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Is there a point where too much CO2 is bad for a plant? Basically when there is too much CO2 in the air can a plant get sick?

Since plants photosynthesize and need CO2 to generate glucose and store starch, and since chemical reactions are pushed toward their end product when the reagent concentrations are increased, one would expect that more CO2 would be better, at least lead to increased growth and survival rates.

Is there a ceiling where CO2 gets toxic?

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    $\begingroup$ I have taken the liberty to add some background. Feel free to roll these changes back. $\endgroup$ – AliceD Mar 13 '18 at 10:49
  • $\begingroup$ I'd question whether CO2 availability is really much of a limiting factor in plant growth. I would think it would be the energy available from sunlight, if growth isn't limited by other factors such as water, temperature, nitrogen & other soil nutrients... $\endgroup$ – jamesqf Mar 13 '18 at 19:30
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    $\begingroup$ Many trees at Mammoth Mountain have died due to CO2 poisoning. See pubs.usgs.gov/fs/fs172-96 for more info from link: "High CO2 concentrations in the soil kill plants by denying their roots O2 and by interfering with nutrient uptake. In the areas of tree kill at Mammoth Mountain, CO2 makes up about 20 to 95% of the gas content of the soil. " $\endgroup$ – ssnobody Mar 13 '18 at 19:55
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Short answer
It has been shown that plants may already suffer from doubling the atmospheric CO2 concentration from 340 to 610 ppm, something that might happen during the next hundred years or so based on current emissions.

Background
A popular science website tells us that an excess of carbon dioxide (CO2) reduces the rate of transpiration of some plants. This is so because the stomata, which are the openings of the leaves (Mansfield & Majernik, 1970) and used for exchanging gases as well as water vapor (transpiration) will close when there is too much CO2 in the air, or other polutants such as SO2. As transpiration drops, the water flow from the soil to the leaves also drops, causing a runoff of water.This in turn stalls nutrient uptake. Indeed, doubling the present-day CO2 concentration to 610 ppm does not necessarily lead to increased growth and may in fact inhibit growth due to excess starch formation in the leaves, indicating it's simply stored as backup energy, nothing else (Coviella & Trumble, 1999). It is believed that plants might be near their saturation point and cannot eliminate CO2 faster than they are doing right now.

Somehow plants also become more susceptible to insect foraging when CO2 concentration increases.

Note however, that CO2 tolerances are species dependent. Most research in this arena has focused on common crops. CO2 tolerance in for example cotton plants is low, and starch buildup has been observed in the entire plant, but especially in the root systems and the stem (Hendrix et al., 1994). Other species, such as wheat and rice are less prone to effects of elevated CO2 (source: Nature).

References
- Coviella & Trumble, Conservation Biology (1999); 13(4): 700–12
- Hendrix et al., Agricult Forest Meteorol (18994); 70(1–4): 153-62
- Mansfield & Majernik, Environmental Pollution (1970); 1(2): 149-54

Source
- Earth untouched

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    $\begingroup$ In case the OP wanted to see an example of CO2 killing plants, there's a place in California where this happens due to volcanic activity: pubs.usgs.gov/dds/dds-81/Intro/facts-sheet/GasKillingTrees.html $\endgroup$ – Reginald Blue Mar 13 '18 at 17:17
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    $\begingroup$ @TheEnvironmentalist Ferns have been found to do well at high (3360 ppm, 10 time pre-industrial levels) CO2. link.springer.com/article/10.1023/A:1006805427943 $\endgroup$ – DavePhD Mar 13 '18 at 18:35
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    $\begingroup$ In situations with extreme excess of CO2, plants experience "excess starch formation" because it is "stored as backup energy" and it "inhibits growth"? So... Global Warming makes plants fat? $\endgroup$ – Kamil Drakari Mar 13 '18 at 19:35
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    $\begingroup$ "... something that might happen during the next hundred years or so due to current global warming." Are you saying that warming increases CO_2 levels? $\endgroup$ – jpmc26 Mar 13 '18 at 23:46
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    $\begingroup$ Good answer, but I think it's missing a critical point - different plants have vastly different tolerance. Many plants benefit from even large increases in CO2 levels (algae love CO2, as do many ferns etc.), while many can be stunted in growth or outright killed by a "mere" doubling. Also, I don't think it's warranted to say the increase in CO2 is "due to global warming" - even if the increase in CO2 levels is due to increases in global temperatures (and not vice versa, for example), it's not something you can just flatly assert without references, and the cause isn't really relevant. $\endgroup$ – Luaan Mar 14 '18 at 10:54
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It depends upon the plant species.

As explained in Algae thrive under Pure CO2 Nature 227, pages 744–745 (15 August 1970):

Cyanidium caldarium (an algae found in Yellowstone National Park) grows much better in pure carbon dioxide than in air.

However, other plants can suffer from acidification of cell fluids at high carbon dioxide concentrations.

In Elevated Atmospheric Partial Pressure of CO2 and Plant Growth Oecologia (1979) 44: 68, it was shown that increasing CO2 concentration to 640 ppm from then-normal 330 ppm caused increased growth for both cotton (a C3 plant) and maize (a C4 plant), but the increase was much greater (100%) for cotton than for maize (20%).

According to The optimal atmospheric CO2 concentration for the growth of winter wheat (Triticum aestivum) J Plant Physiol. 2015 Jul 20; 184:89-97, photosynthesis at 968 ppm CO2 is optimal. At 1200 ppm, photosynthesis is less than optimal, but still much higher than at current ambient CO2 levels of about 410ppm.

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Since you need water to live, seem you would be peachy dropped on the bottom of the ocean ;)

Organisms usually adapt to specific conditions. In particular, plants need oxygen to actually consume the glucose produced in the process of which oxygen is a byproduct. Is it possible to imagine an organism adapted to not having oxygen, by storing its own oxygen for reuse later? It sure is, and it might have been what originally happened in the original oxygen-free atmosphere. But as a matter of fact, in the current oxygen-rich atmosphere this ability would be completely useless, just as it would be for you to bring everywhere a backpack full of air to breathe. As a consequence, no mayor plant does it; and it would take centuries of evolutionary pressure to develop it.

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Some of the information contained in this post requires additional references. Please edit to add citations to reliable sources that support the assertions made here. Unsourced material may be disputed or deleted.

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    $\begingroup$ Most animals have anaerobic metabolic pathways. As you scale up, they tend to be progressively less capable of supplying all the metabolic needs of the organism, but they're there. Many plants evolved during a time with much higher CO2 concentrations in air, and some (like the vast family of algae) have essentially no upper limit on its concentration. Lots of plants need oxygen, but not all of them. As it is, your answer isn't very helpful - it would be better to include examples with references, rather than misleading metaphors and arguments from evolution. $\endgroup$ – Luaan Mar 14 '18 at 11:02

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