We describe the most important crop responses to atmospheric CO₂ concentrations, the problems and uncertainties related to this effect and describe how crop models include the CO₂ fertilization effect. Another section is supposed to describe the problem exactly representing the effect in LPJmL.

[[TOC]]

• decrease in stomatal conductance (reducing crop transpiration -> lower evaporative cooling -> lower crop water use, amelioration of stress during periods of drought)
• increase in net photosynthetic CO₂ uptake (only in C3 plants; RuBisCo is not saturated at current atm. CO₂, and CO₂ inhibits photorespiration) (Ainsworth et al. 2008)
• C4 plants also benefit, but at lower level (only through increased water use efficiency), “no yield increase would be expected for well-watered C4 crops” (Long et al. 2006)
• “The response of plant production to CO₂ is approximately hyperbolic, increasing linearly at subambient concentration and saturating at around 800 to 2000 ppm” (Long et al., 2006)
• “At 25°C, an increase in CO₂ to 550 ppm should increase light-saturated photosynthesis by 36. The average increase observed for C3 crops in FACE was 20 for the daily integral of photosynthetic CO₂ uptake, 17% for total biomass, and just 13% for yield” (Long et al., 2006)
  -> " discrepancy is possibly related to a fact that the crop models usually predict with non-limited supply of water and nutrition and near optimum temperature for crop growth." (Mo et al. 2009)
• “winter wheat production is more sensitive to climatic variability than summer maize. … This can be interpreted in that wheat as a C3 crop is sensitive to the CO₂ fertilizing effects on leaf stomatal conductance, which may compensate for the warming effects on ET, whereas maize as a C4 crop is not so sensitive to CO₂ fertilizing effects.” (Mo et al. 2009)

• DSSAT-CERES constant multipliers for daily total crop biomass under elevated CO2, equally applied to either stressed or unstressed conditions
• EPIC/CropSyst uses multipliers for either daily biomass or final yield, mean enhancement factors are similar to DSSAT-CERES
• AEZ multipliers of final biomass production, translated directly into yield via a harvest index, includes reduction under stress conditions
• DSSAT-CROPGRO and APSIM simulates crop growth via radiation-use efficiency, transpiration efficiency and a range of limiting factors, which are modified under elevated CO2 using leaf-level mechanistic equation (von Cammerer and Farquhar, 1981)

**Figure 1**: Response rations for CO₂ crop response (Tubiello et al. 2007).

Chambers: plants grown in pots alter the response of plants to elevated CO₂, large environmental differences between inside and outside the chamber like e.g. temperature and water vapor pressure deficit -> the effect of the chamber on plants is often greater than that of elevated CO₂ (Long et al. 2006)

FACE: small sample size and large standard errors (Tubiello et al. 2007, McGrath et al. 2011), most of the FACE systems do not elevate CO₂ at night, elevated CO₂ has been suggested to inhibit dark respiration (artifact of earlier measurement systems) (Ainsworth et al. 2005), no FACE experiments in the tropics (Long et al. 2006)

Contrasting conclusions: “chamber studies might be inadequate for predicting future yields” (Long et al. 2006) ?
„crop yield responses to elevated CO₂ are similar across FACE and non-FACE experimental data” (Tubiello et al. 2007)

Nitrogen: “the CO₂ fertilization effect may be small without large additions of N” (Long et al. 2006),“low N tended to reduce the response of light-saturated CO₂ uptake to elevated CO2” (Ainsworth et al. 2005)

Water: „Under wet conditions [for C4 crops], there was no increase in yield with elevated CO₂” (Ainsworth et al. 2005)

Temperature: Interactions of increased temperatures and elevated CO₂ remain yet to be quantified, e.g. heat stress, shorter growing season, temperature optimum

Ozone: “Chamber studies suggest that elevated CO₂ may provide some protection against elevated O₃ and therefore the effects will not be additive, but this has yet to be verified for any crop under open-air field conditions.” (Long et al. 2006)

Tubiello et al. (2007):

• “…necessary to assess response of crops other than the key cereal grains, and in climate regimes other than temperate, especially those of importance to developing countries in the sub-tropics”
• “…more research is needed to increase understanding of the interactions of elevated CO₂ with increasing temperatures, worsening air pollution, changes in moisture availability and mineral nutrition, and altered incidence of pests, diseases and weeds.”

Ainsworth et al. (2005):

• “…more extensive FACE experimentation with the major crops and within the major growing zones”
• “Future FACE experiments should also consider multiple levels of elevated CO₂, ranging from 50 ppm above current ambient to double current ambient CO₂”

• relation between the stomatal conductance and the transpiration rate
• on the stomata level transpiration regulation is well documented, but it’s role could progressively
• decrease if we consider leaf ? plant ? canopy.
• there is often a correlation between photosynthetic capacity and stomatal conductance which tends to keep the ratio of internal to external CO₂ constant

• inceasing leaf area can offset the effect of reduced stomatal conductance

• leaf temperature influences Vmax
• an effect of temperature on the relative reduction in midday stomatal conductance at elevated CO₂ has been examined (not for maize)

• “… Simulations, and measurements in free air carbon dioxide enrichment systems both indicate that the relatively large reductions in stomatal conductance in crops would translate into reductions of <10% in evapotranspiration, partly because of increases in temperature and decreases in humidity in the air around crop leaves.” Bunce (2004)
• The concept of coupling between leaves and atmosphere is described in terms of the water vapour saturation deficit. Leaves are considered to be well-coupled to the atmosphere if the atmospheric saturation deficit is imposed on the leaf surface without local adjustment
• They are poorly-coupled to the atmosphere, if the surface saturation deficit finds its own value by local equilibration.
• Many crops are very poorly-coupled to the atmosphere because they are not very tall ? transpiration from the crops is likely to depend strongly on radiation
• Tall crops are generally well-coupled to the atmosphere ? transpiration is likely to respond sensitively to small changes in stomatal conductance
• Reduction in evapotranspiration in crops is similar to that in other types of vegetation which have smaller relative reductions in stomatal conductance, because of the poorer aerodynamic coupling of the canopy to the atmosphere in crops

**Figure 2**: Change in annual crop harvest in gC/m2 in year x compared to year y **with** CO₂ fertilization effect (prepared from S.Schaphoff). **Figure 3**: Change in annual crop harvest in gC/m² in year x compared to year y **without** CO₂ fertilization effect (prepared from S.Schaphoff).

• Effect of the water vapour diffusion: the damping effect (physiological)
  (stomatal conductance and going through the boundary layer)
• thermal negative feedback effect (physical) -> canopy loses latent heat by transpiration
  -> These two effects are the decoupling between the plant and the environmental conditions

• hydrological negative feedback effects (physical)

**Figure 4**: Boundary layer conductance, a air layer depends on the geometry of the surface Assumption: here the effect of a change in stomatal conductance on transpiration is less. Stomatal conductance and corresponds to the physiological regulation relation of Adt to stomatal conductance keeps constant.

• Experiments have shown that the control exerted by stomata on the transpiration of a crop is poor when the stomatal conductance is large,
• the control of water losses by stomata is effective for small conductance

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6. Tubiello, F. N., Amthor, J. S., Boote, K. J., Donatelli, M., Easterling, W., Fischer, G., Gifford, R. M., et al. (2007). Crop response to elevated CO2 and world food supply: A comment on “Food for Thought…” by Long et al., Science 312:1918–1921, 2006. European Journal of Agronomy, 26(3), 215-223. http://www.sciencedirect.com/science/article/pii/S1161030106001341