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Over a work happy hour tonight, two employees were talking about their different lifestyles -- one likes riding four wheelers, which the other considers to be recreational pollution. Somehow the fact that the four wheel rider plants trees got brought up and another employee chimed in with this (paraphrased, but pretty close):

You know what? It's a common myth that we need trees. If all the trees were chopped down in the whole world, grass alone would produce three times the oxygen that humans need to breathe to survive.

I've never heard anything like this and wanted to ask here. One can at least find references that trees are important for the production of oxygen. I'm wondering, now, how important they are.

If all of the trees were chopped down, would there be sufficient CO2 -> oxygen conversion taking place with grass alone?


Sorry for the lack of any other sources; I tried to find ones, but just couldn't. Hopefully that isn't sufficient justification for not asking the question. His use of "three times" seemed to indicate that he'd heard it elsewhere, otherwise I don't understand the reason for using a specific quantity.

In my searching, I did find a different article that suggests he might have been confused and instead of grass, it's algae that provides the most oxygen?

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    Your quoted claim only says there would be enough oxygen for humans, but the title, and IMO, the more interesting question is, would there be enough oxygen to support all oxygen-requiring life on earth. – Flimzy Oct 28 '11 at 5:32
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Plants are oxygen neutral

The question wording is a bit misleading, as – surprise – plants do not produce oxygen, when you consider their whole life cycle, including their decomposition. They produce oxygen only as long as they are growing, binding carbon in its mass. In long term all plants are oxygen neutral, as all oxygen which is created by them is again consumed when they dissolve, burn or are eaten, as carbon stored in them reacts with oxygen back to CO2. The more correct view how to describe this is not that plants produce oxygen, but that they store carbon.

This is described in detail for tropic rainforests in Et tu, O2?:

the Earth's forests do not play a dominant role in maintaining O2 reserves, because they consume just as much of this gas as they produce. In the tropics, ants, termites, bacteria, and fungi eat nearly the entire photosynthetic O2 product. Only a tiny fraction of the organic matter they produce accumulates in swamps and soils or is carried down the rivers for burial on the sea floor.

As forest has a lot larger mass than a meadow on the same area, it is quite likely the trees bind a lot more carbon than a grass, but how would this impact Earth O2 levels is something which would require further computation - the impact is one time only, however, there is no "out of balance, producing less O2 then consumed" caused by this.

Plants we eat produce O2 in the same numbers as C

It is obvious (as Christian tries to show in his answer) that any O2 you will consume by breathing can produce CO2 only when it reacts with C. The C comes (directly or indirectly) from the same photosynthesis as O2, and in the same amount, therefore the balance is maintained by definition, and any O2 we breath was produced when the stuff we eat was grown.

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    -1 for now: The Wikipedia article you quote incorrectly summarizes the results of the two articles it cites. Those articles in fact support the fact that there is net O2 production from the tropical rainforests (albeit small). While the tropical rainforests may not be "Earth's lungs", they are by no means "oxygen neutral". Also, human/animal respiration isn't the only source of CO2 in the atmosphere; there's combustion of fossil fuels, which throws off the balance. – ESultanik Nov 1 '11 at 21:52
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    +1 "The more correct view how to describe this is not that plants produce oxygen, but that they store carbon." I never thought of it that way. – Alain Dec 8 '11 at 20:56
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    @suma -1 for two reasons: a) as Wally says in the reference "a tiny fraction of the organic matter they produce accumulates" and that tiny fraction is enough to be responsible for maintaining O2 in the atmosphere over geological time. b) plants release O2 and fix C; non-plants (humans, fungi, etc) consume O2 and release CO2. The O2 is consumed in the process of supporting life. c) you don't seem to answer the question. – David LeBauer Dec 10 '11 at 6:10
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    According to your logic, burning fossil fuels doesn't produce CO_2, it just releases previously stored CO_2. – Acccumulation Jan 20 at 22:28
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    @Acccumulation Right. What originally produced our oxygen was the burial of organic matter in marine sediments and swamps. – Keith McClary Jan 21 at 2:07
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Here is a great article from the University of Michigan that covers some of the topics relevant to your question.

The article deals with the Net Primary Production (NPP) of CO2 in the world related to human consumption. NPP is the amount of CO2 that is "fixed" (i.e., processed) by plants through photosynthesis minus the amount of CO2 that is produced by organisms through respiration. Therefore, to simplify things, the higher the NPP the lower the amount of CO2 in the atmosphere.

The article cites the following table from Atjay et al. 1979 and De Vooys 1979 which breaks down NPP by the type of ecosystem:

(The numbers in the table are amount of surface area on Earth in km2 x 106 and the second number is the NPP in petagrams.)

  • Forest: 31 / 48.7
  • Woodland, grassland, and savanna: 37 / 52.1
  • Deserts: 30 / 3.1
  • Arctic-alpine: 25 / 2.1
  • Cultivated land: 16 / 15.0
  • Human area: 2 / 0.4
  • Other terrestrial (chapparral, bogs, swamps, marshes): 6 / 10.7
  • Lakes and streams: 2 / 0.8
  • Marine: 361 / 91.6

The article uses these figures to calculate the amount of NPP that is currently co-opted for human consumption. For example, they assume that all of the NPP associated with cultivated land goes toward human consumption. They conclude that 30.7% of the terrestrial NPP and only 2.2% of the aquatic NPP is co-opted by humans. These numbers are of course based off of ~30 year old studies, but I think it's still safe to conclude that there's still room for more human-based CO2 production before NPP goes to zero.

According to this 2002 study by Randerson, et al., terrestrial heterotrophs (i.e., organisms that need to breathe oxygen, like humans) produce 82–95% of the CO2 represented by the NPP. Let's be conservative and assume the higher amount: 95% NPP. That means that as long as the forests account for fewer than 5% of the total NPP then we should be fine. The forests produce 48.7 Pg, however, which is a little under 22% of the total NPP.

But wait! 13.6 Pg of NPP associated with forests is co-opted for human consumption (e.g., getting wood for building houses, &c.). If we were to get rid of all of the forests we'd also get rid of that percentage that has already been co-opted. Therefore, if we were to get rid of all of the forests there would be a net loss of 48.7 Pg - 13.6 Pg = 35.1 Pg, which is about 16% of total NPP. That's just low enough to meet the lower bound of 82% CO2 production.

Therefore, there is a small chance that there will be enough NPP after getting rid of all of the trees for human consumption, but it is likely not the case. Furthermore, if we were to get rid of all NPP producers other than grass there would certainly not be enough NPP for human survival.

It's also important to note that, by far, the most productive producers of NPP are the open ocean, tropical rainforest, and temperate forest (see Figure 5 of the University of Michigan reference), so by deforesting as opposed to de-grassing we would be greatly reducing the efficiency of the global ecosystem. Also, as I mentioned in a comment above, there is also the matter of carbon storage. Trees store a good amount of the carbon from the CO2 they process in their trunks where it stays for a long time. Grass, on the other hand, releases its carbon back into the system shortly after it dies and rots away. Therefore, even if grass does produce enough oxygen for life, it probably wouldn't have the same greenhouse gas reducing capabilities as trees.

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  • what does "most efficient producers of npp" mean, and do you have a reference for this claim? Also,presumably if the trees were removed, they could be replaced by grass. So i dont think that your assumptions or conclusions are valid. – David LeBauer Nov 1 '11 at 4:40
  • @David: the "most efficient producers of NPP" claim is from the University of Michigan reference (see Figure 5). "Efficient" is probably the wrong word there, though, it should really be "productive". I'll edit that. With respect to replacing the trees by grass, I interpreted the question as asking what would happen if all of the trees were cut down and nothing else changed. – ESultanik Nov 1 '11 at 14:20
  • you forget to take into account that were all forests to disappear, that land will be covered (at least in part) with other vegetation, and from your own data it appears that grass is as effective at generating O2 as are trees (per acre), thus only the area lost to vegetation completely needs to be taken into consideration. – jwenting Nov 1 '11 at 15:39
  • @jwenting: As I mentioned in my previous comment, I interpreted the question as asking what would happen if all of the trees disappeared and everything else stayed exactly the same. Anyway, vegetation that would replace the trees wouldn't appear instantaneously; if eliminating the trees would produce an oxygen deficit, by the time the vegetation grew everyone might already be dead. – ESultanik Nov 1 '11 at 21:40
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    I was torn between this answer and the other. I thought the other answer highlighted a slightly different approach, which essentially dissolved the question. It's not really about oxygen production, but carbon storage. That's what I liked about it. I realize you have this as well, but the other directly takes this approach. I very much appreciate the answer. – Hendy Dec 3 '11 at 1:56
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No.

The pure reason why some areas (with a particular set of meteorological and soil conditions) nurture trees (and establish themselves as stable forests) is because they are in general more efficient photosynthesizers (on a spatial basis) in that area. That is just pure survival of the fittest. To validate my claims, isolating "plants" from the whole ecosystem only, what you should be looking for is net Primary Production (NPP = photosynthesis - autotrophic respiration). A more positive NPP means more O2 is generated.

In this paper, in tropical region (trying to equalize latitude to make the comparison fairer), it was estimated that NPP in Tropical evergreen forest, Tropical deciduous forest & Tropical savannah are 964, 759 & 661 gC/m^2/yr respectively. Same trend is found in temperate regions.

In another paper, at a certain precipitation level, tree‐dominated systems had significantly higher NPP (∼100–150 gC/m^2/yr) than non‐tree‐dominated systems.

I am sorry to say the two answers with highest votes are both incorrect (one talked about NPP but the NPP definition and interpretation are completely wrong). To answer such a question as to actively satisfying O2 need/demand, we need to know the rate of O2 production/supply (which is NPP), talking about the O2/CO2 balance over a geological time-scale is irrelevant. If you were to chop down all trees (all other things remain the same), as shown by the two well-cited papers above, it is highly unlikely grass will have a higher NPP than trees in the same environment.

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  • Welcome to Skeptics! "That is just pure survival of the fittest" isn't a very strong argument. It is an appeal to common sense. This answer would be improved by links to the definition of NPP, direct quotes from the paper, some reason to believe that - even if someone accepts grass has a lower NPP, that it is insufficient for life, and a reason to not accept the other arguments (e.g. that plants are Oxygen neutral over their lifecycle) – Oddthinking Jan 25 at 23:23
  • Comments are not for extended discussion; this conversation has been moved to chat. – Oddthinking Jan 26 at 4:51

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