Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 18, 2005
Publication Date: July 15, 2005
Citation: Ziska, L.H., Reeves III, J.B., Blank, R.R. 2005. The impact of recent increases in atmospheric co2 on biomass production and vegetative retention of cheatgrass (bromus tectorum): implications for fire disturbance. Global Change Biology. 11:1325-1332. Interpretive Summary: Cheatgrass (Bromus tectorum) is a recognized invasive annual weed of the Western United States that reduces fire return times from decades to less than five years. The interaction between rising carbon dioxide and changes in the biomass or fuel load of this invasive weed is unknown. In the current experiment we determined quantitative and qualitative factors that contribute to standing biomass or fuel load of cheatgrass for recent and near-term changes in the concentration of carbon dioxide. The carbon dioxide values used corresponded to the concentrations in the earth's atmosphere that existed at the beginning of the 19th century, that during the 1960s, current values, and those projected for the year 2020. From 25 until 87 days after sowing (DAS), above-ground biomass for three different populations of cheatgrass increased 1.5-2.7 gram per plant for every 10 part per million (ppm) increase above the 270 ppm pre-industrial baseline. On the qualitative side, rising carbon dioxide decreased the digestibility and potential decomposition of cheatgrass. Consequently, in addition to stimulation of biomass, rising carbon dioxide may also increase the above ground retention of cheatgrass biomass by decreasing removal by animals or bacteria. Overall, these data suggest that recent and ongoing increases in atmospheric carbon dioxide may have contributed significantly to cheatgrass productivity and fuel load with subsequent effects on fire frequency and intensity. This information will be of interest to land managers, ranchers, ecologists and weed scientists who are interested in the interaction between rising carbon dioxide and fire frequency, particularly in the western United States.
Technical Abstract: Cheatgrass (Bromus tectorum) is a recognized invasive annual weed of the Western United States that reduces fire return times from decades to less than five years. To determine the interaction between rising carbon dioxide (CO2) and fuel load, we characterized potential changes in biomass accumulation, C:N ratio and digestibility of three cheatgrass populations from different elevations to recent and near-term projections in atmospheric CO2. The experimental CO2 values (270, 320, 370, 420 ppm) corresponded roughly to the CO2 concentrations that existed at the beginning of the 19th century, that during the 1960s, the current level, and the near-term CO2 projection for 2020, respectively. From 25 until 87 days after sowing (DAS), above-ground biomass for these different populations increased 1.5-2.7 g per plant for every 10 ppm increase above the 270 ppm pre-industrial baseline. Carbon dioxide sensitivity among populations varied with elevational origin with populations from the lowest elevation showing the strongest productivity. Among all populations, the undigestible portion of above-ground plant material (acid detergent fiber ADF, mostly cellulose and lignin) increased with increasing CO2. In addition, the ratio of C:N increased with plant age, with CO2 and was highest for the lower elevational population. These CO2-induced qualitative changes could, in turn, result in potential decreases in herbivory and decomposition with subsequent effects on the above ground retention of cheatgrass biomass. Overall, these data suggest that increasing atmospheric CO2 above pre-ambient levels may have contributed significantly to cheatgrass productivity and fuel load with subsequent effects on fire frequency and intensity.