-west and north-south expanding and contracting flora, fauna, and cultures for millennia. Even an island of old-growth prairie carries with it immeasurable ecological memory. We can kindle that and facilitate its recovery through stewardship and by building connections among prairie places and prairie people…especially if we can get our hands on more porcupine grass and Leiberg’s panic-grass!
1 Curtis, J. 1959. The vegetation of Wisconsin University of Wisconsin Press. Madison, WI.
2 Hayden, A. 1919. Notes on the floristic features of a prairie province in central Iowa. Proceedings of the Iowa Academy of Science 25: 369-389.
3 Brotherson, J. 1969. Species composition, distribution, and phytosociology of Kalsow Prairie, a mesic tall-grass prairie in Iowa. Dissertation, Iowa State University.
4 Kennedy, R. 1969. An analysis of tall-grass prairie vegetation relative to slope position, Sheeder Prairie. M.S. Thesis, Iowa State University.
5 Glenn-Lewin, D. 1976. The vegetation of Stinson Prairie, Kossuth County, Iowa. Proceedings of the Iowa Academy of Science 83: 88-93.
6 Dziadyk, B. and G. Clambey. 1980. Floristic composition of western Minnesota tallgrass prairie. Proceedings of the Seventh North American Prairie Conference: 45-54.
7 Weaver, J., and F. Clements. 1938. Plant ecology. McGraw-Hill Book Company, Inc. New York and London.
8 Weaver, J., and A. Aldous. 1935. Role of fire in pasture management. Ecology 16:651–654.
9 Towne, G., and C. Owensby. 1984. Long-term effects of annual burning at different dates in ungrazed Kansas tallgrass prairie. Journal of Range Management 37: 392-397.
10 10 Ott, J., Klimešová, J., and D. Hartnett. 2019, The ecology and significance of below-ground bud banks in plants. Annals of Botany 123: 1099-1118.
11 Hensel, R. 1923. Recent studies of the effect of burning on grassland vegetation. Ecology 4: 183-188.
12 Weaver, J. and N. Rowland. Effects of excessive natural mulch on development, yield, and structure of native grassland. Botanical Gazette 114: 1-19.
13 Gagnon, P., K. Harms, K., Platt, W., Passmore, H., and J. Myers. 2012. Small-scale variation in fuel loads differently affects two co-dominant bunchgrasses in a species-rich pine savanna. PLoS ONE 7: e29674.
14 Haile, K. 2011. Fuel load and heat effects on northern mixed prairie and four prominent rangeland graminoids. Thesis, Montana State University-Bozeman.
15 Dana R. 1991. Conservation management of the prairie skippers Hesperia dacotae and Hesperia ottoe. Minnesota Agricultural Experiment Station Bulletin 594, University of Minnesota.
16 Weaver, J. 1954. North American prairie. Johnsen Publishing Company. Lincoln, NE.
17 Klimešová J, and L. Klimeš. 2007. Bud banks and their role in vegetative regeneration – a literature review and proposal for simple classification and assessment. Perspectives in Plant Ecology, Evolution and Systematics 8: 115–129.
18 Bowles, M. and M. Jones. 2013 Repeated burning of eastern tallgrass prairie increases richness and diversity, stabilizing late successional vegetation. Ecological Applications 23: 464-478.
19 Alstad, A., Damschen, E., Givnish, T., Harrington, J., Leach, M. and D. Rogers. The pace of plant community change is accelerating in remnant prairies. Science Advances. 2: e1500975