Elevated CO2 induces substantial and persistent declines in forage quality irrespective of warming in mixed grass prairie

Authors: Augustine, David; Blumenthal, Dana; Springer, Timothy; Lecain, Daniel; Gunter, Stacey; Derner, Justin

Submitted to: Ecological Applications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/11/2017
Publication Date: 5/18/2018
Citation: Augustine, D.J., Blumenthal, D.M., Springer, T.L., Lecain, D.R., Gunter, S.A., Derner, J.D. 2018. Elevated CO2 induces substantial and persistent declines in forage quality irrespective of warming in mixed grass prairie. Ecological Applications. 28(3):721-735. https://doi.org/10.1002/eap.1680.
DOI: https://doi.org/10.1002/eap.1680

Interpretive Summary: Two of the most important aspects of global climate change are ongoing increases in the concentration of carbon dioxide (CO2) in the atmosphere, and associated increases in global temperature. We conducted a field experiment under realistic field conditions to examine how increasing C02 concentration in the atmosphere (from the current 400 ppm to a simulated future level of 600 ppm) and increasing temperature (by 1.5°C during the day and 3°C at night), both alone and in combination, will affect the productivity and quality of forage for livestock in the northern mixed-grass prairie. Our study was conducted in southeastern Wyoming, near the capitol of Cheyenne, beginning in 2007 and continuing through 2013. The grassland we studied is dominated by two cool-season grasses (needle and thread grass and western wheatgrass) and one warm-season grass (blue grama), but has >20 plant species that are commonly grazed by cattle. In addition to measuring the production of forage plants in this rangeland, we measured the degree to which these plants can be digested by cattle. We collected forage plants from the experimental plots whcn they reached peak biomass in July each year, and conducted laboratory-based measurements of the rate at which they could be digested. Overall, we found that with increased atmospheric CO2 combined with warming, total forage production increased (by an average of 38% over the 7-year study period), while forage quality declined. Dry matter digestibility of western wheatgrass declined from an average 63.3% to 61.1%, and crude protein content of western wheatgrass declined from 7.8% to 6.5%. These changes could significantly reduce the rate at which individual cattle gain weight during the growing season. Given the predicted increases in forage productivity, reduced forage quality could potentially be mitigated by increasing stocking rates, and through management such as prescribed burning, fertilization at low rates, and legume interseeding to enhance forage quality.

Technical Abstract: Increasing atmospheric [CO2] and temperature are expected to affect the productivity, species composition, biogeochemistry, and therefore the quantity and quality of forage available to herbivores in rangeland ecosystems. Both elevated CO2 (eCO2) and warming affect plant tissue chemistry through multiple direct and indirect pathways, such that the cumulative outcomes of these effects are difficult to predict. Here, we report on a 7-year study examining effects of CO2 enrichment (to 600 ppm) and infrared warming (+1.5°C day/3°C night) under realistic field conditions on forage quality and quantity in a semiarid, mixed-grass prairie. For the 3 dominant forage grasses, warming effects on in vitro dry matter digestibility (IVDMD) and tissue [N] were detected only in certain years, varied from negative to positive, and were relatively minor. In contrast, eCO2 substantially reduced IVDMD (2 most abundant grasses) and [N] (all 3 grass species) in most years, except the two wettest years. Furthermore, eCO2 reduced IVDMD and [N] independent of warming effects. Reduced IVDMD with eCO2 was related both to reduced [N] and increased acid detergent fiber content of grass tissues. For the 6 most abundant forage species (representing 96% of total forage production), combined warming+eCO2 increased forage production by 38% and reduced forage [N] by 13% relative to ambient climate. While the absolute magnitude of the decline in IVDMD and [N] due to combined warming+eCO2 may seem small (e.g. from 63.3 to 61.1% IVDMD and 1.25 to 1.04% [N] for P. smithii), such shifts could have substantial consequences for the rate at which ruminants gain weight during the primary growing season in the largest remaining rangeland ecosystem in North America. With forage production increases, declining forage quality could potentially be mitigated by increasing stocking rates, and through management such as prescribed burning, fertilization at low rates, and legume interseeding to enhance forage quality.