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Grazing Effects on the Mixed-Grass Prairie Seed Bank in the Coteau Region of North Dakota

Methods



Experimental Site

The study was conducted at the Central Grasslands Research Extension Center (CGREC), located on the Missouri Coteau, 7.5 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, but are punctuated by overflow and other range sites. The common soil taxonomic unit of the silty range sites is the Williams Series, which is a fine loam glacial till. The area has a continental climate, with warm summers and very cold winters. The January mean temperature is 90 F and the frost penetration range is 4.6 ft to 6.6 ft. Summer temperatures are highest in July, with a mean of 700 F, and the average freeze-free period is 132 days. The mean annual precipitation for the study area is 17.5 inches, of which 70% is received typically as rainfall during the growing season (May though September).


Selection of Sampling Sites and Grazing Intensities

The grazing intensity trial in 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 other than the control 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 aboveground biomass remaining at the end of the grazing season on the light, moderate, heavy, and extreme grazing treatments, respectively. We selected only two grazing intensities for this study: light and extreme grazing. Within the two grazing intensities, two range sites, silty and overflow, were chosen for this study.


Experimental Procedures

Soil samples were taken in early winter of 2004 by removing the soil from a plot measuring 10 inches ´ 10 inches ´ 4 inches. There were 10 repetitions for each site under different grazing intensities along a definite transect. One soil sample was split into two sub-samples. One part was used for seed germination in the greenhouse in the winter of 2004 (winter: winter-WW); another part was started in spring of 2005 (winter: spring-WS). Soil samples were taken in the early spring of 2005, too. The 2005 samples were also split into two sub-samples. Only one sub-sample was used to germinate in the greenhouse in the spring of 2005 (spring: spring-SS). The other sub-sample was discarded.


The flats (20 inches × 10 inches × 3.5 inches) were placed in a greenhouse at CGREC. The soil sample depth was about 1.5 inches. We watered as needed using a mist nozzle. The greenhouse temperature varied from 77°F in daytime and 59°F at night in winter and was similar to conditions outside in spring. As seedlings became identifiable, they were counted and pulled. Plants that could not be identified as seedlings were pressed and taken to others for identification. Evaluations ended after 3 months.

 


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