The idea that the world’s forests are the sole source of the air we breathe is a widely held belief that simplifies the complex global oxygen cycle. Trees and plants produce a tremendous volume of oxygen every second through photosynthesis, but they are not the primary long-term contributors to the Earth’s atmospheric oxygen reservoir. Understanding the true source of our breathable air requires distinguishing between the gross oxygen produced by an ecosystem and the net amount it actually adds to the atmosphere. The scientific reality points toward a more balanced picture of how life maintains the stable oxygen concentration necessary to support animal life.
The Global Oxygen Contribution of Terrestrial Plants
Terrestrial plants, including all forests, trees, shrubs, and grasses, are significant players in the global photosynthetic process. Photosynthesis occurring on land accounts for roughly half of the total gross oxygen production worldwide, with the other half occurring in aquatic environments. Forests and rainforests are estimated to contribute around 28% of the world’s gross oxygen production before accounting for the oxygen they immediately consume.
Tropical forests, especially the Amazon, are particularly productive ecosystems due to their year-round growing seasons and high density of plant life. The Amazon rainforest alone is responsible for approximately 16% of all photosynthesis occurring on land. A large fraction of the oxygen produced by land plants comes from these concentrated, high-biomass areas. This production represents the total amount of oxygen released, not the amount that newly enters and stays in the atmosphere for global use.
The Primary Global Oxygen Source
The vast majority of the Earth’s long-term, breathable oxygen supply originates not on land, but in the oceans. Tiny, single-celled organisms collectively known as phytoplankton are the primary source of global oxygen generation. These microscopic marine organisms perform photosynthesis just like trees, converting sunlight and carbon dioxide into sugars and releasing oxygen as a byproduct.
Phytoplankton, which include cyanobacteria, diatoms, and coccolithophores, are estimated to produce between 50% and 80% of the oxygen in the atmosphere. They are incredibly efficient because they lack the woody structures and non-photosynthetic tissues that characterize large land plants. Their sheer volume and rapid turnover across the vast surface of the world’s oceans make them the dominant biological source of atmospheric oxygen.
Understanding Net Oxygen Balance
The difference between gross and net oxygen production is central to understanding the role of forests in the global cycle. Gross oxygen production is the total amount of oxygen released during photosynthesis, often measured as Gross Primary Productivity (GPP). However, plants must also consume oxygen to fuel their growth and metabolic processes through a process called respiration.
The net oxygen balance is determined by subtracting the oxygen consumed by the ecosystem—through plant respiration and microbial decomposition—from the gross amount produced. In a mature, stable forest, the rate of oxygen consumption is almost equal to the rate of oxygen production. When a tree or plant dies, microbes and fungi consume the decaying organic matter, a process that requires oxygen and effectively recycles it back into carbon dioxide.
This near-perfect equilibrium means that a mature forest, even one as large as the Amazon, is in a state of balance, recycling the oxygen it produces rather than adding a significant net quantity of new oxygen to the atmosphere. The small amount of oxygen that does accumulate over geological timescales comes from organic matter that is buried and removed from the decomposition cycle. Consequently, the atmospheric oxygen level is a stable reservoir that is not reliant on the day-to-day balance of contemporary forest ecosystems.
The Essential Value of Trees Beyond Oxygen
While forests may not be a major source of new atmospheric oxygen, their functions are irreplaceable for maintaining a habitable planet. One of the most important services trees provide is carbon sequestration, which involves absorbing and storing atmospheric carbon dioxide in their biomass. Forests, especially tropical ones, act as massive carbon sinks, mitigating the increase of greenhouse gases that contribute to a warming climate.
Climate and Ecological Regulation
Trees also play a fundamental part in regulating the water cycle through evapotranspiration. They release large amounts of water vapor into the atmosphere, which contributes to cloud formation and rainfall patterns. Beyond climate and water regulation, forests house an estimated 80% of all terrestrial biodiversity, providing complex habitats for countless species.