Does grass alone produce enough oxygen for life?

Question

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?

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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?

Answer 1

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.

Answer 2

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.

Answer 3

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.