South American tree rings show declining δ¹³C trend

A composite, 290‐year tree‐ring δ¹³C chronology was developed from a site in Chile where 5 Fitroya cupressoides (alerce) trees were sampled, 2 increment cores per tree, and the holocellulose component was analyzed in 5‐year ring groups. This chronology shows a decreasing δ¹³C trend of approximately 1.2‰, primarily since the turn of this century. This δ¹³C decline is similar to that of major tree‐ring studies in the Northern Hemisphere, but it is the only major Southern Hemisphere study which clearly exhibits such a δ¹³C trend. This is the first evidence for any interhemispheric reproducibility of tree‐ring δ¹³C chronologies, and furthermore, the Fitzroya δ¹³C trend conforms well to that of δ¹³C of atmospheric CO₂ determined from ice cores and direct measurements. This correspondence suggests the alerce δ¹³C trend has not been substantially influenced by systematic changes in environmental factors such light, relative humidity and soil moisture or by changing atmospheric CO₂ concentration, all of which are, in theory, capable of altering C_i/C_a ratios and obscuring the atmospheric δ¹³C record contained in the tree rings.