All living things need oxygen in order to stay alive but plants, algae and a group of bacteria called cyanobacteria, can also make oxygen. These plants, algae and cyanobacteria make more oxygen than they use and the extra oxygen goes back into the air. So, the oxygen is sort-of recycled, rather than used up.
Plants like trees use their leaves to make oxygen. But what if there were no trees left on Earth – would there still be enough oxygen in the air for us to breathe?
Living things take oxygen from the air and use it in a process called ‘respiration‘ to make the energy they need to keep their bodies working. Respiration involves a series of chemical reactions that use glucose (a kind of sugar) and oxygen to make water, carbon dioxide and energy.
And plants, algae and cyanobacteria take energy from sunlight and use it to convert water and carbon dioxide back into glucose and oxygen through a process called ‘photosynthesis‘. They do this as a way of making their own food (glucose). But the oxygen that is made by photosynthesis is released back into the air.
So how much of the oxygen in the air comes from trees? Well, according to this article one tree can make about 100kg of oxygen each year. And an average person breathes 740kg of oxygen per year, which is 7-8 trees’ worth!
But most of the oxygen in the air doesn’t come from trees, it comes from a different kind of plant, called ‘phytoplankton‘. Phytoplankton are teeny-tiny plants that float around in the oceans and seas. They can photosynthesise and are the biggest producers of oxygen on Earth. In fact, scientists believe that phytoplankton make between 50% and 85% of the oxygen in the air.
So, even if there were no trees, there would still be oxygen in the air for us to breathe. (But the Earth would be a very different type of place for us to live if there were no trees left.)