BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Andropogon gerardii var. gerardii | Big Bluestem 



GENERAL BOTANICAL CHARACTERISTICS : 
Big bluestem is a native, tall, erect, perennial, warm-season, usually
rhizomatous, sod-forming grass [47,117].  The flowering stalks are
stout, coarse, and solid, and can reach a height of over 10 feet (3 m)
in a favorable growing season [51].  The inflorescence consists of two
to six digitate racemes (most commonly three, hence one of its common
names "turkey foot") which are 2 to 4 inches (5-10 cm) long [47,117].
Spikelets are in pairs along the raceme axis and are approximately equal
in length (0.25 to 0.4 inch [7-10 mm]) [47,117,131].  One is sessile,
usually fertile, plump, and awned.  The other is pedicillate, sterile,
slender, and awnless [82,131].  Big bluestem stands display a variety of
colors from steel gray to wine red [14].

Rhizomes are generally 1 to 2 inches (2.5-5 cm) below the soil surface
but are common to a depth of 4 inches [3,133].  They are coarse, 0.1 to
0.2 inch (3-6 mm) thick, excessively branched, and are frequently
continuous for 6 to 10 inches (15-24.5 cm) [133].  These rhizomes form
an open network, which is branched by the multiple roots which arise
from them.  On mature plants, roots are coarse, with laterals extending
2 to 6 inches (5-15 cm) [133].  The main roots extend downward almost
vertically to depths of 6 to 10 feet (2-3 m), and taper very gradually
[132,133].  A large percentage of the roots are found in the A horizon
independent of soil type or horizon depth.  Seventy-eight to ninety
percent of root biomass has been found in A horizons ranging from 7 to
20 inches (18-50 cm) [136].

Big bluestem exibits wide ecotypic variation.  It is generally shorter
at maturity in the northern portion of its range, and taller in the
southern portion of its range [59].  This variation may also be found
within a local area.  Weaver [132] observed flower stalks varying in
height from 3 feet (0.9 m) on moist slopes to 7 to 12 feet (2.1-3.6 m)
on rich bottomland sites.  Throughout the tallgrass and mixed-grass
prairie regions, big bluestem normally is a rhizomatous, sod-forming
grass.  But when found in more arid locations, or on drier slopes, its
appearance is more similar to a bunchgrass [132].


RAUNKIAER LIFE FORM : 
   Geophyte


REGENERATION PROCESSES : 
Although big bluestem is capable of both vegetative and sexual modes of
regeneration throughout its range, vegetative regeneration is most
prevalent.  Vegetative regeneration occurs with the initiation of growth
from rhizomes.  Parent tillers consist of a rhizome, four to six
adventitious roots, and an aerial shoot, and develop mature buds in leaf
axils on their rhizomes [84].  New tillers begin as buds on the rhizomes
of their parent tillers.  These buds grow horizontally from their parent
before emerging from the soil surface.  In Kansas, rhizome buds begin to
swell and break open in late March, and by the end of April, growth of
tillers and elongation of the rhizomes occurs and continues throughout
the summer until August [105].  McKendrick and others [84] observed that
each tiller averaged two tiller offspring during the succeeding growing
season, with about half of these second-year tillers being vegetatively
reproductive but producing no seed.  Once a clone is produced, it can
live and reproduce for an unknown number of years, perhaps centuries
[105].

In undisturbed stands only a small proportion of tillers flower each
year.  This has been attributed to the irregularity of moisture and
temperatures at blooming time [51,68].  Drought severly reduces
flowering, but flowering increases during a wet year following a
drought.  Big bluestem also flowers abundantly following fire [68].

Abrams [1] found only 2.5 viable seeds per 1,352 cubic centimeters of
soil in undisturbed stands where big bluestem comprised 77 percent of
the canopy cover.  This almost total absence of buried seeds suggests a
high dependence on vegetative reproduction.  The limited germination in
big bluestem may be attributed to the following:  (1) seed predation by
birds, mammals, or insects either after having fallen or while still on
the plant; (2) fungus and bacteria during wet seasons which cause the
seeds to rot; and (3) the very limited space for germination in prairie
stands.  Carter and others [16] reported that 40 percent of the seed
crop of the big bluestem cultivar 'Pawnee' was destroyed by a midge in
certain seed production fields.  This midge is widespread in North
America and occurs in many areas where native stands of big bluestem
remain.


SITE CHARACTERISTICS : 
Big bluestem occurs on a variety of sites throughout its range but
develops best on fertile silt and clay loam soils of lowlands.  Soil
moisture seems to be the most important factor determining big
bluestem's importance in a community.  It develops best on mesic sites
in the true prairie and mixed-grass prairie regions.  In the mixed-grass
prairie where precipitation is limited, it occupies ravines, lower
portions of gentle slopes, and well-watered lowlands where the soil
moisture content is high due to some surface runoff [3,135].  Along the
major rivers and their tributaries in the mixed-grass prairie, big
bluestem can comprise up to 90 percent of the vegetation over extensive
areas [135].  Here its chief associates are sideoats grama (Bouteloua
curtipendula), dropseeds (Sporobolus spp.), switchgrass (Panicum
virgatum), indiangrass (Sorghastrum nutans), and Canada wildrye (Elymus
canadensis).  Farther west toward the Rocky Mountains, big bluestem
becomes even more dependant upon mesic sites.  In the Rocky Mountain
foothills of eastern Wyoming, big bluestem was found only on
south-facing slopes of steep ravines, and always growing immediately
adjacent to large rocks which apparently acted as micro watersheds,
substantially increasing the usable soil moisture from summer rains
[49].

In the true prairie region, big bluestem occurs on upland sites and is
associated with little bluestem (Schizachyrium scoparium), but generally
comprises only 5 to 20 percent of the cover [133].  Big bluestem seems
to be less tolerant than little bluestem to low soil moisture.  This is
apparently due to big bluestem's deep, coarse, root system which is
effective at extracting moisture from the silty or clayey soils of sites
it usually dominates [79].  Even in the true prairie where precipitation
is abundant, big bluestem is most plentiful on low mesic sites
[58,122,133].  Here it is most often associated with two other
tallgrasses:  indiangrass (Sorgastrum nutans) and switchgrass (Panicum
virgatum).  In moist stands, individual stems in well-established sod
are spaced about 0.5 inch (1 cm) apart [108,132].  Yet the foliage is so
dense and spreading that light is reduced at the soil surface by as much
as 58 percent [70], making establishment of other species difficult.  In
drier habitats, big bluestem grows as a bunchgrass, occurring in
isolated bunches [32,108].  Big bluestem is shade tolerant, and its
leaves are able to remain green and function with only 5 to 10 percent
of full sunlight [108].

Big bluestem is believed to have been the prevailing grass present
during the formation of prairie soils [5].  These soils are generally
silt loam to silty clay loam with soil horizons greater than 10 inches
(25 cm).  Big bluestem grows best on fertile silt and clay loam soils of
lowlands and ravines but is also common on calcareous blackland soils
[131].  Generally it grows well in silty or clayey soils, fair in sandy
soils, and poor in gravelly soils or dense clay [126,132].  In Texas,
big bluestem frequency has been shown to be positively correlated with
percent clay and organic matter of soil [25].

Elevational ranges in some western states are as follows [28,42]:

        CO     3,400 to 9,500 feet (1,036-2,896 m)
        NM     3,500 to 9,000 feet (1,067-2,743 m)
        MT     3,300 to 4,000 feet (1,006-1,219 m)
        WY     3,600 to 9,000 feet (1,067-2,743 m)


SUCCESSIONAL STATUS : 
Obligate Climax Species

Self-perpetuating stands of big bluestem are indicative of climax
grassland communties.  Its ability to dominate the prairie is due to its
rapid growth, dense sod-forming habit, great stature, and shade
tolerance [108,132].


SEASONAL DEVELOPMENT : 
Big bluestem grows very rapidly, producing large amounts of foliage in
just a few short months.  A warm-season grass, big bluestem begins
growing in mid to late spring, several weeks after cool-season grasses.
As the season progresses, plants develop a dozen or more leaves from a
central tiller [105].  In Kansas, new growth begins between April 15 and
May 1, and by early July heavy foliage often exceeds 20 inches (50 cm)
[3].  Under favorable conditions, leaves may grow 0.75 inch (2 cm) a
day, attaining a mature height of 2.5 to 3 feet (0.75-0.9 m) by
midsummer [133,134].  After vegetative growth is complete, the flower
stalks begin to develop, and elongate 1.5 to 3 inches (3.8-7.6 cm) per
day [133].  Most flower stalks are produced in mid to late summer, with
plants from the northern portion of big bluestem's range flowering
earlier than those from the southern.  During a drought, discontinuous
flowering may occur.  In Illinois, a drought in July and early August
caused the tips of the culms to die back.  But following rains of above
normal precipitation in August, new inflorescences developed on the
culms below the withered ones [64].

The timing of nine phenological stages for eight varieties (cultivars or
test seed) of big bluestem are shown below [95].

Phenological stage:

1.  First emargence of inflorescence, 10 culms or more
2.  First anthesis, 10 culms or more
3.  Fifty percent emergence of the inflorescence
4.  Fifty percent anthesis
5.  First seed mature
6.  Fifty percent seed maturity
7.  Seed mature - starting to shatter
8.  Most seed shattered
9.  Complete dormancy

Phenolog. stage   1     2     3     4     5     6     7     8     9

origin   seed                                       
nw MN   ND-3784 Jul 11  14    19    27  Aug 20  29   Sep 4  14  Oct 13
nw MN   ND-3785 Jul 12  15    22    30  Aug 20  29   Sep 4  15  Oct 15
   ND   ND-4    Jul 13  14    22    29  Aug 18  Sep 2  14   29  Oct 15
   SD  'Bonilla'Jul 27 Aug 1  18    23  Sep 11  29   Oct 9  ---   ---
   SD   SD-43   Aug 9   10    18    29  Sep 24  ---   ---   ---   ---
   NE  'Champ'  Aug 13  13    27    30    ---   ---   ---   ---   ---
   NE  'Pawnee' Aug 16  16  Sep 4    7    ---   ---   ---   ---   ---
   KS  'Kaw'    Aug 31 Sep 2  13    16    ---   ---   ---   ---   ---

Flowering times for several states are as follows [28,71,94,114,134]:

Location        Begining of flowering       End of flowering

  CO                    July                     Sept
  IL                    July                     Aug
  IO                    July                     Sept
  KS                    July                     Sept
  MO                    July                     Sept
  MT                    Aug                      Sept
  ND                    July                     Aug
  NE                    July                     Sept
  SD                    July                     ----
  WY                    July                     Sept

Related categories for Species: Andropogon gerardii var. gerardii | Big Bluestem