Carbon starvation in glacial trees recovered from the La Brea tar pits, southern California

The Rancho La Brea tar pit fossil collection includes Juniperus (C-3) wood specimens that C-14 date between 7.7 and 55 thousand years (kyr) B.P., providing a constrained record of plant response for southern California during the last glacial period. Atmospheric CO2 concentration ([CO2]) ranged between 180 and 220 ppm during glacial periods, rose to approximate to 280 ppm before the industrial period, and is currently approaching 380 ppm in the modern atmosphere. Here we report on delta(13)C of Juniperus wood cellulose, and show that glacial and modern trees were operating at similar leaf-intercellular [CO2] (c(i))/atmospheric [CO2] (c(a)) values, As a result, glacial trees were operating at ci values much closer to the CO2-compensation point for C-3 photosynthesis than modern trees, indicating that glacial trees were undergoing carbon starvation. In addition, we modeled relative humidity by using delta(18)O of cellulose from the same Juniperus specimens and found that glacial humidity was approximate to 10% higher than that in modern times, indicating that differences in vapor-pressure deficits did not impose additional constrictions on c(i)/c(a) in the past. By scaling ancient ci values to plant growth by using modern relationships, we found evidence that C-3 primary productivity was greatly diminished in southern California during the last glacial period.