Background
The authors write that "at the peak of the last glacial period, atmospheric CO2 concentrations ranged between 180 and 200 ppm, which are among the lowest concentrations that occurred during the evolution of land plants (Berner, 2006)," and they say that "when grown at glacial vs. modern CO2, modern C3 plants show 40-70% reductions in photosynthesis and biomass production (Polley et al., 1993; Sage and Coleman, 2001), 20-30% lower survival (Ward and Kelly, 2004), and may even fail to reproduce (Dippery et al., 1995)."

What was done
Physiological responses to changes in atmospheric CO2 concentration that are manifest in stable carbon isotope ratios of alpha-cellulose extracted from individual tree rings were determined for glacial-age Juniperus (juniper) trees preserved in the Rancho La Brea tar pits of Los Angeles, California (USA), which were ¹⁴C dated to 14.5-47.6 kyr BP (thousand years before present), after which these responses - which ultimately led to determinations of intercellular/atmospheric CO2 ratios (ci/ca) - were then compared to those of modern-age Juniperus trees growing in the Angeles National Forest and higher elevation sites in the San Bernardino National Forest, both of which sites are located close to the La Brea tar pits.

What was learned
Gerhart et al. report that Juniperus trees "showed constant mean ci/ca between the last glacial period and modern times, spanning 50,000 years," and on the basis of modern ca values obtained from direct atmospheric measurements (Keeling et al., 2009) and the Taylor Law Dome ice core (Etheridge et al., 1996) and glacial ca values derived from the Vostock and EPICA Dome C ice cores, they discovered that glacial Juniperus trees "exhibited low ci that rarely occurs in modern trees," i.e. 106 ppm vs. 168 ppm.

What it means
In describing the significance of their findings, the five researchers say that their study "provides some of the first direct evidence that glacial plants remained near their lower carbon limit until the beginning of the glacial-interglacial transition," which lower limit appeared to them to be 90 ppm CO2, below which value it appeared that "juniper trees may not maintain a positive carbon budget for basic physiological functions for survival," as is also suggested by the corroborative work of Campbell et al. (2005).

References
Berner, R.A. 2006. GEOCARBSULF: a combined model for Phanerozoic atmospheric O2 and CO2. Geochimica et Cosmochimica Acta 70: 5653-5664.

Campbell, C.D., Sage, R.F., Kocacinar, F. and Way, D.A. 2005. Estimation of the whole-plant CO2-compensation point of tobacco (Nicotiana tabacum L.). Global Change Biology 11: 1956-1967.

Dippery, J.K., Tissue, D.T., Thomas, R.B. and Strain, B.R. 1995. Effects of low and elevated CO2 on C3 and C4 annuals. I. Growth and biomass allocation. Oecologia 101: 13-20.

Etheridge, D.M., Steele, L.P., Langenfelds, R.L., Francey, R.J., Barnola, J.-M. and Morgan, V.I. 1996. Natural and anthropogenic changes in atmospheric CO2 over the last 1,000 years from air in Antarctic ice and firn. Journal of Geophysical Research 101: 4115-4128.

Keeling, R.F., Piper, S.C., Bollenbacher, A.F. and Walker, J.S. 2009. Atmospheric CO2 records from sites in the SIO air sampling network. In: Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tennessee, USA.

Polley, H.W., Johnson, H.B., Marino, B.D. and Mayeux, H.S. 1993. Increase in C3 plant water-use efficiency and biomass over glacial to present CO2 concentrations. Nature 361: 61-64.

Sage, R.F. and Coleman, J.R. 2001. Effects of low atmospheric CO2 in plants: more than a thing of the past. Trends in Plant Science 6: 18-24.

Ward, J.K. and Kelly, J.K. 2004. Scaling up evolutionary responses to elevated CO2: Lessons from Arabidopsis. Ecology Letters 7: 427-440.