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 2008, and on cespitose (bunch) grasses from 1992 to 2008. Basal cover was sampled on plant species in 1988, 1990, 1992, 1993, 1996, 1999, 2002, 2005 and 2008, and basal cover of litter and bare ground was sampled in 1996, 1999, 2002, 2005 and 2008. 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 2008 (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 2008, 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. If you click on the plant name in either of these tables it will lead you to a graph of the changes in abundance over time on each of the different treatments.
On overflow sites, total density of non-rhizomatous grasses has become greatest on the extreme grazing treatment and least on the ungrazed treatment. 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. 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. 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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