Get reliable science

An extensive sensitivity analysis of a model for the Simulation of Production and Utilization on Rangelands (SPUR) was conducted. A perturb and observe approach was employed in a series of fractional factorial experiments. State variables selected as sensitivity indicator variables included: peak standing crop, maximum plant nitrogen to carbon ratio, integrated year-long mineralization of soil nitrogen, integrated season-long plant death, integrated season-long carbon assimilation, integrated effect of soil moisture on net photosynthesis, integrated season-long forage intake by steers and cummulative season-long change in steer weight. The 1st of three stages of the analysis showed dynamics which affect simulated warm and cool-season grasses, warm and cool-season forbs and shrubs were similar in the absence of grazing. Therefore, one plant functional group (warm-season grasses) was used in subsequent stages, allowing a greater number of plant species specific parameters to be examined. In the 2nd stage, also without grazing, optimum, minimum and maximum temperatures for plant activity, the day senescence ends and their joint effects had the largest impacts on the plant component. Characteristics of the soil and soil-water relationships had only minor effects on plant-related indicators. Grazing caused many of the interactions which previously affected plant-related indicators to become less important. Effects associated with day senescence ends were greatly reduced in both magnitude and importance. Steer forage intake and weight change were sensitive to stocking rate and the parameter which converts the percent nitrogen of the forage into total digestible nutrients. Plant parameters which affected the relative quantities of carbon and nitrogen in plant biomass, tended to mediate the effects of livestock-related parameters.