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 was collected each year of the study. Density was collected on forbs and shrubs in 1988 and 1990 to 2005, and on cespitose (bunch) grasses from 1992 to 2005. 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 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 8 lists the percent frequency of plant species in 25 x 25 cm frames in 1988 and 2005 (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 it seems to be favored by moderate grazing. These are species that would increase as grazing pressure goes from ungrazed to moderately grazed, but decrease as grazing pressure goes from moderate to extreme. “Increase” indicates that the species seems to be favored by heavy grazing and “invader” would indicate species that only appear on the site after heavy grazing. By comparing the values across all treatments in 1988 with those in 2005, 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 for which the grazing response is not obvious, on overflow range sites: Aster ericoides increased on all treatments from 1988 to 1993 then decreased from 1994 to 2004. It 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 it has been least abundant on the ungrazed treatment. Galium boreale decreased on extreme from 1995 to 1997, but then increased from 2002 to 2003. It increased on the light treatment from 1997 to 2003. It increased the most on the light treatment. Agropyron caninum has fluctuated in abundance on all treatments. It increased from 1991 to 1994 then decreased from 1994 to 1998, during this period it increased the most on the extreme treatment.
On overflow sites, total forb density and total plant density (includes forbs, bunchgrasses and shrubs but not rhizomatous grasses) have become greatest on the extreme treatment and least abundant on the ungrazed treatment and total plant basal cover has increased on the extreme treatment. Shrub density decreased on the extreme and heavy treatments from 1990 to 2005. On silty sites, total forb density and total plant density tend to increase as the grazing intensity increases. Total grass density has decreased on all but the heavy treatment but it decreased the most on the ungrazed and light treatments. Total shrub density decreased on the heavy treatment and increased on the ungrazed treatment between 1999 and 2005. Also 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.
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