5.4.1.1 Effects of elevated CO₂ on plant growth and yield

Plant response to elevated CO₂ alone, without climate change, is positive and was reviewed extensively by the TAR. Recent studies confirm that the effects of elevated CO₂ on plant growth and yield will depend on photosynthetic pathway, species, growth stage and management regime, such as water and nitrogen (N) applications (Jablonski et al., 2002; Kimball et al., 2002; Norby et al., 2003; Ainsworth and Long, 2005). On average across several species and under unstressed conditions, recent data analyses find that, compared to current atmospheric CO₂ concentrations, crop yields increase at 550 ppm CO₂ in the range of 10-20% for C₃ crops and 0-10% for C₄ crops (Ainsworth et al., 2004; Gifford, 2004; Long et al., 2004). Increases in above-ground biomass at 550 ppm CO₂ for trees are in the range 0-30%, with the higher values observed in young trees and little to no response observed in mature natural forests (Nowak et al., 2004; Korner et al., 2005; Norby et al., 2005). Observed increase of above-ground production in C₃ pastures is about +10% (Nowak et al., 2004; Ainsworth and Long, 2005). For commercial forestry, slow-growing trees may respond little to elevated CO₂ (e.g., Vanhatalo et al., 2003), and fast-growing trees more strongly, with harvestable wood increases of +15-25% at 550 ppm and high N (Calfapietra et al., 2003; Liberloo et al., 2005; Wittig et al., 2005). Norby et al. (2005) found a mean tree net primary production (NPP) response of 23% in young tree stands; however in mature tree stands Korner et al. (2005) reported no stimulation.

While some studies using re-analyses of recent FACE experimental results have argued that crop response to elevated CO₂ may be lower than previously thought, with consequences for crop modelling and projections of food supply (Long et al., 2005, 2006), others have suggested that these new analyses are, in fact, consistent with previous findings from both FACE and other experimental settings (Tubiello et al., 2007a, 2007b). In addition, simulations of unstressed plant growth and yield response to elevated CO₂ in the main crop-simulation models, including AFRC-Wheat, APSIM, CERES, CROPGRO, CropSyst, LINTULC and SIRIUS, have been shown to be in line with recent experimental data, projecting crop yield increases of about 5-20% at 550 ppm CO₂ (Tubiello et al., 2007b). Within that group, the main crop and pasture models, CENTURY and EPIC, project above-ground biomass production in C₃ species of about 15-20% at 550 ppm CO₂, i.e., at the high end of observed values for crops, and higher than recent observations for pasture. Forest models assume NPP increases at 550 ppm CO₂ in the range 15-30%, consistent with observed responses in young trees, but higher than observed for mature trees stands.

Importantly, plant physiologists and modellers alike recognise that the effects of elevated CO₂ measured in experimental settings and implemented in models may overestimate actual field- and farm-level responses, due to many limiting factors such as pests, weeds, competition for resources, soil, water and air quality, etc., which are neither well understood at large scales, nor well implemented in leading models (Tubiello and Ewert, 2002; Fuhrer, 2003; Karnosky, 2003; Gifford, 2004; Peng et al., 2004; Ziska and George, 2004; Ainsworth and Long, 2005; Tubiello et al., 2007a, 2007b). Assessment studies should therefore include these factors where possible, while analytical capabilities need to be enhanced. It is recommended that yield projections use a range of parameterisations of CO₂ effects to better convey the associated uncertainty range.