American Bison - Buffalo Field Campaign

6.2.1.2.1 Northern mixed grasslands
In the absence of fire, bison have been observed making
extensive use of prairie dog colonies in the northern mixed
grasslands ecoregion, where colonies may have covered 2-
15% of the short grasslands (Knowles et al. 2002; Virchow and
Hygnstrom 2002). Bison utilise the forb-dominated centres of
prairie dog colonies for resting and wallowing, but feed at the
graminoid-dominated periphery of colonies rather than at the
colony centre (Coppock and Detling 1986; Krueger 1986). Bison
use of prairie dog towns peaks during the summer and declines
in the autumn (Krueger 1986) when the available forage biomass
is low or the vegetation is senescent (Coppock et al. 1983a;
1983b). Bison use of colony sites also declines when recently
burned grasslands are available (Coppock and Detling 1986).
Grasses and sedges were almost 90% of the year-round bison
diet, and sedges formed 7 to 37% of the seasonal diet in the
northern mixed grassland ecoregion (Table 6.1). Bison selected
foraging sites containing more than 75% warm season (C4)
grasses during the summer growing season (Steuter et al.
1995). C4 grasses were approximately 33% of the diet in June,
and a maximum of 40% of the bison diet in late summer, but
C4 grasses were less in the bison diet in autumn, winter, and
spring (Plumb and Dodd 1993). Conversely, cool season grasses
formed approximately 50% of the summer diet, but increased to
80% of the diet in September (Plumb and Dodd 1993).
6.2.1.2.2 Central shortgrass prairie
In a lightly grazed site, bison almost exclusively consumed
grasses, but consumed more than 10% woody plants in the
autumn and winter at a heavily grazed central shortgrass prairie
site shared with cattle and sheep (Table 6.1). Three C4 grasses
accounted for 65 to 75% of the bison diet (Peden et al. 1974;
Schwartz and Nagy 1976).
6.2.1.2.3 Tall grasslands prairie and
southern shortgrass prairie
Bison in the tall grasslands prairie and southern shortgrass
prairie ecoregions utilised only recently burned areas in spring,
but selected areas burned annually throughout the year (Shaw
and Carter 1990; Vinton et al. 1993). Bison grazing and regrazing
can maintain areas with a low vegetative cover and standing
crop (Coppedge and Shaw 1998; Vinton et al. 1993). Areas
grazed by bison were characterised by a lower abundance of
C4 grasses, a higher abundance of C3 grasses, and greater
overall plant species diversity (Hartnett et al. 1996). These
characteristics were more pronounced in areas burned annually
(Hartnett et al. 1996), which is consistent with greater bison use
of annually burned sites (Shaw and Carter 1990; Vinton et al.
1993). Bison grazed little bluestem (Schizachyrium scoparium)
more frequently post-burning, probably in response to removal
of standing dead tillers by fire (Pfieffer and Hartnett 1995). The
greater overall plant species diversity in burned areas was linked
to increased nitrogen cycling and availability (Bakker et al. 2003;
Johnson and Matchett 2001).
C3 grasses were the most common dietary item in winter
(Coppedge et al. 1998). Dietary quality, as measured by faecal
nitrogen, peaked in May and June, coincident with a peak in C3
grasses productivity (Post et al. 2001). Up to 39% of the spring
diet was sedges (Coppedge et al. 1998).
6.2.1.2.4 Northern fescue grasslands
Understanding contemporary trophic ecology of bison in this
ecoregion is confounded somewhat by a management-imposed
rotational grazing, by which bison are moved throughout the
National Bison Range (NBR) National Wildlife Refuge, Montana
(McCullough 1980). When occupying lower elevation areas of the
NBR, bison utilised level to undulating open grasslands. Once
herded to higher elevation portions of the range, bison continued
to utilise the more level open areas available (McCullough 1980).
The year-round distribution of bison was away from higher
elevation steep-slope areas. Bison showed no selection for
aspect, as they tended to use the more level areas available
throughout the year. Bison fed almost exclusively on grasses
(Table 6.1; McCullough 1980).
6.2.1.2.5 Rocky Mountain forest
In the high topographical relief of the Rocky Mountains
the heterogeneity of herbaceous productivity and standing
crop is caused by the spatial distribution of moisture on the
landscape. Herbaceous above ground net primary productivity
(ANPP) is influenced by site-specific topographic position
relative to moisture distribution and aspect (Burroughs et al.
2001). Herbaceous ANPP is lower at low elevations with less
precipitation and at the highest elevations due to a shorter
growing season attributable to lower temperatures than at mid-
elevations (Coughenour 2005). In general, herbaceous ANPP
occurs as a pulse of nitrogen rich vegetation that sequentially
follows an elevational gradient from the lower elevation winter
ranges to the higher elevation summer ranges. This pattern
of ANPP makes young nutritious and concentrated forage
available to bison for up to six months of each year (Frank
and McNaughton 1992). Summer movements of bison to
higher elevation areas reduces vegetation utilisation at lower
elevations and thereby enhances the availability of vegetation
at lower elevations during the non-growing season (Frank and
McNaughton 1992).
Bison on Yellowstone’s northern range forage on sedges within
more mesic sites in winter (Meagher 1973) to the extent that
the winter diet is more than 95% grasses, sedges, and rushes
(Table 6.1; Singer and Norland 1994). Similarly, bison utilising
the Yellowstone central range during winter primarily feed on
sedges along the edges of thermally influenced drainages and
American Bison: Status Survey and Conservation Guidelines 2010 45