13.3.4  Phenology, temperature and CO2

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Mean global surface temperatures are expected to increase by approximately 1.5–4°C with a doubling of the atmospheric CO2 concentration from 350 to 700 µmol CO2 mol–1. This may appear inconsequential in comparison with the common occurrence of temperature fluctuations of up to 30°C in a single day. However, small temperature rises over an entire season will have a significant effect on crop yield and pasture growth largely because of the acceleration of developmental rates. Temperature increases and CO2 enrichment independently accelerate development of annual crop species. In wheat and rice, successive leaves appear faster and time to flowering is shortened by increases in both temperature and CO2. In determinant species such as wheat (Triticum aestivum L.) and rice (Oryza sativa L.), yield is often dependent on whole-plant biomass production. Consequently, hastened development (faster phenology) decreases the time available for radiation interception prior to maturity. This decreases biomass accumulation and yield.

The scale of temperature × CO2 interactions on yield is thus set by phenology, and isolines that differ primarily in the duration of their life cycle provide useful test material. The near-isogenic wheat varieties Hartog and Late Hartog provide a good example of a genetically dependent phenology. Using these two varieties, experiments were carried out (H. Rawson, pers. comm. 1997) to determine whether duration could increase the advantage gained by higher temperatures at elevated CO2 concentrations. Temperatures were increased by 2°C above ambient in both winter and summer growing conditions (Figure 13.8).


Figure 13.8 Grain yield response of wheat (Hartog and Late Hartog) to increasing CO2 concentration from 350 µmol mol-1 to 700 µmol mol-1, expressed as a ratio of yield from enriched to yield under ambient CO2 concentrations. Symbols represent two different sites (H. Rawson, unpublished data)

In summer, the mean daily maximum was approximately 23°C. The maximum daily temperature was over 30°C, with temperatures exceeding 40°C on 17 occasions. Crop yield responded dramatically under these conditions. Doubling the ambient atmospheric CO2 concentration led to a biomass and grain yield increase of over 30% in the shorter duration genotype (Hartog). As a result of its slower developmental rate and subsequent increased radiation interception, biomass production was greater in Late Hartog than Hartog in two consecutive summer studies. In winter, when mean temperature was around 10°C, the increase in yield in response to CO2 enrichment ranged from 8% to 12%.

Based on responses to a doubling of the current CO2 concentration over a range of temperatures, a 1.8% increase in biomass production and yield with each °C rise in temperature can be anticipated (Figure 13.8). These data imply a considerable benefit in terms of grain yield in a future CO2-enriched environment, but extrapolation to areas where summer temperatures are already marginal for production would be misleading. In those cases, cereal yield will be constrained by environmental stresses despite potential benefits from higher CO2.