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I have a friend who likes to promote hemp on Facebook, and one of the posts had this "factoid":

enter image description here

Looks highly doubtful to me but I can't find any real supporting/disputing info.

Is there anything that would support/refute this claim?

Best I can find that even remotely tries to use real data is https://comebackdaily.co/how-is-hemp-even-better-than-trees-for-producing-oxygen/

Jason S
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    Note that there is no shortage of oxygen, so although academically interesting, it has little to no practical implication if true. The claim probably seeks to imply that the additional oxygen production is advantageous, even if not stated. – gerrit Jul 21 '20 at 09:00
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    @gerrit but production of oxygen here is equivalent to removal of CO2, so the implication that oxygen production is advantageous has quite a lot of popular support. – HugoRune Jul 21 '20 at 10:10
  • @HugoRune I suppose so (although I don't know if long-term sequestration may differ still), but that's not the claim. – gerrit Jul 21 '20 at 10:21
  • Many member of the grass family use C4 photosynthesis. Despite its nickname, cannabis is not a grass. All cannabis species use C3 photosynthesis, the same as most trees. – David Hammen Jul 21 '20 at 11:58
  • There is a flaw in one argument in the story: "Hemp, on the other hand, is a fast-growing plant that can grow up to four meters in about 100 days. From there it’s harvested before it has time to grow old. The capacity of hemp to produce oxygen does not deteriorate, therefore." This overlooks the fact that after harvesting, the plants are producing *no oxygen at all*. The next crop will be producing little if any yet, either. – Weather Vane Jul 21 '20 at 13:33
  • To mod: Please do not change the focus of my question! I don't care about the linked article that much, what I care is about the facebook image claim. – Jason S Jul 21 '20 at 14:22
  • How much carbon is removed is less important than how long it is removed. When plants die, they rot and most of the carbon returns to the atmosphere. Hemp dies in less than a year, so all the carbon they contain is what they could extract during a single year. Most trees live several decades, so the amount of carbon they keep out of the atmosphere is many dozen times their yearly carbon uptake. – Tgr Jul 21 '20 at 14:55
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    (It should be obvious just from common sense that forests are much more effective carbon reservoirs than hemp fields - they contain much more biomass.) – Tgr Jul 21 '20 at 14:56
  • From a common sense point of view, suppose C4 photosynthesis is more efficient than is C3 photosynthesis, by a factor of 25. If this was true there would be no C3 plants anywhere. C4 photosynthesis is marginally more efficient than is C3 photosynthesis in some environments, which has enabled grasslands to predominate in areas where C3 photosynthesis doesn't perform as well. But in areas that don't have drought problems, C3 photosynthesis outdoes C4 photosynthesis. Hence we still have forests. – David Hammen Jul 21 '20 at 15:11
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    I made a number of improvements to your question. You have reversed them. I guess we need to go through each one one-by-one until we can knock this question into shape. – Oddthinking Jul 21 '20 at 16:35
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    You use the word "factoid" in quotes. A lot of people don't seem to know that that word implies it is false. Hence you are prejudging the answer without evidence. I suggest you remove that word. – Oddthinking Jul 21 '20 at 16:35
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    You link to ComebackDaily. Your link is a raw URL. I suggest you inline it to make it more readable, and more clear why you might click on it. – Oddthinking Jul 21 '20 at 16:36
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    You claim that Comeback Daily (remotely) tries to use real evidence. It isn't at all clear why you say that. You should quote the relevant sections that do what you claim. – Oddthinking Jul 21 '20 at 16:38
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    The claim appears in an image, which means it cannot be found by people searching for the words. It cannot be read by screen-readers. Please make it more available by including the actual text in the question. – Oddthinking Jul 21 '20 at 16:40
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    The claim that you quote is **25x**. You changed it to 1x in the title (but nowhere else). It isn't clear why you did this. The quote that I included from Comeback Daily supported this by providing an actual claim, but you omitted it. – Oddthinking Jul 21 '20 at 16:41
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    You specify "1 acre" and then "equivalent amount" which is overly complex. Why not just specify one acre twice, or drop the area and just say an equivalent amount? – Oddthinking Jul 21 '20 at 16:42
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    The question is clearly about biology, but you removed the tag. I suggest you return it. – Oddthinking Jul 21 '20 at 16:43
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    @Tgr: Forests take longer than 6/12 months to grow. If you multiply the biomass of hemp every 6 months by the time it takes to grow a forest, the result may be different. (This is why we don't accept common sense arguments here!) – Oddthinking Jul 21 '20 at 16:46
  • @Oddthinking I don't see how that's relevant, unless you want to claim that replacing a forest with a hemp farm, while being bad in the long run, is beneficial for carbon retention in the very short term (a few years). That might well be true but it's a different claim (and an uninteresting one, global warming being a long-term problem). – Tgr Jul 22 '20 at 22:40
  • @Tgr: The question isn't about how much biomass there is, or how long it retains it. – Oddthinking Jul 23 '20 at 00:01
  • @Oddthinking as HugoRune pointed out earlier the claim tries to mislead people into thinking that hemp is more effective at reducing CO2 in the atmosphere, even though it does not actually state that (since oxygen production on its own is entirely uninteresting from an environmentalist point of view). Exposing and evaluating hidden implications of a claim is a good practice IMO. – Tgr Jul 23 '20 at 00:53

1 Answers1

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Comback Daily argues that with two crops per year and with more efficient C4 photosynthesis ...

The claim is based on the false premise that Cannabis species use C4 photosynthesis.

Several of the grasses use C4 photosynthesis. Despite its nickname, Cannabis species are not members of the grass family. They are members of the Cannabaceae family, all of which use C3 photosynthesis (the same mechanism used by most trees). No member of the Cannabaceae family is listed in a very extensive list of C4 plants [Sage], and the photosynthesis in Cannabis sativa is consistent with C3 photosynthesis rather than C4 photosynthesis [Chandra].

References:

Chandra, Suman, et al. "Photosynthetic response of Cannabis sativa L., an important medicinal plant, to elevated levels of CO 2." Physiology and Molecular Biology of Plants 17.3 (2011): 291-295.

Sage, Rowan F. "A portrait of the C4 photosynthetic family on the 50th anniversary of its discovery: species number, evolutionary lineages, and Hall of Fame." Journal of Experimental Botany 68.2 (2017): e11-e28.

David Hammen
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  • I also found several reddit and quora posts that claimed that *cannabis* species use C3 photosynthesis, but those are not credible sites. The few sites that I did find that claim that *cannabis* species do indeed use C4 photosynthesis are all non-credible sites, and the claims therein are unreferenced. – David Hammen Jul 21 '20 at 13:22
  • This unfortunately wasn't my question; one of the mods edited my question to something with a different intent focused on the article I linked to. – Jason S Jul 21 '20 at 14:22
  • @JasonS -- This does answer your question; you just don't know it. Oddthinking picked out the key claim (which is false) that underlies the claim that an acre of hemp produces more oxygen than 25 acres of forest. That claim, buried in the combackdaily.co article, is that hemp uses C4 photosynthesis. I suggest you reverse your reversal. – David Hammen Jul 21 '20 at 14:59
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    This picks *one* feature of the argument apart, but not really the relativist aspect of real world data comparison in the claim. If hemp and tree are both C3, we still would like to know how 1 hectare of each produces how much O2? – LangLаngС Jul 21 '20 at 16:07