You’ve probably heard that trees produce oxygen, but have you ever wondered just how much oxygen one tree makes? The amount of oxygen produced by a tree depends on several factors, including its species, age, health, and surroundings. A tree produces a different amount of oxygen in summer compared to winter. So, there is no definitive value.
Trees release oxygen when they use energy from sunlight to make glucose from carbon dioxide and water. Like all plants, trees also use oxygen when they split glucose back down to release energy to power their metabolisms. Averaged over a 24-hour period, they produce more oxygen than they use up; otherwise there would be no net gain in growth.
It takes six molecules of CO2 to produce one molecule of glucose by photosynthesis, and six molecules of oxygen are released as a by-product. A glucose molecule contains six carbon atoms, so that’s a net gain of one molecule of oxygen for every atom of carbon added to the tree. A mature sycamore tree might be around 12m tall and weigh two tonnes, including the roots and leaves. If it grows by five per cent each year, it will produce around 100kg of wood, of which 38kg will be carbon. Allowing for the relative molecular weights of oxygen and carbon, this equates to 100kg of oxygen per tree per year.
A human breathes about 9.5 tonnes of air in a year, but oxygen only makes up about 23 per cent of that air, by mass, and we only extract a little over a third of the oxygen from each breath. That works out to a total of about 740kg of oxygen per year. Which is, very roughly, seven or eight trees’ worth.
Here are some typical calculations:
“A mature leafy tree produces as much oxygen in a season as 10 people inhale in a year.”
“A single mature tree can absorb carbon dioxide at a rate of 48 pounds per year and release enough oxygen back into the atmosphere to support two human beings.”
“One acre of trees annually consumes the amount of carbon dioxide equivalent to that produced by driving an average car for 26,000 miles. That same acre of trees also produces enough oxygen for 18 people to breathe for a year.”
“A 100-foot tree, 18 inches diameter at its base, produces 6,000 pounds of oxygen.”
“On average, one tree produces nearly 260 pounds of oxygen each year. Two mature trees can provide enough oxygen for a family of four.”
“Mean net annual oxygen production (after accounting for decomposition) per hectare of trees (100% tree canopy) offsets oxygen consumption of 19 people per year (8 people per acre of tree cover), but ranges from nine people per hectare of canopy cover (4 people/ac cover) in Minneapolis, Minnesota, to 28 people/ha cover (12 people/ac cover) in Calgary, Alberta.”
The bigger the tree the more total oxygen it produces and more is its own requirement for oxygen for survival. Net offset Oxygen, as opposed to total Oxygen produced, is the oxygen returned back to the atmosphere: this depends more on the rate of growth of a tree. If a tree weighs 1000kg, and if it adds 10% (100kg) to its weight then it would have released (offset) roughly 100kg of Oxygen. The tree produces more than that, but much of the oxygen it produces will be offset by its own requirement for oxygen. Yes a tree breathes in Oxygen too. But the overall offset can be calculated by the amount of change in mass of a plant or tree. To compare: an average human needs about 750kg of Oxygen per year. All of humanity consume about 7 billion tonnes then! Luckily Oxygen is among the most abundant of elements on earth.