Simulating Carbon Dioxide Effects on Range Plant Growth and Water Use with GPFARM-Range Model

Steadily rising carbon dioxide (CO2) in the Earth's atmosphere has the potential to increase plant biomass production and reduce plant transpiration in semiarid rangelands. Incorporating results from field CO2-enrichment experiments into process-based simulation models enhances our ability to project climate change impacts on these rangelands. In this study, we added algorithms for computing changes in plant biomass growth and stomatal resistance under elevated [CO2] to the GPFARM-Range (Great Plains Framework for Agricultural Resource Management in Rangelands) model, a newly developed stand-alone software package for rangeland management. The GPFARM-Range model was tested against 5 yr (1997-2001) of soil water and plant biomass data from CO2-enrichment (720 ppm) field experiments conducted in shortgrass steppe in northern Colorado. Simulated results for both peak standing crop biomass and soil water for both ambient and elevated [CO2] treatments had a percent bias within ± 10%, Nash-Sutcliffe efficiency ≥ 0.5, and index of agreement > 0.70. The model also captured the observed trend of increased C3 grass biomass and reduced plant transpiration under elevated [CO2]. The model was used to evaluate the separate effectiveness of elevated [CO2] on plant growth rate (C3 grasses only) and stomatal resistance (both C3 and C4 grasses). Two separate simulations showed that increased growth rate and stomatal resistance due to elevated [CO2] enhanced total plant biomass gain (C3 + C4) by 22% and 17%, respectively. The results indicate the algorithms used to simulate the impacts of elevated [CO2] on range plant growth and water use are reliable and can be used to evaluate rangeland production for predicted increases in [CO2]. However, further studies are necessary because the reduction in plant transpiration under elevated [CO2] was underestimated, and increase in nitrogen use efficiency due to elevated [CO2] is not included.