Central Grasslands REC, Streeter


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Long-Term Grazing Intensity Research in the Missouri Coteau of North Dakota - 2012 Annual Report

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 2012 and on cespitose (bunch) grasses from 1992 to 2012. Basal cover was sampled on plant species in 1988, 1990, 1992, 1993, 1996, 1999, 2002, 2005, 2008 and 2011 and basal cover of litter and bare ground was sampled in 1996, 1999, 2002, 2005, 2008 and 2011. 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 9 lists the percent frequency of plant species in 25 x 25 cm frames in 1988 and 2012 (first and current year of the study) on each treatment on the loamy overflow ecological sites that averaged at least ten percent frequency across all treatments in all years or that showed a response to grazing. It also lists the grazing response. Table 10 lists the same information for loamy ecological 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 "Invader" indicates species that only appear on the site after heavy grazing. By comparing the values across all treatments in 1988 with the values in 2012, 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.

On loamy overflow sites, total density of non-rhizomatous grasses has increased the most on the extreme grazing treatment and has decreased on the ungrazed treatment. Total forb density has tended to increase with grazing intensity and has become greatest on the extreme treatment and least on the ungrazed treatment. Total plant density (including forbs, bunchgrasses, and shrubs, but not rhizomatous grasses) has also tended to increase with grazing intensity. Total plant basal cover has increased on the extreme and heavy treatments and decreased on the ungrazed and light treatments.

On loamy sites, total forb density has become highest on the extreme treatment and lowest on the light and ungrazed treatments. Total plant density has increased more on the extreme treatment than on the ungrazed or light treatments. Total grass density decreased from 1994 to 2002. From 2004 to 2009, total grass density decreased the most on the ungrazed and light treatments and has not recovered on those treatments, while there has been a steady increase in grass density on the moderate, heavy and extreme treatments. Also on loamy ecological sites, total plant basal cover decreased on all treatments between 2002 and 2008 but it decreased less on the extreme than on the other treatments. In addition to the changes listed for plant species, litter has decreased on loamy ecological sites and bare ground has increased on both loamy and loamy overflow ecological sites under heavy grazing.

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