Submitted to: Biological Systems Simulation Group Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: March 6, 2006
Publication Date: April 11, 2006
Citation: Morgan, J.A., Derner, J.D., Andales, A.A., Dunn, G.H. 2006. Water relations and management of semi-arid grasslands: current status, further critical research, and the useful role of models. pp. 30-31. 36th Annual Biological Systems Simulation Conference. April 2006, Ft. Collins, CO. Technical Abstract: The productivity and distribution of grasslands worldwide are driven primarily by aspects of the climate which influence available soil water. Similarly, the type of grassland vegetation, e.g. tallgrass prairie vs. shortgrass steppe, and how it functions are first and foremost dependent on soil water. As such, grasslands are particularly susceptible to weather and climate. We already know a fair amount about the consequences of relatively short-term (years) extreme weather events like droughts on rangeland productivity and ranching enterprises; even a few years of drought can be devastating. A recent example of this is the drought which visited the region in 2000, and some would say still lingers today in some corners of Colorado. This followed the decade of the 1990’s with its abundant precipitation, and likely surprised some of the less experienced producers, no doubt forcing some out of business. A more sinister weather-related phenomenon is global climate change. The term ‘global climate change’ refers to the anthropogenic release of greenhouse gases into Earth’s atmosphere and the resulting effects of those gases on the climate. We cannot see these gases that are being emitted in greater quantities than in any recent time, so there is a tendency to be indifferent. Yet there is increasing evidence that these gasses have already caused significant changes in Earth’s climate, particularly altered temperature and precipitation patterns, and are likely to induce greater changes in years ahead. Rangelands may be particularly susceptible to climate change. So what is the current state of rangeland management in regard to water and our understanding concerning its limitations on productivity? Most cow-calf producers in the shortgrass steppe and northern mixed-grass prairie rangeland ecosystems use ‘average’ forage production to determine initial stocking rates and number of cows for the ‘base cow herd’. Because intra- and interannual variability in precipitation is high in these semi-arid grasslands, it is problematic for land managers to properly adjust stocking rates between and within years to achieve desired livestock production and sustainable use of these lands. Growing season precipitation has been used for coarse-level decisions as this metric is often highly correlated with forage and livestock production in these rangelands. However, there is little within-year utility of forage productivity information based simply on early-to-mid growing season precipitation. Therefore, land managers often are relegated to making reactionary management decisions regarding stocking rates and grazing season length during the growing season. We suggest that land managers need to increase their flexibility in livestock classes and numbers to facilitate management decisions, within- and between-years. To increase flexibility, livestock owners can 1) reduce their base cow herd and increase yearling numbers which can be sold when forage is limiting without having to sacrifice genetic resources of the cow herd, 2) change to a later calving date to reduce grazing pressure on summer pastures due to smaller calves, 3) change to an earlier weaning date to reduce grazing pressure on summer pastures and increase marketing options at non-traditional selling dates, and 4) some combination of the previous options. To help in their decision-making, producers need to be able to accurately estimate the next year’s forage production, and the sooner they can do that, the better. Improved forecasts of forage production may be obtained by combining estimates of initial soil water content in spring with simulation modeling of the soil water balance and forage growth using forecasted weather (see figure). The difficulty in predicting forage production lies in forecasting the weather for the coming growing season. The Climate Prediction Center (CPC) of the National Weather Service provides outlooks (0.5 to 12.5 months lead-time) for both temperature and precipitation for the Continental U.S. from which precipitation and temperature inputs to the model can be generated. Although previous CPC seasonal precipitation forecasts were shown to have limited agricultural utility in the northern Great Plains, improvements in climate forecasting skill should increase the utility of these forecasts in the future. More localized forecasts may also be available from proximate institutions. Because of uncertainty in weather and subsequent forage production, predictions of forage production are expressed in terms of probabilities of occurrence. What about the longer-term issues related to global climate change? One of the more consistent predictions is that future weather is likely to become more extreme, e.g. droughts and storms will become more pronounced. Management options discussed above relate directly to this problem, and underscore the necessity to develop management practices which deal with uncertainty. A second global change scenario is that enhanced forage production under elevated CO2 will lead to N limitations and declining forage quality, some of which may be due to shifts in plant community composition. This suggests a need to find economic means to add N to rangelands. Rangeland management is essentially water management. Uncertainties about future weather patterns, whether they occur next year or next decade, undermine the ability of rangeland managers to optimize their operations. Our challenge as scientists is two-fold: one, to synthesize present knowledge of grassland responses to weather/climate, particularly in regard to impacts on hydrology; and two, to convert this information into tools ranchers can apply with confidence.