How Many Trees Do You Need to Live?

...to make the oxygen you breathe and store the carbon you produce?

Jul 03, 2024

Trees make oxygen, and humans need oxygen to breathe. Have you ever wondered how many trees it takes to produce the oxygen you need to breathe? We're about to answer that in detail and also examine how many trees it takes to absorb the carbon dioxide produced during the production of everything you need in your life.

Humans breathe in air that contains 21% oxygen. The oxygen is then combined with sugars stored in the body from food and a chemical reaction called respiration (or metabolism) occurs. This is how the human body gets energy and survives. Carbon dioxide and water vapor are then exhaled as waste products.

Coincidentally, the reverse reaction of respiration is photosynthesis, where plants take in carbon dioxide and water to produce sugars and oxygen.

Not all of the carbon dioxide a tree takes in stays in the tree.

Gross primary productivity, or GPP, is the rate at which solar energy is captured in sugar molecules during photosynthesis. Producers such as plants use some of this energy for metabolism/photorespiration and some for growth (building tissues and accumulating biomass).

Net primary productivity, or NPP, is gross primary productivity minus the rate of energy loss to metabolism and maintenance. In other words, it's the rate at which energy is stored as biomass by plants or other primary producers and made available to the consumers in the ecosystem.

It's this rate of biomass accumulation that we need to know since that can tell us a tree's rate of net oxygen production. NPP is generally about 50-75% of GPP, since a quarter to half of the captured solar energy is used for metabolism and maintenance.

Biomass Accumulation

A study by Michael Köhl examined the biomass accumulation rates of individual trees of 3 species in the tropical country of Suriname.

To summarize these findings, old trees accumulate much more carbon and produce more oxygen than young ones. If we take the average carbon accumulation rates of these three tree types over their lifetimes and average them together, we get 12.5 kg of carbon per tree per year. This study only looked at above-ground carbon, and about 80% of a tree is above ground, which means the total carbon accumulation is 15.6 kg per tree per year. Since carbon represents about 50% of a tree's biomass (the rest is hydrogen and oxygen), the total biomass accumulation of these trees is 31.2 kg per tree per year.

How Many Trees Do You Need to Breathe?

Math time!

The average human needs about 550 liters (145 gallons) of oxygen (O2) per day. That works out to 287 kg (632 lbs) of oxygen per year.

Now, we need to find out how much oxygen a tree produces per year. We can account for the net primary productivity of trees after the photorespiration and maintenance processes are considered by looking at the products of photosynthesis (glucose and O2) rather than the reactants (CO2 and H2O). This way we can relate the amount of biomass and carbon accumulation in trees to the amount of oxygen they produce.

Since carbon represents about 50% of a tree's biomass in the form of cellulose, lignin, and hemicellulose, but the product of photosynthesis is glucose, which is only 40% carbon by mass (72 amu* carbon divided by 180 amu glucose), we need to determine how much glucose the tree needs to produce in order to accumulate 15.6 kg of carbon. So, we take 15.6 kg divided by 0.4 to find that the tree produces 39.0 kg of glucose per year.

Now, we can calculate oxygen production from the amount of glucose production. Trees produce 6 molecules of O2 for every molecule of C6H12O6 (glucose). The molecular mass of glucose is 180 amu and the molecular mass of O2 is 32 amu, so to determine the mass of oxygen produced we take 39.0 kg of glucose divided by 180 amu and multiply it by 192 amu (the mass of 6 O2 molecules.) We find the tree produces 41.6 kg of O2 per year.

Since a person needs 287 kg of oxygen per year and each tree produces 41.6 kg of oxygen per year, a single person needs the oxygen from 6.9 trees to breathe.

It's important to note that this can vary widely depending on a variety of factors, such as tree species, age, and growing conditions.

Interestingly, from this math we can derive some rules of thumb.

For every kilogram of biomass of trees and plants, 1.33 kg of oxygen gas is produced, and 0.5 kg of carbon is stored.
Each person needs 216 kg of biomass production from plants per year in order to breathe. That's 1.3 lbs of plant biomass per day.

If a tree isn't growing in size, it isn't producing oxygen overall.

How Many Trees Does It Take to Absorb Carbon Emissions from One Person?

The worldwide average carbon emission per person per year (from fossil fuels and industry alone) is 4,700 kg.

That means 9,400 kg of biomass needs to be produced to absorb one person's carbon emissions each year. That's the equivalent of 301 mature trees.

Since annual carbon emissions per person in the US are about 14,900 kg (32,780 lbs), 955 mature trees are needed to absorb the CO2 from a single American.

Other Oxygen Sources

Trees aren't the only organisms that absorb carbon and produce oxygen. In fact, trees and plants on land only produce about 20-50% of the oxygen on Earth. 50-80% of the oxygen is produced by marine phytoplankton, microscopic plants and organisms that live in the ocean and other bodies of water. These include diatoms, dinoflagellates, cyanobacteria, green algae, and coccolithophores.

How many trees would you have expected to be required to produce enough oxygen for one person?
How can this knowledge be used to move towards a sustainable civilization?