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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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Unfortunately, grass doesn't cast a very useful shadow. –  Jonas Oct 28 '11 at 4:47
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
your claim also does not take into consideration that trees may/do not produce all or even the majority of oxygen not produced by grass today. –  jwenting Oct 28 '11 at 6:01
Well, there's 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. –  ESultanik Oct 28 '11 at 14:56
@Hendy, yep, algae makes more sense. Not only do they produce the majority of O2 currently, they are responsible for putting O2 in the atmosphere in the first place, a pre-requisite to the colonization of land by plants (and indeed to the existence of multi-cellular organisms at all). See scotese.com/precamb_chart.htm –  David Nov 1 '11 at 15:13

3 Answers 3

up vote 9 down vote accepted

Plants are oxygen neutral

The question wording is a bit misleading, as – surprise – plants do not produce oxygen. 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
not having read the articles, how do they propose plants producing oxygen out of thin air (literally)? Any O2 released by them would be absorbed again when the plant dies and rots/burns. The only way I can think of a net O2 production is if the forest is harvested and the wood conserved for construction purposes, thus carbon being extracted from the ecosystem and not counted towards its oxygen balance when and if it is later destroyed and its carbon released (thus the system leaks, a general problem with most theories proposed by environmentalists and non-scientists in general). –  jwenting Nov 2 '11 at 7:55
+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
@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 Dec 10 '11 at 6:10
@David, I know this is creeptastic, but I noted that you referred to Wally as Wally so I figured you were one of my kind. Your website tells me that not only are you a fellow climate scientist, but that I am not the only ESSer who enjoys this stackexchange. :) Good work here. –  Tacroy Jun 16 '12 at 7:35

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 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
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

Are we saying that there are only humans that need oxygen to survive? What about all the other mammals, amphibians and birds?

Even if grass was planted in all of the lost forest area, it would not produce enough oxygen for survival of the planet (Ref: US Dept of Energy Q&A site).

Oxygen is only a small part of the air we breathe. Trees absorb other gaseous pollutants that, if they were not here, would skew the air concentration and change the air gasses ratio. We are adapted to survive on 19% to 21% oxygen to survive; anything above or below would not be good for human survival.

It is long accepted that the rain forests are essentially the lungs of the world and if they are cutdown the critical mass to buffer and protect the atmospheric conditions of life on earth would be lost. (Ref: Rainforest Information Centre).

The problem is we have lost so much rain forest that the toxic gas by-products of industries and land degradation are not able to be absorbed through the flora. Every organism is adapted to living in a closed neutral cycle space which humanity has destroyed through its greed for industrial advancement over the last 200 years and grass only is not the answer to life survival on earth.

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Welcome to Skeptics! This answer needs some better references. The ones cited are not peer-reviewed, and they don't cover all of the claims. (e.g. that we can't thrive at 18% or 22% oxygen. That every organism is asapted to a "closed neutral cycle space", whatever that means. The jump from the Q&A site answer to your conclusion needs further justification. Even blaming pollution on "greed for industrial advancement" should probably have a reference, so it doesn't read as a rant. –  Oddthinking Jun 16 '12 at 6:28

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