Learning from the past: how low [CO₂] studies inform plant and ecosystem response to future climate change.

Atmospheric [CO2] over the past 800 000 yr has varied generally as a function of glacial periods, with minima (c. 170–200 ppm) during glacial periods and maxima (c. 280–300 ppm) during inter-glacial periods (Luthi et al., 2008). During the Last Glacial Maximum (LGM; 18 000–20 000 yr ago), atmospheric [CO2] ranged from 180 to 200 ppm, which is approximately half the current [CO2] (392 ppm), and among the lowest [CO2] observed during the evolution of vascular land plants over the past 350 million yr. While it has been observed that low atmospheric [CO2] directly limits photosynthesis (Tissue & Lewis, 2010), with subsequent reductions in biomass production (Lewis et al., 2010), reproduction (Dippery et al., 1995), and survival (Ward & Kelly, 2004), these studies have primarily been conducted on modern plants grown for a single generation in low [CO2] (see review by Gerhart & Ward, 2010). Subsequently, they do not address the potential evolutionary adaptive responses to low [CO2] which would only become evident in plants growing for long-time periods and many generations under these environmental conditions.