INTERANNUAL RESPONSES OF GLACIAL AND MODERN AGATHIS AUSTRALIS TO CHANGES IN ATMOSPHERIC [CO2] SINCE THE LAST GLACIAL MAXIMUM

During the Last Glacial Maximum, atmospheric [CO₂] was as low as 180 ppm and has since risen to a modern value of ~400 ppm. In order to understand how changing [CO₂] influenced the growth patterns of trees, we analyzed ring widths of individual tree rings from glacial Agathis australis specimens preserved in peat bogs in Northland, North Island, New Zealand (aged >50 kyr BP). Modern trees from the same region were also analyzed to compare effects of changing precipitation, temperature and atmospheric [CO₂] levels. Carbon isotopic signatures were measured for individual rings of each specimen, from which the c_i/c_a ratio (inter-cellular [CO₂] to atmospheric [CO₂]) could be calculated. Ring widths were also measured, including raw width and standardized indices (calculated as raw width divided by a 20-year sliding mean) and compared across individuals.

The c_i/c_a ratio did not significantly differ between glacial and modern A. australis. Constant c_i/c_a ratios resulted in severely low c_i levels in glacial trees, suggesting a reduction in growth capabilities in glacial trees. Despite low ci levels, however, glacial and modern trees showed no significant differences in mean growth and maximum growth levels. Both glacial and modern A australis trees showed high variation in maximum raw growth, suggesting that microsite conditions and interannual variation in climate play major roles in growth patterns. 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.