Plant Community Dynamics

Changes in the plant communities are monitored by sampling the percent frequency of occurrence, density per unit area, and percent basal cover of all plant species as well as sampling the weight of herbage produced. Frequency of occurrence is sampled by placing a frame on the ground fifty times along a transect at each sampling site. Every time it is placed on the ground all the plant species which occur in the frame are recorded. The number of frames a species occurs in, divided by the total number of frames, multiplied by 100, is the percent frequency of that species. The frequency value obtained from sampling is dependent on the size of the frame. If the frame is too small, the species will rarely be recorded and if the frame is too large, the species will be present in almost every frame. Because species may differ in their abundance on a particular site, and a particular species may differ in its abundance on different sites, it is not possible to select a single frame size that is optimum for all species on all sites. For this study, a 25 x 25 cm frame with 5 x 5 cm and 10 x 10 cm frames nested within it, is being used.

Density is determined by counting all the individual plants of each species in each of the frames. Density data were only collected on shrubs and forbs from 1988 to 1991. In 1992 we began collecting density data on cespitose (bunch) grasses and sedges. Density data are not collected on rhizomatous grasses and grass-like plants because of the difficulty of counting individual plants.

Basal cover is sampled by repeatedly placing a 10 point frame on the ground and recording each time one of the points strikes the base of a plant. The number of times a point hits the base of a plant divided by the number of points read, times 100, is the percent basal cover of that species. Basal cover is stable from year to year because it doesn't change unless old plants die or new plants are established. Also the point frame can be used to sample the amount of litter or bare ground as well as total plant basal cover. However, it requires too large a sample size to provide reliable data for all but the dominant species of a plant community. The point frame was used to sample litter and bare ground in 1996 and 1999. Frequency can be sampled more quickly than density or basal cover which makes it an ideal method for monitoring range trend or the direction of change in range condition. These three methods of monitoring vegetation change can be complementary and are used together in this study. Frequency data from 1988 to 1999, density data from 1988 and 1990 to 1999 and basal cover data from 1988, 1990, 1992, 1993, 1996 and 1999 were compared for each sample site using analysis of variance. The change in abundance of species between years was determined for each site. The arcsine transformation was applied to frequency and basal cover data to convert it from a binomial distribution to a nearly normal distribution. Analysis of variance was performed to determine if there was a change in species abundance across all sites which might indicate a response to weather, or if there was a change in response to the different grazing treatments. All tests were performed at the P=0.05 level. Table 10 lists those species which appear to be favored by no grazing or light grazing. Table 11 lists those species which appear to be favored by moderate grazing, and Table 12  lists those species which appear to be favored by heavy grazing. Table 12 also indicates species which appear to be invasive. These are species which have not been found or are extremely rare on the ungrazed or lightly grazed treatments. On silty range sites total plant basal cover has decreased on the ungrazed and lightly grazed treatments between 1992 and 1999 and increased on the extreme and heavy treatments between 1996 and 1999. In addition to the changes listed for plant species, litter has decreased and bare ground has increased on both silty and overflow range sites under heavy grazing.

The value of the plant community for grazing depends on the plant species present and their forage quality. As livestock select plants of high palatability in their diet, they give a competitive advantage to plants of lower palatability and cause a shift in the composition of the plant community. An ungrazed pasture may consist of high quality forage but it is of no use to a livestock producer unless the cattle are allowed to graze it. Likewise, a pasture which has been continually overgrazed may consist of plants of low forage quality and would be of little use to a livestock producer.

Table 10. Plant species which appear to be favored by no grazing or light grazing (P 0.05.)
Overflow Range Sites
Bromus inermis
Helianthus rigidus
Rosa arkansana
Tragopogon dubius
smooth brome
stiff sunflower
prairie rose
goat's beard
Silty Range Sites
Ambrosia psilostachya
Artemisia absinthium
Helianthus rigidus
Medicago sativa
Melilotus officinalis
Poa pratensis
Psoralea esculenta
Stipa comata
Tragopogon dubius
western ragweed
wormwood
stiff sunflower
alfalfa
yellow sweetclover
Kentucky bluegrass
breadroot scurf-pea
needle-and-thread
goat's beard

Table 11. Plant species which appear to be favored by moderate grazing (P 0.05.)
Overflow Range Sites
Ambrosia psilostachya
Anemone canadensis
Anemone cylindrica
Campanula rotundifolia
Carex lanuginosa
Galium boreale
Poa pratensis
Sisyrinchium montanum
Solidago rigida
western ragweed
meadow anemone
candle anemone
harebell
wooly sedge
northern bedstraw
Kentucky bluegrass
blue-eyed grass
stiff goldenrod
Silty Range Sites
Agropyron repens
Anemone cylindrica
Artemisia ludoviciana
Astragalus flexuosus
Cirsium flodmanii
Dichanthelium wilcoxianum
Erysimum inconspicuum
Psoralea argophylla
Ratibida columnifera
Sisyrinchium montanum
Solidago mollis
Stipa curtiseta
quackgrass
candle anemone
cudweed sagewort
pliant milk-vetch
Flodman's thistle
Wilcox dichanthelium
smallflower wallflower
silver-leaf scurf-pea
prairie coneflower
blue-eyed grass
soft goldenrod
needlegrass

Table 12. Plant species which appear to be favored by heavy grazing (P 0.05).
Overflow Range Sites
Achillea millefolium
Agropyron caninum
Agropyron smithii
Agrostis hyemalis
Androsace occidentalis
Astragalus agrestis
Aster ericoides
Carex heliophila
Cerastium arvense
Cirsium flodmanii
Conyza canadensis
Erysimum inconspicuum
Euphorbia serpyllifolia
Geum triflorum
Grindelia squarrosa
Juncus interior
Lithospermum incisum
* Medicago lupulina
Oxalis stricta
Penstemon gracilis
Polygonum convolvulus
* Polygonum ramosissimum
Potentilla norvegica
* Potentilla pensylvanica
Solidago missouriensis
Stipa viridula
Taraxacum officinale
Vicia americana
Viola pedatifida
western yarrow
slender wheatgrass
western wheatgrass
ticklegrass
western rock jasmine
field milk-vetch
heath aster
sun sedge
prairie chickweed
Flodman's thistle
horse-weed
smallflower wallflower
thyme-leaved spurge
prairie smoke
curly-cup gumweed
inland rush
yellow puccoon
black medic
yellow wood sorrel
slender beardtongue
wild buckwheat
bushy knotweed
Norwegian cinquefoil
Pennsylvania cinquefoil
Missouri goldenrod
green needlegrass
common dandelion
American vetch
larkspur violet
Silty Range Sites
Achillea millefolium
* Agrostis hyemalis
Androsace occidentalis
Artemisia frigida
Aster ericoides
Carex eleocharis
Carex heliophila
Cerastium arvense
Chenopodium desiccatum
Draba nemorosa
Erysimum asperum
Euphorbia serpyllifolia
Grindelia squarrosa
Hedeoma hispidum
Koeleria pyramidata
Lepidium densiflorum
*Medicago lupulina
Oxalis stricta
Plantago patagonica
* Polygonum ramosissimum
Potentilla pensylvanica
Symphoricarpos occidentalis
Taraxacum officinale
Vicia americana
western yarrow
ticklegrass
western rock jasmine
fringed sagewort
heath aster
needle-leaved sedge
sun sedge
prairie chickweed
narrow-leaved goosefoot
yellow whitlowort
western wallflower
thyme-leaved spurge
curly-cup gumweed
rough false pennyroyal
junegrass
peppergrass
black medic
yellow wood sorrel
wooly plantain
bushy knotweed
Pennsylvania cinquefoil
buckbrush
common dandelion
American vetch
* Invaders, these species have not been found, or are extremely rare, on the ungrazed and lightly grazed treatments.