No, All Plantlife Doesn't Die at 150 ppm CO2

Frequently I'm told that the increase in CO2 from fossil fuel emissions is good for the planet because, before the industrial revolution, CO2 levels were dangerously low. Gregory Wrightstone, for instance, claims that "before we began adding CO2 to the atmosphere, we weren’t sure that we wouldn’t cross that critical 150-ppm threshold during the next glacial period" when nearly all "terrestrial plant life cannot exist." According to him, "advanced plant life could not survive without at least 150 ppm" CO2. He calls this a "line of death." [1] I've even read people claiming that currently the atmosphere is "starved" of CO2. It's hard to know where this 150 ppm value originates; I haven't been been able to find it anywhere in the scientific literature, but I did read once that in greenhouses, daytime photosynthesis can lower CO2 concentrations to about 150 ppm. However, that does not justify any conclusion that plants cannot survive at concentrations lower than 150 ppm. There is, however, plenty of evidence investigating the impacts of low CO2 on vegetation.

Laboratory Experiments

In one study from 2012, plants were grown in CO2 concentrations ranging from 30 ppm to 800 ppm CO2. As you can see in the above graph, C3 plants could grow at CO2 levels as low as 30 ppm (but did not do well) and growth increase significantly as concentrations increased to 800 ppm. C4 plants did better at lower CO2 levels, and growth did not increase as dramatically with increasing CO2. In neither case was there a "line of death" at 150 ppm or even 100 pm CO2. From this experiment, lower CO2 levels would favor C4 plants over C3 plants, but CO2 levels at 150 ppm would not cause the extinction of plant life.

A second study[3] examined a C3 plant Arabidopsis thaliana at ambient CO2 (left column) and at 100 ppm CO2 (right column). After 4 weeks (A) and 6 weeks (b), this species grew much better at ambient CO2, but it did not die at 100 ppm. 

Still another study from 1995[4] observed the growth of Abutilon (C3) and Amaranthus (C4) plants under various partial pressures of CO2 - at below the Pleistocene minimum (150 ppm), preindustrial levels (270 ppm), 1995 levels (350 ppm) and estimated future levels (700 ppm). The C3 Abutilon theophrasti did not grow well at below Pleistocene minimum, but the C4 Amaranthus retroflexus showed very little difference between the lowest and highest levels of CO2. Evidence from studies like these indicate that since C3 plants do not grow as well under lower CO2 levels, climates with low CO2 levels may well have prompted the development of C4 plants. But these results are all in controlled experiments. Do we have evidence that plant life could survive long term at CO2 levels at 150 ppm or lower? It turns out that yes, we do.

Early Permian Low CO2

At the end of the Devonian, trees began to proliferate throughout the continents. These were strange looking trees, some exceeding 160 feet tall. These forests grew tall, but they lacked deep root systems, so they fell over fairly easily. These trees took great quantities of carbon out of the air and pumped oxygen into it. Over ~100 million years, carbon was sequestered in fossil beds from which we now are our sources of fossil fuels. With the long-term sequestration of CO2, Oxygen levels also rose. The excess oxygen allowed for insects to get large—dragonflies with a wing span of 2.5 feet lived at this time. The reduction of CO2 levels cooled the planet to what was probably the coldest temperatures of the entire Phanerozoic.

By the beginning of the Permian, CO2 levels decreased to ~100 ppm or so, possibly lower, and glaciation reached its maximum.[5][6][7][8] However, while plants were certainly stressed, they obviously did not go extinct. While almost certainly adaptations occurred to allow plants to survive low CO2 levels, photosynthesis did not stop. We know this because plant life continues to exist today.

According to the proxy evidence, the Earth spent about 1 million years below 150 ppm CO2, suggesting that the laboratory observations can actually be observed in the real climate of the early Permian. I'm not suggesting that 100 ppm CO2 would be a good thing, but the claim that 150 ppm CO2 is a "line of death" below which photosynthesis ends is clearly false. Low CO2 would heavily favor C4 plants and severely stress C3 plant life, but plant life would survive. And plant life was just fine at preindustrial levels of 280 ppm. The 50% increase in CO2 concentrations above preindustrial levels did not save the Earth from any mass extinction from lower CO2 levels. 

References:

[1] Wrightstone, Gregory. INCONVENIENT FACTS: The science that Al Gore doesn't want you to know . Mill City Press. Kindle Edition.

[2] Von Caemmerer, S., Quick, W. P., & Furbank, R. T. (2012). The Development of C4 Rice: Current Progress and Future Challenges. Science, 336(6089), 1671–1672. doi:10.1126/science.1220177
https://pubmed.ncbi.nlm.nih.gov/22745421/

[3] Yuanyuan Li, Jiajia Xu, Noor Ul Haq, Hui Zhang, Xin-Guang Zhu, Was low CO2 a driving force of C4 evolution: Arabidopsis responses to long-term low CO2 stress, Journal of Experimental Botany, Volume 65, Issue 13, July 2014, Pages 3657–3667, https://doi.org/10.1093/jxb/eru193

[4] Dippery, J. K., Tissue, D. T., Thomas, R. B., & Strain, B. R. (1995). Effects of low and elevated CO2 on C3 and C4 annuals. Oecologia, 101(1), 13–20. doi:10.1007/bf00328894

https://link.springer.com/article/10.1007/BF00328894

[5] Georg Feulner. Coal formation and global glaciation. Proceedings of the National Academy of Sciences Oct 2017, 114 (43) 11333-11337; DOI: 10.1073/pnas.1712062114.
https://www.pnas.org/content/114/43/11333

[6] Montañez et al 2016 finds that the low CO2 values correlate with a global eustatic fall and the apex of glaciation during the time period.

Montañez, I., McElwain, J., Poulsen, C. et al. Climate, pCO2 and terrestrial carbon cycle linkages during late Palaeozoic glacial–interglacial cycles. Nature Geosci 9, 824–828 (2016). https://doi.org/10.1038/ngeo2822

[7]  The estimates in this study are revised from his earlier estimates, which can be found in Montañez et al 2007.

Montanez, I. P., Tabor, N. J., Niemeier, D., DiMichele, W. A., Frank, T. D., Fielding, C. R., … Rygel, M. C. (2007). CO2-Forced Climate and Vegetation Instability During Late Paleozoic Deglaciation. Science, 315(5808), 87–91. doi:10.1126/science.1134207

 
[8] Breeker wrote a computer program that allowed for more accurate estimates of atmospheric CO2 levels from the proxy data that Montañez used in his 2007 paper.

Breecker, D. O. (2013), Quantifying and understanding the uncertainty of atmospheric CO2 concentrations determined from calcic paleosols, Geochem. Geophys. Geosyst., 14, 3210–3220, doi:10.1002/ggge.20189.