Do Trees Release Carbon Dioxide or Oxygen?

Trees release both carbon dioxide and oxygen, and the balance between these two gases shifts constantly depending on the time of day, the season, and which part of the tree you’re looking at. During daylight hours, leaves pull in carbon dioxide and push out oxygen through photosynthesis, and this process dominates the balance so heavily that terrestrial photosynthesis removes about six times more carbon dioxide each year than all fossil fuel emissions combined.¹ But every living cell in a tree also breathes, consuming oxygen and releasing carbon dioxide around the clock. The interplay between these opposing processes is what makes the question more interesting than a simple textbook answer suggests.

Two Opposite Processes Running Simultaneously

A tree performs photosynthesis in its green tissues, primarily the leaves. Sunlight powers a reaction that splits water molecules, releasing oxygen as a byproduct while incorporating carbon dioxide into sugars the tree uses for growth and energy. The molecular machinery that splits water is remarkably intricate, involving a metal cluster in a protein complex called photosystem II that cycles through several states as it assembles the oxygen atoms that eventually pair off and escape as O₂ gas.² That oxygen is what you breathe. It’s a waste product of the tree’s effort to capture carbon.

At the same time, every cell in the tree runs cellular respiration: it burns sugars for energy, consuming oxygen and releasing carbon dioxide. This is the same basic chemistry your own body uses to stay alive. The trunk, the branches, the roots, even the leaves themselves all respire continuously. During the day, photosynthesis in the leaves outpaces this respiration by a wide margin, so the net output is oxygen. But respiration never stops.

What Happens at Night

Once the sun goes down, photosynthesis shuts off entirely. The tree’s leaves, along with every other tissue, continue respiring. That means a tree in the dark is a net emitter of carbon dioxide, just like you are when you sleep. Measurements in temperate forests show that nighttime leaf respiration rates run roughly twice as high as daytime leaf respiration rates, with day respiration averaging about 53 to 56 percent lower than night respiration across several tree species.³ This isn’t because the cells ramp down deliberately in sunlight; part of the reduction comes from how the photosynthetic machinery interacts with the respiration pathway during the day, partially suppressing it.

The nighttime carbon dioxide release is real, and it’s the reason you sometimes hear people say you shouldn’t keep houseplants in your bedroom. In practice, the amount of carbon dioxide a single plant releases overnight is negligible compared to what you exhale yourself. But on the scale of an entire forest, the nightly pulse of carbon dioxide is measurable. It’s one reason ecosystem-level carbon budgets are so complicated to pin down.

Trunks and Branches Are Carbon Dioxide Sources

When people picture a tree exchanging gases, they usually think of leaves. But the trunk, branches, and roots are metabolically active too, and they don’t photosynthesize in any meaningful way. Bark is opaque; the wood beneath it is deep in the dark. These tissues respire all day and all night, steadily leaking carbon dioxide.

In Scots pine, stem respiration accounted for 77 to 79 percent of the total carbon dioxide leaving the trunk surface during the growing season.⁴ In mature specimens of the same species, stem respiration represented about 95 percent of the observed carbon dioxide escaping into the atmosphere.⁵ A small fraction of that carbon dioxide gets recycled by green cells in the bark or dissolved in the sap stream and carried upward, but most of it simply diffuses outward into the air.

Tropical trees show a similar pattern but with an interesting twist. In one study of a tropical species in the Amazon, carbon dioxide leaving the trunk surface was reduced by up to 35 percent during the daytime, likely because the upward flow of water through the wood dissolved and carried away some of the respired carbon dioxide before it could escape through the bark.⁶ In young poplar trees, researchers found that the concentration of dissolved carbon dioxide in the sap stream varied throughout the day and spiked at night and during rain, making stem carbon dioxide efflux harder to predict from temperature alone.⁷ The carbon dioxide dissolved in sap doesn’t vanish; it eventually escapes through the bark higher up the trunk or exits through the leaves. But it does complicate the picture of when and where a tree is emitting.

Elevation matters too. In an Ecuadorian mountain rainforest, average daily carbon dioxide release from tree stems dropped more than sixfold between 1,050 meters and 3,050 meters elevation, largely because colder temperatures slow down cellular respiration.⁸ Trees at high altitude still respire, but their carbon dioxide output is dramatically lower than their lowland relatives.

The Net Balance Over a Year

Despite all that respiration, a healthy, growing tree is a net absorber of carbon dioxide over the course of a year. The carbon it captures through photosynthesis during daylight hours exceeds the carbon dioxide it releases through respiration at all hours and from all tissues. The excess carbon ends up locked into the tree’s physical structure: the cellulose in its wood, the lignin in its bark, the starch reserves in its roots. That’s carbon storage, and it’s the reason forests are considered carbon sinks.

How big a sink? The scale is enormous. Terrestrial photosynthesis removes roughly six times more carbon dioxide annually than humanity’s fossil fuel emissions.¹ But most of that captured carbon cycles right back. Respiration by the trees themselves, respiration by the fungi and microbes in the soil, and decomposition of dead leaves and wood return much of it to the atmosphere. The net amount that stays locked away in living biomass and soil is a small fraction of the gross uptake. That fraction is what climate scientists track so carefully, because it’s the buffer between what we emit and what stays in the atmosphere.

Young Trees Versus Old Trees

A common claim is that old-growth forests stop absorbing carbon and become carbon-neutral. The reality is messier. Young, vigorously growing trees invest heavily in new wood, new leaves, and new roots, which means they pack away a lot of carbon per unit of leaf area. Comparisons between young and old ponderosa pine ecosystems show that young stands can maintain roughly twice the rate of carbon dioxide exchange as old stands, partly because younger sites carry more leaf area and experience more favorable conditions for photosynthesis across the year.⁹

But old-growth forests are not zero. Large old trees still photosynthesize, and the soil beneath them continues to accumulate organic matter. The rate of net carbon uptake does tend to slow, and some old-growth stands hover close to carbon neutrality where the carbon released through respiration and decomposition roughly matches what the canopy absorbs. Whether a given stand is a net sink or source depends on the species composition, the climate, the disturbance history, and the health of the soil microbiome. Blanket statements about old forests being “done” sequestering carbon overstate the case.

When Trees Die, the Carbon Comes Back

A standing dead tree or a fallen log is essentially a slow-release carbon dioxide emitter. Deadwood decomposition releases carbon dioxide and methane into the atmosphere as fungi, bacteria, and insects break down the wood.¹⁰ The rate depends on the tree species, the size of the trunk, and the climate. Tropical deadwood decomposes faster than temperate or boreal deadwood, and thinner trunks break down faster than thick ones. Models of tropical dead-tree decay suggest you can estimate the release rate fairly well just from the tree’s size at death and its wood density.¹¹

This is a critical point for thinking about forests as climate solutions. A tree that grows for a century and then burns or rots releases most of the carbon it stored. Researchers have explored methods like “wood vaulting,” where harvested wood is buried in conditions that prevent decomposition, keeping the carbon locked away for thousands of years. Excavation of 3,775-year-old buried wood confirmed that this approach can work in principle.¹ Without some intervention, though, the default fate of tree carbon is to return to the atmosphere eventually.

Drought and Heat Shift the Balance

Environmental stress can tip a tree from net oxygen producer to net carbon dioxide emitter, at least temporarily. Drought is the biggest lever. When water runs short, trees close the pores on their leaf surfaces to avoid drying out. This stops water loss but also blocks carbon dioxide from entering the leaf, which stalls photosynthesis. In one experiment with Chinese fir saplings, drought slashed photosynthetic rates by about 75 percent regardless of whether carbon dioxide levels in the air were normal or elevated.¹² That’s a drastic reduction. Meanwhile, respiration continues, so the tree is burning through stored carbon without replacing it.

Studies of mature deciduous trees tell a similar story. When researchers separated the effects of drought from the effects of leaf aging, stomatal closure accounted for about 75 percent of the total limitation on carbon uptake during dry periods.¹³ Some additional impairment came from damage to the photosynthetic machinery itself, beyond just closing stomates. A tree under severe drought can become a net carbon dioxide source for weeks or months, releasing more through respiration than it absorbs through its stunted photosynthesis.

Heat alone also accelerates respiration faster than it boosts photosynthesis, which narrows the gap between the two and reduces net carbon storage. This is one of the worrying feedback loops in climate science: warming temperatures push forests to respire more, releasing more carbon dioxide, which drives further warming.

Can Trees Adjust to a Warmer World?

There is some encouraging news on that front. Trees can partially acclimate their respiration rates to warmer temperatures. In a canopy-warming experiment in a Panamanian tropical forest, researchers heated upper canopy leaves by about 3°C for a week and measured how leaf respiration responded. Respiration initially jumped but then partially settled back down, a response called thermal acclimation. When the researchers fed those acclimation rates into a model of tropical forest carbon balance over the coming century, the acclimation response led to a projected 21 percent greater increase in net primary productivity and 18 percent greater biomass carbon storage compared to a scenario with no acclimation at all.¹⁴

The acclimation wasn’t perfect, though. Respiration still increased with temperature; it just increased less than expected. And the response varied between tissue types. In a separate experiment, leaves and green stems showed little thermal acclimation, while non-photosynthetic tissues like roots and woody stems acclimated more readily, adjusting their respiration rates so that they were similar across a range of temperatures.¹⁵ Since stems and roots are responsible for a large share of whole-tree respiration, their ability to acclimate matters a great deal. But the picture is incomplete and based on short-term experiments. Whether decades of gradual warming produce the same acclimation response remains an open question.

Wildfires Turn Forests Into Carbon Sources

Fires represent the most dramatic way a forest can flip from carbon sink to carbon source. A wildfire rapidly oxidizes decades or centuries of stored carbon, releasing it as carbon dioxide in hours or days. The aftermath leaves dead wood and damaged soil that continue releasing carbon for years.

This is especially concerning in boreal forests, the vast belt of conifers stretching across Canada, Alaska, and Siberia. As the climate warms and fire seasons lengthen, modelers project that wildfires in boreal North America alone could contribute a cumulative net release of nearly 12 gigatonnes of carbon dioxide by mid-century, an amount representing about 3 percent of the remaining global carbon dioxide budget associated with keeping warming within the 1.5°C Paris Agreement target.¹⁶ Fire management strategies could reduce those emissions, but the scale of the problem is sobering. Forests that have been net carbon sinks for millennia can become net sources if fire frequency or severity exceeds what the regrowth can offset.

What the Soil Under a Tree Is Doing

The ground beneath a tree is its own carbon dioxide factory. Soil respiration comes from two sources: the tree’s own roots and the army of microorganisms breaking down organic matter. Separating these two contributions is one of the persistent challenges in forest carbon science.¹⁷ Roots breathe to fuel their growth and nutrient uptake. Fungi and bacteria breathe as they decompose fallen leaves, dead roots, and anything else organic in the topsoil. The carbon dioxide bubbling up from forest soil can rival or exceed the carbon dioxide released by the tree’s above-ground tissues. In many forests, soil is the single largest source of ecosystem-level carbon dioxide emissions, even while the canopy overhead is busily absorbing it.

Temperature and moisture govern how fast this underground respiration runs. Warm, moist soils in tropical forests decompose organic matter quickly, returning carbon to the atmosphere faster than in cold boreal soils where decomposition slows to a crawl. That slow decomposition is why boreal forests and peatlands have historically accumulated enormous carbon stores in their soils, and why thawing permafrost is such a concern: warming unlocks centuries of stored carbon for microbial decomposition all at once.

Choosing the Right Trees for Cities

If you’re interested in the practical side of trees and carbon, urban forestry is where the action is right now. City trees provide a suite of benefits: they cool streets, filter air, manage stormwater, and sequester carbon. But species choice matters enormously. An analysis of Polish cities found that modifying the species composition of urban forests could increase annual carbon sequestration rates by roughly 48 to 114 percent, depending on the strategy.¹⁸ Fast-growing, large-canopy species store more carbon per year than slow-growing ornamentals, but they may also have higher water demands and shorter lifespans. Cities that optimize their tree plantings for carbon uptake face tradeoffs with drought tolerance, maintenance costs, and pest resistance. Still, the magnitude of improvement available through smarter species selection is striking for something as simple as choosing one tree over another.

Urban trees also face unique stresses, from compacted soil and limited rooting space to reflected heat from pavement. These stresses reduce photosynthetic performance and can push city trees closer to carbon neutrality than their rural counterparts. A stressed city tree with stunted growth and dying branches is doing less carbon work than a healthy forest tree of the same species. Adequate watering, soil management, and canopy space aren’t just about aesthetics; they directly affect how much carbon dioxide a city tree can pull from the air.

The Seasonal Swing

If you’ve ever seen the famous “Keeling Curve” showing rising atmospheric carbon dioxide, you’ve noticed the sawtooth pattern layered on top of the long-term upward trend. That zigzag is forests breathing. During the Northern Hemisphere spring and summer, trees in North America, Europe, and Asia pull enough carbon dioxide out of the atmosphere to visibly bend the global concentration downward. During the fall and winter, as leaves drop, growth stops, and decomposition dominates, the concentration ticks back up. The annual swing is about 6 parts per million at Mauna Loa, and it’s larger at higher northern latitudes where seasonal forests are more abundant.

Deciduous forests amplify the seasonal swing because their leaves are only present for part of the year, concentrating all their photosynthetic work into roughly six months. Evergreen forests maintain some photosynthetic capacity year-round, but cold temperatures and short winter days drastically reduce the rate. In Scots pine, stem respiration peaked in June when growth was greatest, contributing up to 79 percent of the total stem carbon dioxide flux during the growing season.⁴ So even the respiratory cost of growing new wood follows a seasonal rhythm, spiking in the warm months when photosynthesis is also at its highest.

This seasonal cycle is why “trees produce oxygen” and “trees release carbon dioxide” are both true statements that tell you nothing useful on their own. The whole story is about timing, magnitude, and net balance. Over a full year, a healthy growing forest absorbs more carbon dioxide than it emits, producing a net surplus of oxygen. Over a single winter night, that same forest is exhaling carbon dioxide into the dark. Both facts coexist in the same tree.