Growth responses of glacial and modern trees to changes in atmospheric [CO2] since the Last Glacial Maximum

Background/Question/Methods

During the Last Glacial Maximum, atmospheric [CO₂] was as low as 180 ppm and has currently risen to a modern value of 393 ppm as a result of fossil fuel combustion and deforestation. Past research has shown that glacial trees had c_i values (inter-cellular CO₂ concentrations) less than half that of modern trees, suggesting a severe carbon limitation with potentially significant effects on productivity. In order to understand how changing [CO₂] influenced the growth patterns of trees over the last 50,000 years, we analyzed ring widths of individual tree rings from glacial Juniperus specimens preserved in the Rancho La Brea tar pits in southern California (aged 14-49 kyr BP) and Agathis australis specimens from peat bogs in North Island, New Zealand (aged >52 kyr BP). Modern trees from each region were also analyzed to compare effects of changing precipitation, temperature and atmospheric [CO₂] levels. For each specimen, raw width and standardized indices (calculated as raw width divided by a 20-year sliding mean) were calculated, and then compared across individuals.

Results/Conclusions

Mean raw width was not significantly different between glacial and modern trees of Juniperus or Agathis. In addition, mean raw maximum width in each species was also not significantly different between glacial and modern specimens. Interannual variation in ring width index also did not show significant differences between glacial and modern trees. Juniperus and Agathis trees showed wide variation in maximum raw growth in both glacial and modern specimens, suggesting that microsite conditions and interannual variation in climate play major roles in growth trends. Taken together, these results suggest that glacial trees were adapted to maintain growth despite lower carbon availability. These adaptations may constrain the ability of modern trees to fully utilize increases in atmospheric [CO₂], resulting in limited growth enhancements with rising CO₂ in long-lived species. These results have significant implications for increasing our understanding of the adaptation of trees to changing [CO₂] and the impacts such adaptations may have on responses of plants to future climatic change.