As we were all taught in middle school science, trees produce oxygen through photosynthesis, making them one of the main oxygen producers on land. But what many of us misunderstand is the percentage of trees that contribute to oxygen production when it comes to the Earth as a whole.
As you know, trees shed their leaves in autumn. This raises the question: How can a tree without leaves produce oxygen? Or does it produce? If not, how can we continue to breathe? The answer to these questions is not that simple, because there are many factors that affect the global oxygen cycle, and trees are only a small part of it.
First, let’s learn the shocking truth: Trees are not our planet’s chief oxygen producers.
One of the first questions we should ask is how much oxygen trees actually produce, and how does that compare to the total amount of oxygen in the atmosphere. By some estimates, trees undertake about 30% of terrestrial photosynthesis, that is, they produce about 30% of the oxygen released by land plants. The rest comes from other plant species such as grasses, crops, shrubs and mosses.
More importantly, terrestrial photosynthesis is responsible for only half of global photosynthesis. The other half comes from marine photosynthesis by algae, phytoplankton and cyanobacteria in the oceans. These microscopic organisms are often referred to as the “lungs of the planet” because they produce about 50% of the oxygen we breathe.
So when we add up all the oxygen production sources on Earth, we can roughly say that trees contribute about 15% of the total oxygen supply. In other words, the fact that trees shed their leaves and stop producing oxygen for a few months does not cause a noticeable change in oxygen levels. But that doesn’t mean trees are unimportant, of course.
Another question we need to ask is how much oxygen the trees consume and how this affects the net oxygen balance.
Like all living organisms, trees need oxygen for respiration; which means they use oxygen to break down sugars and release energy for growth and maintenance. Trees respire all year long, even when dormant or leafless in winter.
The amount of oxygen consumed by trees; it actually depends on various factors such as their size, age, species and environmental conditions. But as a general rule, trees consume about half of the oxygen they produce. This shows us that trees have a net positive effect on oxygen balances, but not as much as we thought.
For example, a mature leafy tree can produce enough oxygen for 10 people in a season for a year, but it also has the capacity to consume enough oxygen for 5 people in a season for a year. So the net effect is that a tree can support 5 people for a year with its oxygen production. In addition, trees spend their leafless period by reducing their own respiration rate and consuming less oxygen. This has a positive effect on the net oxygen balance.
What about other sources of oxygen production and consumption? Is there a time when they stop too?
It turns out that the seasons have a similar effect on them. For example, grasses and crops also stop growing and photosynthesizing in winter, but they also reduce respiration, like trees. Similarly, algae and phytoplankton in the oceans have seasonal cycles of growth and decline, depending on light availability and food source.
So, overall, the seasons affect both oxygen production and consumption in a balanced way, meaning that oxygen levels don’t change much throughout the year. In fact, atmospheric oxygen levels have been relatively stable since 1991, fluctuating between 20.94% and 20.96%, according to measurements by the Scripps Institution of Oceanography in California.
(Don’t be surprised either, nitrogen/nitrogen makes up about 78% of Earth’s atmosphere.)
What other factors affect the Earth’s oxygen level?
Of course, the seasons aren’t the only factor affecting oxygen levels. There are many natural and man-made factors that affect the global oxygen cycle, such as volcanic eruptions, forest fires, burning of fossil fuels, land use change and climate change. Some of these factors have a positive effect on oxygen levels, while many have a negative effect.
volcanic eruptions
Volcanic eruptions can release large amounts of carbon dioxide and sulfur dioxide into the atmosphere, which can then react with oxygen, lowering oxygen levels. Not only that, but the volcanic ash and dust released in the eruption also reduces photosynthesis because they block sunlight.
Forest fires
Similarly, wildfires can consume large amounts of oxygen by burning biomass, but they can also encourage new growth and photosynthesis by removing dead wood and releasing nutrients. Combustion of fossil fuels can also consume large amounts of oxygen by burning coal, oil and gas, but can also increase carbon dioxide levels enough to increase plant growth and photosynthesis.
Land use change
Land use change can also affect oxygen levels by changing the amount and type of vegetation on the Earth’s surface. For example, felling trees can reduce oxygen production, but it can also reduce oxygen consumption by removing decomposing organic matter. In the opposite scenario, i.e. planting trees with afforestation, oxygen production can be increased, but adding decomposing organic matter can also increase oxygen consumption.
climate change
Finally, climate change can also affect oxygen levels by altering temperature, precipitation, and wind patterns that affect photosynthesis and respiration. High temperatures can increase respiration rates and lower oxygen levels, but they can also increase evaporation rates and water vapor levels, which can trap heat and increase photosynthesis rates.
As you can see, there are many factors that affect oxygen levels in complex and sometimes counterintuitive ways. However, the overall impact of these factors is really small compared to the total amount of oxygen in the atmosphere. In fact, according to a study by University of Michigan climate scientist Chris Poulsen, oxygen levels have changed between 10% and 35% in the last 500 million years, and very little in the last 10 million years.
In conclusion, we can say that the fact that trees shed their leaves and decrease oxygen production does not affect our oxygen balance.
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