Productivity and water use of wheat under free‐air CO₂ enrichment

Abstract

A free-air CO2 enrichment (FACE) experiment was conducted at Maricopa, Arizona, on wheat from December 1992 through May 1993. The FACE apparatus maintained the COj concentration, [CO2], at 550 jimol mor' across four replicate 25-m-diameter circular plots under natural conditions in an open field. Four matching Control plots at ambient [CO2](about 370 jimol mol"^) were also installed in the field. In addition to the two levels of [COjI, there were ample (Wet) and limiting (Dry) levels of water supplied through a subsurface drip irrigation system in a strip, split-plot design.Measurements were made of net radiation, R^; soil heat flux, GQ,-soil temperature; foliage or surface temperature; air dry and wet bulb temperatures; and wind speed. Sensible heat flux, H, was calculated from the wind and temperature measurements. Latent heat flux, XET, and evapotranspiration, ET, were determined as the residual in the energy balance. The FACE < treatment reduced daily total R^ by an average 4%. Daily FACE sensible heat flux, H, was higher in the FACE plots. Daily latent heat flux, XET, and evapotranspiration, ET, were consistently lower in the FACE plots than in the Control plots for most of the growing season, about 8% on the average. Net canopy photosynthesis was stimulated by an average 19 and 44% in the Wet and Dry plots, respectively, by elevated [CO2] for most of the growing season. No significant acclimation or down regulation was observed. There was little above-ground growth response to elevated [CO2] early in the season when temperatures were cool. Then, as temperatures warmed into spring, the FACE plants grew about 20% more than the Control plants at ambient [CO2], as shown by above-ground biomass accumulation. Root biomass accumulation was also stimulated about 20%. In May the FACE plants matured and senesced about a week earlier than the Controls in the Wet plots. The FACE plants averaged 0.6 °C wanner than the Controls from February through April in the well-watered plots, and we speculate that this temperature rise contributed to the earlier maturity. Because of the acceleration of senescence, there was a shortening of the duration of grain filling, and consequently, there was a narrowing of the final biomass and yield differences. The 20% mid-season growth advantage of FACE shrunk to about an 8% yield advantage in the Wet plots, while the yield differences between FACE and Control remained at about 20% in the Dry plots.

Lessons from climate modeling on the design and use of ensembles for crop modeling

“…On the other hand, a major argument against weighting of climate models based on hindcast error is that it assumes that the response to future radiative forcing will be consistent with that in the historical period, which cannot be tested. Crop models on the other hand can at least be tested against field experiments that impose higher atmospheric CO 2 concentrations (Kimball et al 1995;Ewert et al 2002) and higher temperatures (Wall et al 2011) than are observed today.…”

Modelling the impacts of weather and climate variability on crop productivity over a large area: A new process-based model development, optimization, and uncertainties analysis

“…Extensive controlled-environment experiments such as the Free-Air Concentration Enrichment experiments (e.g., Kimball et al, 1995;Ainsworth et al, 2002;Leakey et al, 2004;Kim et al, 2006Kim et al, , 2007 have showed that increases in both mean and extremes of temperature and elevated [CO 2 ], under predicted climate change scenarios, can impact the growth and development of crops in several ways. Sustained temperature increases over the season will change the growing period of a crop (e.g., IPCC, 2001), whereas short episodes of high temperature during the critical flowering period of a crop can impact yield independently of any substantial changes in mean temperature (e.g., Matsui and Horie, 1992;Wheeler et al, 2000).…”

“…All other effects of elevated [CO 2 ] on plants and ecosystems are derived from these two fundamental responses . Rising CO 2 would increase the photosynthesis rate, especially for C 3 crops (Kimball et al, 1995). Although C 4 crops may not show a direct response in photosynthesis activity, an indirect increase in water use efficiency in water-stressed environments via reduction in stomatal conductance may still increase yield .…”

“…Under elevated CO 2 , stomatal conductance in most species will decrease, which may result in less transpiration per unit leaf area (Sionit et al, 1984;Atkinson et al, 1991). Water loss by transpiration is not only affected by the conductivity of the stomata, but also by the driving forces for exchange of the water vapour from the leaf surface to the surrounding atmosphere (i.e., V PD ; McNaughton and Jarvis, 1991;Kimball et al, 1995). With all other factors being equal, the existing V PD between stomatal cavity and surrounding air -the boundary layer -will increase at a reduced transpiration rate and feedback to stimulate transpiration.…”

“…The important consideration is that experimentally observed crop physiological responses to climate change variables at plot and field levels (e.g. Kimball et al, 1995;Ainsworth et al, 2002;Leakey et al, 2004;Kim et al, 2006Kim et al, , 2007 are too simplified in current crop models (Tubiello et al, 2007a). As a consequence, the potential for negative surprises is not fully explored, thus reducing the level of confidence in regional and global projections (Tubiello et al, 2007a).…”