Atmospheric [CO2 x Temperature Interactions

Since short-term stimulation of Pn is increased at higher temperatures (Long, 1991), a large positive interaction between effects of increased [CO2] and temperature on biomass might be expected (Rawson, 1992). While a relatively consistent positive interaction was measured in wheat experiments conducted by Rawson (1995), no interaction was found by Mitchell et al. (1995) and Batts et al. (1997, 1998a,b). There was a non-significant positive interaction of [CO2] with seasonal mean daily temperature in the multi-site OTC experiments (Bender et al., 1999; van Oijen and Ewert, 1999). This is an area where simulation models have proved useful for analysis. While the Farquhar and von Caemmerer (1982) model predicts strong temperature x [CO2] interaction for effects on Pn (see Long, 1991), wheat models which use this as a sub-model show that the interactive effect on biomass is often not large. This is because increasing temperature means the growing season ends earlier, so that less of the growth takes part in the hottest part of the year. Using 30-year averages of temperature and radiation for different European sites, van Oijen and Ewert (1999) showed that models predicted no interaction between temperature and [CO2] in absolute yield terms. If the seasonal variation in temperature is less in the Australian climate, this would explain the [CO2] x temperature interaction observed there.

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