Trees are widely recognized as oxygen producers, but their contribution to the atmosphere is not a constant flow. The production of oxygen depends entirely on the tree’s biological activity and life stage. This makes the overall atmospheric balance a dynamic and nuanced system.
How Trees Create Oxygen
Trees produce oxygen through photosynthesis, a light-driven process occurring primarily in the leaves’ green chloroplasts. This reaction uses energy from sunlight to convert carbon dioxide and water into energy-rich sugar molecules for the tree. Carbon dioxide enters the leaf through tiny pores called stomata, and oxygen is released as a byproduct of this conversion via the same stomata. Because this process is entirely dependent on sunlight, the rate of oxygen creation is highest during bright days and ceases completely when light is absent.
The Tree’s Need for Oxygen
For a tree to grow and maintain its tissues, it must constantly break down the sugars created during the day, a process that requires oxygen. This is accomplished through cellular respiration, which is the reverse of photosynthesis, converting stored glucose into chemical energy that powers all life functions. Unlike photosynthesis, cellular respiration occurs in every living cell of the tree—in the roots, trunk, and leaves—and continues around the clock. During the daytime, photosynthesis significantly exceeds respiration, resulting in a net release of oxygen; however, when the sun sets, the tree becomes a net consumer of oxygen, releasing carbon dioxide as a waste product.
When Oxygen Production Stops
A tree’s annual contribution is measured by its net oxygen output: the total oxygen produced minus the total oxygen consumed. A living, growing tree stores carbon in its wood, resulting in a positive net oxygen contribution over its lifetime. However, once a tree dies or enters a state of significant decline, its role as an oxygen producer ends, and it shifts to becoming a net oxygen consumer. This shift is due to the process of decomposition, which is driven by fungi, bacteria, and insects that break down the stored wood. These organisms rely on aerobic respiration to consume the carbon-rich material, consuming oxygen and releasing carbon dioxide. For a mature forest where new growth is balanced by decomposition, the net oxygen output can approach zero.
The Planet’s Primary Oxygen Source
While trees perform the function of long-term carbon storage, terrestrial plants are not the primary source of the Earth’s atmospheric oxygen. The vast majority of the oxygen we breathe, estimated to be between 50 and 85 percent, is produced by marine organisms in the ocean. This production is largely carried out by microscopic, single-celled organisms known as phytoplankton. These tiny marine algae drift near the water’s surface and perform photosynthesis on a global scale. One species in particular, Prochlorococcus, is the smallest known photosynthetic organism, yet it is responsible for generating a substantial percentage of the oxygen in the biosphere. The oxygen they produce is released into the ocean water and then diffuses into the atmosphere.