Trees, often called the lungs of the planet, interact with the atmosphere in ways far more complex than the simple exchange of carbon dioxide and oxygen taught in early science classes. This relationship involves a continuous negotiation of gases, liquids, and chemical compounds that affects climate, air quality, and the global water cycle. Understanding what trees truly breathe in and out reveals their profound role not just as passive absorbers, but as active regulators of the Earth’s atmosphere.
The Fundamental Exchange of Gases
The most recognized atmospheric interaction is the dual process of photosynthesis and respiration, which govern the intake of carbon dioxide (CO2) and the release of oxygen (O2). During daylight hours, photosynthesis is the dominant process, where the tree takes in carbon dioxide and water to create carbohydrates, releasing oxygen as a byproduct. This daytime carbon capture is important for regulating the global carbon cycle.
Trees also respire continuously, both day and night, just like every other living organism. Respiration involves taking in oxygen to break down stored sugars for energy, which releases carbon dioxide and water vapor. While the net effect during the day is a significant release of oxygen, the ebb and flow of CO2 and O2 shows that trees are engaged in continuous gas exchange with the air around them.
The Role of Water Vapor
The largest gas a tree releases into the atmosphere, by volume, is water vapor, through a process called transpiration. This process begins with water drawn up from the soil through the roots and transported through the xylem vessels, carrying dissolved mineral nutrients. The water then evaporates out of microscopic pores on the leaves called stomata, which are regulated by guard cells.
This loss of water vapor creates a negative pressure, known as transpirational pull, which is the primary force driving water upward against gravity. Transpiration is a consequence of photosynthesis, as the stomata must open to let in CO2, which simultaneously allows water vapor to escape. This water loss is substantial, accounting for approximately three-quarters of the water vaporized from the global land surface.
The evaporation of water from the leaf surface provides evaporative cooling for the plant itself, similar to sweating in animals. This cooling effect is also transferred to the surrounding environment. Urban trees can significantly mitigate the “urban heat island” effect by lowering local air temperatures. The plumes of water vapor released from forests contribute significantly to atmospheric moisture, influencing cloud formation and regional rainfall patterns.
Atmospheric Emissions: Volatile Organic Compounds
Beyond the simple gases of water, oxygen, and carbon dioxide, trees actively emit a wide array of complex chemical compounds known as Biogenic Volatile Organic Compounds (BVOCs). These compounds include molecules like Isoprene and various Terpenes, such as pinene and limonene. Global BVOC emissions from vegetation often exceed the total amount of volatile organic compounds released by human activities.
These emissions serve several purposes for the tree, acting as a chemical defense mechanism against herbivores and pathogens. They also help the plant cope with heat and oxidative stress. Isoprene is emitted in large quantities by many broadleaf species, while Terpenes, which give coniferous forests their characteristic scent, are released by species like pines and spruces. Once released, these chemicals become highly reactive in the atmosphere, playing a significant role in atmospheric chemistry.
BVOCs react with other atmospheric components, particularly nitrogen oxides, to contribute to the formation of ground-level ozone, a harmful air pollutant. They also oxidize to form Secondary Organic Aerosols (SOAs), which are tiny airborne particles that can scatter sunlight and act as nuclei for cloud droplet formation. These chemical signals influence regional air quality, visibility, and the microclimate.
Trees as Air Filters: Absorbing Pollutants
Trees serve as effective natural filters, actively absorbing or intercepting harmful trace atmospheric pollutants. This cleansing occurs through a dual mechanism involving both the leaves and the surfaces of the plant. Gaseous pollutants, such as sulfur dioxide (SO2), nitrogen oxides (NOx), ozone (O3), and carbon monoxide (CO), are taken into the leaf through the stomata.
The tree essentially mistakes these gaseous pollutants for carbon dioxide, which it is trying to absorb for photosynthesis. Once inside the leaf’s interior, these toxic gases dissolve in the water and can be converted into less harmful compounds or sequestered within the plant’s tissues. This uptake process is a continuous, direct removal of harmful substances from the air.
The second mechanism involves the physical removal of particulate matter, which are tiny solid and liquid particles floating in the air. The rough surfaces of the bark, waxy cuticles, and fine hairs on leaves act as passive traps, intercepting these particulates. While much of this matter may be washed to the ground by rain, this temporary capture is a significant factor in improving local air quality, particularly in urban areas.