Plant Community Dynamics
Changes in the plant community are monitored by sampling the 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 data were collected each year of the study, starting in 1988, one year prior to the beginning of grazing. Density was collected on forbs and shrubs in 1988 and from 1990 to 2007, and on cespitose (bunch) grasses from 1992 to 2007. Basal cover was sampled on plant species in 1988, 1990, 1992, 1993, 1996, 1999, 2002, and 2005, and basal cover of litter and bare ground was sampled in 1996, 1999, 2002, and 2005. 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 whether species abundance changed across all sites, which might indicate a response to weather, or whether species abundance changed in response to the different grazing treatments. All tests were performed at the P=0.05 level.
Table 8 lists the percent frequency of plant species in 25 x 25 cm frames in 1988 and 2007 (first and current year of the study) on each treatment on the overflow range sites that averaged at least 10 percent frequency across all treatments in all years or that showed a response to grazing. It also lists the grazing response. Table 9 lists the same information for silty range sites.“Decrease” indicates that the species seems to be favored by rest. “Increase-decrease” indicates that the species seems to be favored by moderate grazing. These are species that increase as grazing pressure increases from ungrazed to moderately grazed, but decrease as grazing pressure increases from moderate to extreme. “Increase” indicates that the species seems to be favored by heavy grazing, and “invade” indicates species that only appear on the site after heavy grazing. By comparing the values across all treatments in 1988 with the values in 2007, it is obvious why most species are assigned to a particular grazing response, but frequency fluctuates with weather, so for many species the grazing response was more obvious when comparing intermediate years.
Of those plant species on overflow range sites for which the grazing response is not obvious, Symphyotrichum ericoides increased on all treatments from 1988 to 1993, then decreased from 1994 to 2004. This species increased the most on the extreme treatment. Artemisia ludoviciana increased on all treatments from 1988 to 1995, then decreased from 1995 to 2004. Since 1998, Artemisia ludoviciana has been least abundant on the ungrazed treatment. Elymus caninum has fluctuated in abundance on all treatments. It increased from 1991 to 1994, and then decreased from 1994 to 1998, during which time it increased the most on the extreme treatment. Polygonum convolvulus has also fluctuated in abundance. It increased from 1988 to 1992 then decreased from 1992 to 1997, during this period it increased the most on the heavy and extreme treatments. Draba nemorosa has never been very abundant and it fluctuates in abundance, but since 1997 it has only occurred on the extreme and heavy treatments. Senecio plattensis has only been found on the light, moderate and heavy treatments in 1995, 1997 and 1998. Lepidium densiflorum was not found before 1992; it has not occurred on ungrazed and has only been found once on the light grazing treatment. It increased the most on the extreme grazing treatment. Lithospermum incisum is uncommon but was found on extreme in 1994 and 1999. On silty sites: Lactuca tatarica increased from 1989 to 1996 then decreased from 1997 to 2005, it increased the most on the heavy and extreme treatments. Elymus repens increased from 1992 to 1999 then decreased from 1999 to 2002, it increased the most on the light treatment. Erysimum asperum increased from 1988 to 1992 on grazed treatments, and then decreased from 1992 to 1995. It did not change significantly on the ungrazed treatment. Potentilla norvegica has fluctuated in abundance; it was most abundant on moderate in 1994, light in 1995 and extreme in 2000 and 2005. Psoralea esculenta is uncommon but tends to be most abundant on the ungrazed treatment when it is present.
On overflow sites, total forb density and total plant density (including forbs, bunchgrasses, and shrubs, but not rhizomatous grasses) have become greatest on the extreme treatment and most sparse on the ungrazed treatment. Total plant basal cover has increased on the extreme treatment. Shrub density decreased on the heavy treatments from 1990 to 2005 and on the extreme treatment from 1990 to 2006. On silty sites, total forb density and total plant density tend to increase as the grazing intensity increases. Total grass density decreased from 1994 to 2002 then it increased on the heavy and extreme treatments from 2002 to 2006 and on moderate from 2004 to 2006. It decreased the most on the ungrazed and light treatments and has not recovered significantly on those treatments. Total shrub density decreased on the heavy treatment between 1999 and 2006 and increased on the ungrazed treatment between 1998 and 2006. Also on silty range sites, total plant basal cover 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.
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