During the Last Glacial Maximum, atmospheric [CO₂] was as low as 180 ppm and has currently risen to a modern value of 385 ppm as a result of fossil fuel combustion and deforestation. In order to understand how changing [CO₂] influenced the physiology and evolution of trees over the last 50,000 years, we analyzed carbon isotope ratios of individual tree rings from juniper wood specimens from the Rancho La Brea tar pits in southern California and kauri wood specimens from peat bogs in New Zealand (North Island). Modern trees from different altitudes were included to account for changes in precipitation and temperature through time in order to isolate the effects of changing [CO₂]. Using carbon isotope ratios, we calculated the ratio of c_i/c_a (intercellular [CO₂]/atmospheric [CO₂]) for each annual ring of each tree. From c_i/c_a, we calculated c_i values using c_a data from the Vostok and Taylor Dome ice cores.  

Results/Conclusions

In both kauri and juniper trees, mean c_i/c_a values remained constant throughout 50,000 years despite major climatic and [CO₂] changes, indicating there is a long-term physiological set point for c_i/c_a in these groups.  We also observed that no ice age Juniper trees experienced c_i values below 90 ppm, suggesting this value may represent a survival compensation point for Juniper. In addition, ice age trees under low [CO₂] conditions showed very low interannual variation in c_i/c_a values compared to modern trees, even though interannual climatic variability was as high during the LGM in this region as it is today. A lack of variability in c_i/c_a values of ice age trees suggests that tree physiology may have been dominated by low [CO₂] as opposed to other climatic variables, since [CO₂] was relatively stable from year to year during the last ice age. Modern trees showed high interannual variation in c_i/c_a, likely since water dominates current physiological responses and varies greatly from year to year. These results have significant implications for increasing our understanding of the adaptation of trees to changing [CO₂] and show that the environmental factors that show a dominant influence on plant physiology may have changed over geologic time scales.