Long-Term Ecological Grazing Intensity Research in the Missouri Coteau of North Dakota

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 2004, and on cespitose (bunch) grasses from 1992 to 2004. Basal cover was sampled on plant species in 1988, 1990, 1992, 1993, 1996, 1999, and 2002, and basal cover of litter and bare ground was sampled in 1996, 1999 and 2002. 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 2004 (first and current year of the study) on each treatment on the overflow range sites that averaged at least 5 percent frequency across all treatments in those two 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 “invade” indicates species that only appear on the site after heavy grazing. By comparing the values across all treatments in 1988 with those in 2004, 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 1994 then decreased from 1994 to 2001. It increased the most on the extreme treatment. Andropogon gerardii increased on the moderate and heavy treatments from 2001 to 2004 but did not change significantly on the other treatments. Cirsium flodmanii increased from 1992 to 1995 then decreased from 1995 to 2003. The increase was on moderate, heavy and extreme. In 1995 it was most abundant on heavy and in 1994 and from 1996 to 2000 it was most abundant on extreme. Aster simplex increased on the light treatment from 1998 to 2000 but has not changed significantly on any other treatment. It has always been most abundant on the light treatment. Solidago mollis was most abundant on ungrazed in 1990 and 2002 and it was most abundant on light from 1994 to 1996 and in 2000. It has never been very abundant on extreme. Comandra umbellata increased on the heavy and extreme treatments from 1991 to 2004. Vicia americana decreased on ungrazed from 1992 to 2001. It increased on extreme from 1992 to 1998, decreased in 1999 but then increased again from 1999 to 2001. It increased on moderate from 1992 to 2002. So it appears to be favored by grazing rather than by rest. Agrostis hyemalis increased from 1991 to 1995 then decreased from 1996 to 2002. It increased the most on the extreme treatment and the least on the ungrazed treatment. Silene antirrhina increased on the extreme treatment from 1990 to 2004. Campanula rotundifolia increased from 1998 to 1999 then decreased from 1999 to 2002. It increased more on the light, moderate and heavy treatments than on ungrazed or extreme. Lithospermum incisum was most abundant on extreme in 1994 and 1999. Potentilla pensylvanica first appeared on overflow in 1990, it has fluctuated in abundance but has generally been most abundant on extreme. Senecio plattensis was not found in sampling before 1995. It has been found on light, moderate and heavy. It has fluctuated in abundance but has been most abundant on light. Sisyrinchium montanum increased from 1992 to 1994 then decreased from 1994 to 2001. It has been most abundant on the moderate and heavy treatments.

 

Of those for which the grazing response is not obvious, on silty range sites: Poa pratensis increased on all treatments from 1988 to 1995, but it increased the most on the ungrazed, light and moderate treatments. Stipa viridula decreased on all treatments but it decreased more on ungrazed and light and less on heavy than on the other treatments. Sphaeralcea coccinea decreased on all treatments from 1988 to 2002, it decreased on the heavy treatment from 1995 to 2000, but it increased on the extreme treatment from 2001 to 2003. Penstemon gracilis increased from 1988 to 1999 then decreased from 1999 to 2001. It decreased more on the light and ungrazed than on the other treatments. Lithospermum incisum increased from 1991 to 1994 then decreased from 1995 to 2002, but it increased on the heavily grazed treatment from 2000 to 2004. Tragopogon dubius increased from 1988 to 1993 then decreased from 1993 to 2003. It increased the most on the ungrazed treatment. Lepidium densiflorum increased on extreme from 1988 to 1999 then decreased from 1999 to 2002. It did not change significantly on the other treatments. Erysimum asperum increased from 1988 to 1992 then decreased from 1993 to 1995. The increase was greatest on the grazed treatments. Erysimum inconspicuum was uncommon in 1988 and 2004 but it has fluctuated in abundance in intermediate years. It was most abundant on the light treatment in 1991 and 1999 and on the moderate treatment in 1996. Orthocarpus luteus increased from 1999 to 2000 then decreased in 2001. It was most abundant on the heavy treatment in 1993, 2000 and 2001. Potentilla norvegica also was uncommon in 1988 and 2004 and fluctuated in abundance in the intermediate years. It was most abundant on moderate in 1994 and on extreme in 2000. Sisyrinchium montanum increased from 1993 to 1996 then decreased from 1996 to 2002. It increased primarily on the heavy and moderate treatments and has not been found on the ungrazed 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 and heavy treatments 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 2004. On silty sites, total forb density and total plant density tend to increase as the grazing intensity increases. 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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