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The carbon-starved world of the last glacial maximum

21 Dec 2022 | Science Notes

From the CO2Science Archive: The authors of this unique study compared “the physiological (stomatal-regulation) and modeled-growth responses of glacial and modern Juniperus trees at La Brea by using stable carbon isotope methodologies,” working with what they describe as “a very rare series of preserved Juniperus (C3) wood specimens that span a large portion of the last glacial period,” which are part of the Rancho La Brea tar pit fossil collection of the George C. Page Museum of La Brea Discoveries in Los Angeles, California, USA. Values of both carbon isotope discrimination and the ratio of internal leaf to atmospheric CO2 concentration (ci/ca) at the time of the last glacial maximum were determined to be the same as those of today, implying that “leaves of full-glacial trees had extremely low calculated ci values (averaging 113 ppm) that were 25% lower than in leaves of postglacial trees (ci of 150 ppm between 7.665 and 12.450 kyr B.P.), and 40% lower than in leaves of modern trees (average ci of 187 ppm).”

Paper reviewed: Ward, J.K., Harris, J.M., Cerling, T.E., Wiedenhoeft, A., Lott, M.J., Dearing, M.-D., Coltrain, J.B. and Ehleringer, J.R. 2005. Carbon starvation in glacial trees recovered from the La Brea tar pits, southern California. Proceedings of the National Academy of Sciences, USA 102: 690-694.

Noting that “previous studies with modern C3 plants indicate that ci values are highly scaleable to photosynthetic rate and growth, with particularly strong correlations occurring at low CO2 conditions (Polley et al., 1993; Sage and Coleman, 2001),” Ward et al. estimated that forest productivity was reduced by about 55% between modern and full-glacial ci values. Viewed from the perspective of forward-evolving history, this finding suggests that most of Earth’s trees have already seen their productivity more than doubled by the aerial fertilization effect of the increase in the air’s CO2 concentration experienced over the past 12.5 thousand years.

What it means
Pointing out that the low Last Glacial Maximum ci values they found “were not unique to southern California, because glacial leaves of Pinus flexilis from the Great Basin exhibited ci values of 110 ppm (van de Water et al., 1994), supporting the notion that trees in nearby regions were also carbon-starved during the last glacial period,” Ward et al. ultimately conclude that “the productivity of ancient trees, along with other C3 species, was greatly diminished during the last glacial period.” The words carbon-starved are their choice for describing this situation, and they appear in several places throughout their paper. Hence, it should be clear to all that the concomitant increases in atmospheric CO2 concentration and air temperature experienced from the Last Glacial Maximum to the present have truly been a godsend. In addition, as the air’s CO2 content continues to climb, with or without a continuation of the life-promoting air temperature increase of the past, it will only continue to enhance the growth and vitality of both terrestrial and marine plant life and, therefore, animal life as well.

Polley, H.W., Johnson, H.B., Marino, B.D. and Mayeux, H.S. 1993. Increases 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 on plants: more than a thing of the past. Trends in Plant Science 6: 323-326.

Van de Water, P.K., Leavitt, S.W. and Betancourt, J.L. 1994. Trends in stomatal density and 13C/12C ratios of Pinus flexilis needles during the last glacial-interglacial cycle. Science 264: 239-243.

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