Paper Reviewed
Walker, S.M. and Ward, J.K. 2018. Interactions between rising CO2 and temperature drive accelerated flowering in model plants under changing conditions of the last century. Oecologia 187: 911-919.

According to Walker and Ward (2018), a number of field surveys have documented accelerations in time-to-flowering (TTF) in various plant species over the past century. And in nearly all instances the accelerated TTF is attributed to rising temperatures.

Such attribution, however, may be problematic. For, as noted by the two University of Kansas researchers, warming treatments in field experiments largely underestimate TTF accelerations that have been observed in long-term field studies. Consequently, Walker and Ward state "there appears to be a major missing factor(s) for explaining accelerations in flowering time that have occurred in natural field settings over the last century," and it was their opinion that rising atmospheric CO2 may well be that missing factor.

To test their hypothesis, Walker and Ward grew eight genotypes of Arabidopsis thaliana in a controlled-environment setting under two temperature and two CO2 treatments. Temperature treatments included a 20.0/13.0°C or 21.3/14.3°C day/night regime to correspond with the preindustrial or modern period, respectively, while the CO2 treatments included a preindustrial value of 270 or a modern value of 380 ppm. Plants were subjected to the temperature and CO2 conditions from seed emergence and monitored for growth rate, TTF and biomass at flowering.

Results indicated that A. thaliana growth rates were enhanced by 14% in the modern temperature/CO2 regime over the preindustrial temperature/CO2 regime. Statistical analysis concluded that this enhancement was driven by an interaction between rising CO2 and temperature rather than by either factor in isolation. Walker and Ward also observed an interactive effect of CO2 and temperature on TTF, such that this parameter was reduced by 3.8 days in response to the 110 ppm increase in CO2 and 1.3°C increase in temperature. And, while there was no significant effect on TTF by CO2 alone, elevated temperature reduced TTF by 1.3 days when it was studied in isolation of CO2. Lastly, the authors report that the biomass at flowering moderately increased in response to elevated CO2 alone, but decreased in response to temperature alone. In combination, elevated CO2 and temperature resulted in no significant change in biomass at flowering.

In discussing their findings, Walker and Ward say that "the interactive effects of rising CO2 and temperature over the last century may explain why there has been observed accelerations in flowering times in long-term field surveys that cannot be replicated in warming-only studies." Indeed, their results suggest as much; an average 3 day reduction in TTF per degree increase in temperature. What remains to be discerned, however, is how this advance may (or may not) impact other plants and the ecosystems in which they reside.