Vegetation Response to Grazing Intensities on Mixed-Grass Prairie in the Coteau Region of North Dakota

Guojie Wang, Shiping Wang, Laboratory of Quantitative Vegetation Ecology, Institute of Botany, the Chinese Academy of Sciences

Bob Patton, Paul Nyren, Xuejun Dong and Anne Nyren, NDSU Central Grasslands Research Extension Center

 



Introduction

Methods

Results and Discussion

Conclusions

Acknowledgements



Introduction


The impact of grazing on community structure and ecosystem processes due to selected grazing, trampling, and nutrients returned from animal excretion is a key issue for range management as well as for conservation (Crawley 1983; Noy-Meir et al.1989). Range managers emphasize the long-term sustainable maximization of livestock production and profitability of the operation, while ecologists seek to maintain high biodiversity (Tilman et al. 1996; Noy-Meir 1998).


The grazing intensity trial began in 1989 on a mixed-grass prairie in the Coteau region at the Central Grasslands Research Extension Center (CGREC). The objectives of the study are to evaluate the responses of individual species and functional groups (grasses, forbs, and shrubs) productivity and diversity, above- and below-ground biomass, and their relationship to different grazing intensities on mixed-grass prairie in the Coteau region of North Dakota.


Methods


The study was conducted at the Central Grasslands Research Extention Center (CGREC), located on the Missouri Coteau 12 miles northwest of Streeter, in south-central North Dakota. The study site is typical of rangeland in the Missouri Coteau, which consists of a mosaic of soil types and range sites. Silty range sites (well-drained soils that are moderately fine in texture) dominate the study sites. The common soil taxonomic unit of the silty range sites is the Williams Series.


This region has a continental climate, with warm summers and very cold winters. The January average temperature is 2.50 F and the frost penetration range is 4 ft to 6 ft. Summer temperatures are highest in August, with an average of 670 F, and the average freeze-free period is 132 days. The average annual precipitation for the study area is 18 inches, 70% of which is received in the growing season (May though September).


The grazing intensity trial at the CGREC was started in 1989 as a completely randomized design with five treatments: light, moderate, heavy and extreme grazing intensities and an ungrazed control. Each treatment was replicated three times on pastures of about 30 acres each. The pastures are stocked in mid-to-late May with the goal of leaving 65%, 50%, 35% and 20% of an average year’s above-ground biomass remaining at the end of the grazing season on the light, moderate, heavy and extreme grazing treatments, respectively. Three grazing intensities, ungrazed, light and extreme, were chosen for this study and only silty range sites were sampled.


Above-ground biomass and species composition was measured from the end of July to early August of 2004. Twenty 0.25 m2 quadrats at light and extreme grazing intensities and ten 0.25 m2 quadrats in the ungrazed exclosure were taken. Litter was collected and above-ground standing living and dead biomass was clipped and separated into species. All samples were oven-dried at 1500F for 48 hours before weighing.


Ten soil samples were collected in each pasture, each sample consisting of a composite of three soil cores taken with a 2.12 inch diameter auger in three layers: 0-4 inches, 4-12 inches, and 12-20 inches. Root material was separated and oven-dried for 48 hours at 1500F.


Species richness is the average number of different species encountered on each of the treatments, and evenness is the distribution of those species throughout the landscape. The Shannon-Wiener index is a mathematical method of combining these two parameters into a single index. The higher the Shannon-Wiener index the higher the biodiversity. All data were analyzed using the ANOVA procedure of the MINTAB general linear model to evaluate overall significance of treatment effects.


Results and Discussion


On the experimental site, Kentucky bluegrass (Poa pratensis L.) was a common dominant plant species on all grazing treatments, but its dominance varied with different grazing intensities and its relative dominance decreased as grazing intensity increased (Table 1).


Table1. Above-ground biomass in lbs/acre of the top three dominant species under different grazing intensities on silty range sites.

Grazing

Intensity

First dominant plant

Second dominant plant

Third dominant plant

Species

Dry weight

Species

Dry weight

Species

Dry weight

Ungrazed

Kentucky bluegrass

(Poa pratensis)

1778

Many-flowered aster

 (Aster ericoides)

  298

Stiff sunflower

(Solidago rigida)

115

Light grazed

Kentucky bluegrass

(Poa pratensis)

1307

Buckbrush

(Symphoricarpos occidentalis)

 407

Smooth brome

(Bromus inermis)

263

Extreme grazed

Kentucky bluegrass

(Poa pratensis)

 238

Sedges

(Carex spp.)

 78

Blue grama

 (Bouteloua gracilis)

47



The forbs and grasses were the main contributors to the palatable biomass consumed by cattle (Table 2). The aboveground biomass of forbs and grasses decreased as grazing intensity increased and was significantly less on the extreme treatment than on the other two treatments. Shrub biomass was significantly greater on the light grazing treatment than on either the ungrazed or extreme grazing treatment. Compared to the ungrazed and extreme grazing treatments, the proportion of forbs and grasses to shrubs was significantly lower on the light treatment.


Table 2. The effects of two grazing treatments on above-ground biomass by functional group in lbs/acre on silty range sites.

Grazing intensity

Forbs

Grass

Shrub

Ungrazed

738 ± 131a

 1919 ± 166a

126 ± 64b

Lightly grazed

435 ± 71a

1553 ± 92

  761 ± 225a

Extreme grazed

145 ± 16b

  444 ± 62b

101 ± 34b

Means in the same column followed by the same letter are not significantly different (P< 0.05)


The amount of above-ground biomass was not significantly different (p>0.05) between the ungrazed and light-grazing treatments; however above-ground biomass was significantly lower at the extreme grazing intensity (Table 3). Litter was highest in the light treatment and the lowest in the extreme treatment (p < 0.05). Compared with light grazing intensity, the extreme grazing intensity significantly decreased below-ground biomass in the 4-12 inch and 12-20 inch levels. Total below-ground biomass for the 0-20 inch depth did not change significantly. The ratio of below-ground biomass to above-ground biomass was 10.4:1 in the light grazing intensity and 38.2:1 in the extreme grazing intensity, probably because more above-ground biomass was removed by cattle or more photosynthate was assigned to roots in the extreme grazing intensity. This data shows that under the extreme grazing treatment while root biomass did not change in the 0-4 inch layer, it significantly decreased in the 4-12 inch and 12-20 inch layers thus indicating a trend towards root concentration at shallower depths. Therefore, in a dry year, plants on extreme grazed pastures will have a limited ability to extract water from deeper soil levels.


Table 3. The influence of grazing intensity on above-and below-ground biomass (lbs/acre)on silty range sites.

Treatment

Above

ground

Biomass

Litter

0-4 in. Below

ground biomass

4-12 in.

Below

ground

biomass

12-20in.

Below

ground

biomass

0-20 in.

Below

ground

biomass

Ungrazed

2783a

 2803b

 

 

 

 

Light grazed

2778a

 3579a

19,426a

6502a

3042a

28,970a

Extreme grazed

  705b

   329c

20,534a

4366b

2203b

27,104a

Means in the same column followed by the same letter are not significantly different (P< 0.05)

 


Both species richness and biodiversity index increased with grazing intensity (Table 4) because grazing decreased the dominance of Kentucky bluegrass in the community and thus the evenness of species in the community increased as grazing intensity increased.

 


Table 4. The influence of grazing intensity on plant biodiversity

Treatment

Species richness

Shannon-Wiener index

Ungrazed

24.67c

2.048c

Light grazed

33.67b

2.592b

Extreme grazed

48.67a

3.510a

Means in the same column followed by the same letter are not significantly different (P< 0.05)

 


Conclusions


With increased grazing intensity, above-ground biomass and below-ground biomass at the 4-12 inch and 12-20 inch depths were significantly reduced. The increase in biodiversity was caused by a decrease in Kentucky bluegrass as grazing intensity increased. Therefore, the results showed that for the mixed-grass prairie in the Coteau region, a more diverse community can be maintained under continuous grazing with higher grazing intensity, but with much lower productivity and poorer root distribution.


Acknowledgments


This research was supported by the NDSU, CGREC. The authors would like to thank Jimmie Richardson, Professor, Department of Soil Science, NDSU, for his help in locating sampling sites.

 


NDSU Central Grasslands Research Extension Center

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