Rising 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. Such changes could have major consequences for the sustainability of livestock management strategies in temperate rangelands containing a mixture of C3 and C4 grasses. 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. 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 semiarid, mixed-grass prairie.
Results/Conclusions
For the 3 dominant forage grasses, warming altered in vitro dry matter digestibility (IVDMD) and tissue [N] only in certain years, and only to a limited extent. In contrast, eCO2 substantially reduced IVDMD and [N] of both C3 and C4 grasses in most 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 forage quality 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 ruminant weight gains in one of the largest remaining rangeland ecosystems 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.