Interaction of Simulated Drought
and Grazing on Mixed Grass Prairie
The study will be conducted on the Central Grasslands Research Extension Center 8 miles northwest of Streeter, in south central North Dakota. Study sites will be located on section 14 (T138N R70W), a long-term grazing intensity study initiated in 1988. The station lies within the mixed grass prairie (Whitman and Wali 1975). Important grass species include western wheatgrass, Kentucky bluegrass, blue grama, needle-and-thread, prairie junegrass, and threadleaf sedge. Some important forbs include white sage, fringed sage, silver-leaf scurf pea, red false globemallow, and goldenrod species. Western snowberry is the dominant low shrub on pastureland.
A grazing intensity study was begun in 1988 on mixed-grass prairie that had a history of intermittent light grazing for ten years. Grazing intensity treatments were defined as the amount of aboveground standing biomass left at the termination of the grazing period. For this research project only the moderate grazing intensity will be sampled. Each year the moderate treatment was grazed between 86 and 181 days beginning in mid-May. Average aboveground biomass remaining at the termination of annual grazing periods averaged 50% for the moderate grazing intensity.
Automated rainout shelters will be installed to control the amount of precipitation received on rainfall treatments. Rainfall treatments will consist of 60-65% of the long-term average (drought), 100% of the long-term average (average), and the natural amount received annually (natural). Three replicate shelters, 3 x 6 m will be randomly assigned to silty range sites for the drought and average rainfall treatments. The shelters will be on rails equipped with electric motors and wet sensitive apparatus, that when wetted will activate the motors to move the shelters across sampling plots. To prevent lateral water movement, each rainout shelter boundary will be trenched to 60 cm depth, lined with plastic and refilled with soil. Drought conditions will be simulated for two consecutive years for the region. Plots subjected to the drought and average treatments will be drip irrigated at two-week intervals using the long-term bimonthly recorded precipitation for the site.
Prior to the grazing season, portable enclosures will be located within each rainfall treatment to prevent livestock grazing. In early July herbaceous production will be estimated by clipping quadrats from each replication of the three rainfall treatments. Herbaceous biomass will be separated into cool and warm-season grasses, grass-like species and forbs. Basal cover will be sampled using the ten-point frame technique (Society for Range Management 1986). Cover estimates will be by major grass species, grass-like, forbs, litter and bare ground. Density of forbs will be determined using 0.25 m2 and nested frequency quadrats. All forbs will be identified and counted.