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The Rancho La Brea tar pit fossil collection includes Juniperus (C3) wood specimens that 14 C 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 Ϸ280 ppm before the industrial period, and is currently approaching 380 ppm in the modern atmosphere. Here we report on ␦ 13 C of Juniperus wood cellulose, and show that glacial and modern trees were operating at similar leaf-intercellular [CO2] (ci)͞atmospheric [CO2] (ca) values. As a result, glacial trees were operating at ci values much closer to the CO2-compensation point for C3 photosynthesis than modern trees, indicating that glacial trees were undergoing carbon starvation. In addition, we modeled relative humidity by using ␦ 18 O of cellulose from the same Juniperus specimens and found that glacial humidity was Ϸ10% higher than that in modern times, indicating that differences in vapor-pressure deficits did not impose additional constrictions on ci͞ca in the past. By scaling ancient ci values to plant growth by using modern relationships, we found evidence that C3 primary productivity was greatly diminished in southern California during the last glacial period. , and may fail to reproduce as a result of carbon limitations (6). These stress responses are due to limiting CO 2 availability, which decreases net photosynthetic rates as a result of reduced CO 2 substrate and increased rates of photorespiration (3). At higher spatial scales, Francois et al. (7) modeled global net primary productivity (NPP) between the last glacial maximum and the recent preindustrial period. The authors estimated that NPP values were only 38 gigatonnes (Gt) of C per year during the glacial maximum and increased to 53 Gt of C per year during the preindustrial period. Francois and coworkers attributed much of the reduction in NPP for the last glacial period to the effects of low [CO 2 ] on vegetation. Furthermore, Harrison and Prentice (8) modeled changes (BIOME4) in global vegetation between the last glacial period and modern times and found that, when climate change only (temperature and precipitation) was considered, the extent of forest cover in temperate, boreal, and, especially, tropical regions was greatly overestimated without the inclusion of low-[CO 2 ] effects on plant physiology.It is critical that we understand what effects the low [CO 2 ] that occurred during the last glacial period had on the physiological responses of actual terrestrial vegetation samples, which will then improve our estimates of ancient primary productivity and biospheric carbon stocks (7-9). If glacial C 3 plants responded to low [CO 2 ] in a manner similar to modern plants, wide-scale reductions in productivity would have occurred during the last glacial period, particularly in regions that were too cold to support C 4 species, which are highly tolerant of low [CO 2 ] (6). Therefore, physiological s...

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