Elevated atmospheric carbon dioxide effects on maize and alfalfa in the Northeast US: A comparison of model predictions and observed data

The main feed crops on dairy farms in the northeastern U.S. are alfalfa (Medicago sativa L.) and maize (Zea mays L.). Projected changes in climate along with increased atmospheric carbon dioxide (CO₂) concentration will affect future production of these crops. We evaluated maize and alfalfa yield and evapotranspiration response to CO₂ enrichment predicted by three process-based cropping system models, CropSyst, DSSAT and IFSM, in six counties of Pennsylvania and New York, using 25 years of daily weather and two concentrations of atmospheric CO₂ (350 and 550 ppm). Positive effects of CO₂ enrichment on crop growth have been documented using plants grown under controlled CO₂ conditions; of particular relevance are results obtained in fields with free-air CO₂ enrichment (FACE) technology. For evaluation, simulation results were compared among models and to FACE experimental data. The three models simulated similar maize response to increasing CO₂ for grain yield, total biomass yield and harvest index, with predicted responses within the ranges reported in FACE experiments. The models may have slightly overestimated the yield response of maize during wet years compared with FACE experiments. The models also simulated increased productivity of alfalfa within the bounds observed in FACE experiments. While DSSAT and IFSM predicted yield increases of about 20%, Cropsyst predicted an increase of 32%, which is in the upper bound of FACE experimental data. For both crops, models simulated a lesser reduction in evapotranspiration under increasing CO₂ than that measured in FACE experiments. The larger response of alfalfa may either shift rotations towards more alfalfa, or allow for a shorter alfalfa phase in a rotation depending on market and other production constraints. While there were overestimations in simulating maize productivity under no water stress and uncertainties in simulating evapotranspiration that are relevant for local and regional hydrology, these models adequately represented crop yield response to changes in atmospheric CO₂ concentration.