Do Trees Do Photosynthesis? How the Process Works

Trees are some of the most powerful photosynthesizers on the planet. Every leaf on every tree, from a backyard maple to a towering rainforest giant, runs the same fundamental chemical reaction that all green plants use: it absorbs sunlight, pulls carbon dioxide from the air, draws water up from the roots, and assembles sugar molecules the tree can burn for energy or use as building material. What makes tree photosynthesis especially interesting is its scale and the clever adaptations trees have evolved to keep the process running under wildly different conditions, from blazing tropical heat to freezing subalpine winters.

How a Tree Turns Sunlight Into Sugar

Photosynthesis in a tree works in two linked stages. In the first, light energy hits chlorophyll molecules inside tiny compartments called chloroplasts, which are packed into leaf cells. That energy splits water molecules, releasing oxygen as a byproduct and generating chemical energy the cell can use. In the second stage, that chemical energy drives a cycle of reactions that grabs carbon dioxide from the air and builds it into simple sugars. The tree then ships those sugars through its inner bark (the phloem) to wherever they’re needed: growing branch tips, roots, flowers, or fruit.

Chloroplasts are the engine of the whole operation, and their condition matters enormously. When a tree has plenty of nitrogen available, its chloroplasts are large, packed with well-organized internal membranes, and loaded with starch granules. Starve the same tree of nitrogen and those chloroplasts shrink, their internal membranes break apart, and starch production drops sharply.¹ This is one reason nutrient-poor soils can stunt tree growth even when sunlight and water are abundant: the photosynthetic machinery itself degrades without the raw materials to maintain it.

Not Just Leaves

Most people picture photosynthesis happening exclusively in leaves, and leaves do handle the vast majority of it. But trees have a second, lesser-known photosynthetic system running in their bark and the green tissues just underneath it. If you’ve ever peeled a thin strip of bark from a young branch and noticed a green layer, you’ve seen these tissues firsthand.

Bark and woody tissue photosynthesis serves a different purpose from leaf photosynthesis. Rather than being a major sugar factory, it acts more like a local life-support system. The dense, metabolically active tissues inside a growing stem consume oxygen faster than it can diffuse inward from the outside air, which can create dangerously low oxygen conditions. When light hits the bark, the green cells underneath start producing oxygen right where it’s needed, preventing the kind of internal oxygen starvation that would slow or kill growing wood cells.² The highest rates of oxygen release happen in the outermost cortex tissues, because light drops off steeply as it tries to penetrate deeper into the stem.

Woody tissue photosynthesis also contributes to stem growth in a measurable way. Research on young poplar trees found that stems allowed to photosynthesize grew about 39% more in diameter than stems that were kept in the dark, under normal atmospheric carbon dioxide levels.³ That’s a substantial boost from a system most people don’t even know exists. Interestingly, when carbon dioxide levels were artificially raised, the advantage disappeared, suggesting that when a tree already has plenty of carbon coming in through its leaves, the extra contribution from bark photosynthesis becomes redundant.

Water, Stomata, and the Balancing Act

Photosynthesis is not just about light and carbon dioxide. Water is the third essential ingredient, and getting it to where it needs to go is one of the most impressive feats of tree engineering. A large oak can move hundreds of liters of water per day from its roots to its canopy through a continuous column of water held under tension inside the xylem, the network of narrow tubes running through the trunk and branches.

The catch is that to let carbon dioxide into their leaves, trees have to open microscopic pores called stomata, and every time a stoma opens, water vapor escapes. Trees face a constant trade-off: open the stomata wide and you get plenty of carbon dioxide for photosynthesis but lose a lot of water, or close them to conserve water but starve the photosynthetic machinery of its carbon supply. This stomatal regulation, combined with the physical structure of the canopy, is what keeps a tree’s photosynthetic system running without the plumbing catastrophically failing during dry spells.⁴

During a drought, many trees will partially close their stomata to avoid losing the water tension in their xylem, a condition called hydraulic failure that can kill branches or entire trees. Photosynthesis slows as a result, but the tree survives to photosynthesize another day. Some species are more aggressive about keeping stomata open and gambling on rain arriving before they dehydrate, while others are more conservative. This spectrum of strategies is a major reason why different tree species thrive in different climates.

Deciduous Trees vs. Evergreens

One of the most visible differences in the tree world is between species that drop their leaves each autumn and those that keep them year-round. This distinction maps directly onto photosynthetic strategy. Deciduous trees invest in relatively thin, high-performance leaves that photosynthesize at high rates during the growing season, then shed them before winter when short days and cold temperatures would make those leaves a liability. Evergreens take the opposite approach: tougher, longer-lived leaves that photosynthesize at lower rates but keep working across multiple seasons.

Measurements across a range of species show the gap clearly. Among broadleaf trees studied under the same conditions, deciduous species achieved peak photosynthetic rates roughly 50% to 170% higher than evergreen species.⁵ Deciduous leaves also open their stomata wider and move carbon dioxide more efficiently through their internal tissues. But this high-performance design comes at a cost: a deciduous tree has to build an entirely new set of leaves every spring, which requires a large upfront investment of stored energy and nutrients.

Leaf nitrogen content is a key part of why deciduous leaves photosynthesize faster. Nitrogen is a core ingredient in the enzymes that drive carbon fixation, and deciduous broadleaf species tend to pack more nitrogen per unit of leaf area than evergreen conifers do.⁶ When a deciduous tree drops its leaves, it first reabsorbs much of the nitrogen back into the branches and trunk, recycling it for next year’s leaves. Evergreens skip this annual rebuild but accept lower per-leaf productivity as the trade-off.

What Happens to Photosynthesis in Winter

Evergreen trees keep their needles through winter, but that doesn’t mean they’re photosynthesizing at full capacity when the temperature drops below freezing. Many conifer species in cold climates undergo a near-complete shutdown of photosynthesis during the coldest months. Their light-harvesting machinery shifts into a protective mode, diverting absorbed light energy into heat rather than letting it damage frozen cellular structures. This phenomenon, sometimes called chronic photoinhibition, is visible as a measurable drop in the efficiency of the photosynthetic reaction centers.⁷

Spring reactivation is not instant. As temperatures warm and liquid water becomes available again, subalpine conifers gradually re-engage their photosynthetic systems. Soil temperature and liquid water availability strongly influence how quickly this happens, because the tree needs to rehydrate and restart its water transport before the full photosynthetic apparatus can safely come back online. The protective pigments that shielded the needles during winter are slowly dismantled as conditions improve.⁸ This transition period can last weeks, which is one reason why early spring snowmelt followed by a hard freeze can be so damaging to evergreen forests: the trees have already begun lowering their defenses when the cold returns.

How Light Varies Within a Single Canopy

A tall tree doesn’t experience uniform light conditions. The leaves at the top of the canopy are bathed in direct sunlight, while leaves deeper in the crown may receive only a small fraction of that light after it’s been filtered by the layers above. Trees deal with this by building different kinds of leaves at different heights. Sun leaves near the top tend to be thicker, denser, and packed with more nitrogen and photosynthetic enzymes per unit area. Shade leaves lower in the canopy are thinner, have less nitrogen per unit area, and are optimized to capture the dim, diffuse light that reaches them.⁹

This isn’t just a passive response to available light. Trees actively adjust the photosynthetic capacity of their leaves based on the light environment each leaf experiences. The maximum rate of photosynthesis a leaf can achieve scales roughly with how much average light it receives, so the tree effectively tunes each leaf to its position in the canopy.¹⁰ From the tree’s perspective, spending nitrogen on high-capacity photosynthetic machinery in a deeply shaded leaf would be wasteful, so it invests more of that nitrogen where the return in carbon gain will be greatest.

Where All That Sugar Goes

A tree doesn’t keep everything it photosynthesizes. A substantial share of the sugar produced by photosynthesis gets burned right back up through respiration, the process by which living cells break down sugars for energy, releasing carbon dioxide back into the atmosphere. Roots respire. Trunks respire. Leaves respire, even while they’re photosynthesizing during the day. The balance between what a tree fixes through photosynthesis and what it loses through respiration determines how much carbon it actually adds to its body each year.

In a pine plantation studied over three years with variable rainfall, gross carbon uptake through photosynthesis ranged from about 17 to 18 tonnes of carbon per hectare per year. But after subtracting respiratory losses, net ecosystem productivity dropped to roughly 5 to 7 tonnes of carbon per hectare per year.¹¹ So roughly 60% to 70% of the carbon the trees captured through photosynthesis was released back through respiration and decomposition. That gap between gross photosynthesis and net carbon storage is a number that surprises many people who assume a tree simply locks away every molecule of carbon dioxide it absorbs.

The picture gets more complex in mixed-species forests. Trees growing in diverse mixtures appear to spend more of their photosynthetic income on respiration than trees growing in single-species stands. One study found that annual respiration in mixed plots was more than three times higher than in monocultures, driven largely by increased respiratory activity in leaves and roots rather than wood.¹² This doesn’t necessarily mean mixed forests store less carbon overall, because the higher respiration might reflect more vigorous growth and competition. But it does mean the relationship between photosynthesis and carbon storage isn’t as simple as “more trees equals more sequestration.”

The Underground Side of Tree Photosynthesis

Trees don’t keep all their photosynthetic output for themselves. A significant portion of the sugars produced by photosynthesis flows down to the roots and is handed off to symbiotic fungi called mycorrhizae, which form dense networks around and within root tips. In exchange, the fungi are traditionally thought to provide the tree with mineral nutrients, especially nitrogen and phosphorus, that their thread-like structures can extract from soil far more efficiently than roots alone.

The reality of this exchange is more complicated than the neat “carbon-for-nutrients” bargain that textbooks describe. Research in boreal forests found that when trees allocated more carbon to their mycorrhizal partners, the trees actually received less nitrogen, not more. Reducing the trees’ carbon supply through shading led to lower fungal biomass on root tips but more nitrogen reaching the tree’s own foliage.¹³ Under nitrogen-limited conditions, the fungi appeared to be keeping a larger share of the nitrogen for their own growth. This suggests that the fungal partnership is not always a straightforward mutual aid society; the fungi have their own metabolic priorities, and those don’t always align with the tree’s best interests.

Still, mycorrhizal networks are deeply important for forest function. They can connect trees to one another underground, and the sugars that photosynthesis produces are the currency that powers the entire network. A tree that photosynthesizes vigorously in a sunny gap may end up subsidizing neighboring trees through shared fungal connections, though the extent and significance of these transfers remains an active area of research.

Why Almost All Trees Use the Same Photosynthetic Pathway

Plants have evolved three main variations of photosynthesis, commonly labeled C3, C4, and CAM. C4 photosynthesis is famously efficient in hot, sunny, dry conditions and is the engine behind crops like corn, sugarcane, and many tropical grasses. CAM photosynthesis is the water-miser strategy used by cacti and some succulents. You might expect at least some trees to have adopted C4 photosynthesis, especially in the tropics. But virtually every tree on Earth uses the C3 pathway.

The only true C4 trees known to science belong to a single group within the genus Euphorbia, a lineage better known for producing small shrubs and succulents. The reasons C4 photosynthesis and the tree growth form are almost mutually exclusive involve several compounding challenges. C4 plants require more energy per unit of carbon fixed, which makes them less efficient in shaded conditions, and the interior of a dense canopy is heavily shaded. C4 plants also show less flexibility in adjusting their photosynthetic machinery to different light environments, which matters when a tree has leaves spanning from full sun to deep shade within a single crown.¹⁴

There may also be a plumbing problem. Trees rely on a passive sugar-transport system through their phloem, driven by pressure differences between where sugars are made (the leaves) and where they’re consumed (roots, growing tips). C4 photosynthesis produces sugars in a slightly different cellular arrangement that could interfere with how efficiently those sugars load into the phloem, and the long transport distances in a tall tree would amplify any inefficiency. This combination of energetic, structural, and transport constraints helps explain why, despite C4 photosynthesis having evolved independently more than 60 times across the plant kingdom, trees have almost never made the switch.

Pollution and Stress Can Quietly Throttle Photosynthesis

Trees in urban and suburban environments face photosynthetic challenges that their forest counterparts don’t. Ground-level ozone, a common air pollutant formed when vehicle emissions and industrial chemicals react in sunlight, can directly damage the photosynthetic machinery inside leaves. In evergreen seedlings exposed to elevated ozone, new leaves showed significant declines in their peak photosynthetic rates, stomatal opening, and the efficiency of their light-harvesting systems. Older leaves on the same trees, however, were largely unaffected.¹⁵ This selectivity matters: it means ozone exposure may not immediately look dramatic, since the older, more visible leaves appear fine while the damage accumulates in new growth that hasn’t fully hardened yet.

Heat stress, drought, and flooding all impair photosynthesis through different mechanisms. Heat can denature the enzymes that fix carbon. Drought closes stomata and cuts off the carbon dioxide supply. Flooding drowns root systems, disrupting the water transport chain that photosynthesis depends on. Climate change is pushing many tree populations into conditions where multiple stressors overlap, creating compounding hits to their ability to photosynthesize and grow. Urban trees face the additional burden of soil compaction, limited root space, and reflected heat from pavement, all of which stack on top of pollution effects.

How Scientists Measure What a Tree Photosynthesizes

Measuring photosynthesis in a single leaf is now a mature, well-standardized technique. Portable instruments clamp onto a leaf, seal a small chamber around it, and track how quickly it takes up carbon dioxide and releases water vapor under controlled light and temperature conditions. These leaf-level measurements have become faster and more precise in recent years thanks to improvements in sensor design and automated environmental controls.¹⁶

Scaling up from a single leaf to a whole tree or an entire forest is where things get harder. Canopy-level measurements use tower-mounted instruments that track the movement of carbon dioxide and water vapor above a forest, calculating how much the ecosystem is absorbing or releasing over the course of a day or a season. More recently, satellite and drone-based sensors that detect reflected light at specific wavelengths, and even the faint fluorescence that chlorophyll emits during photosynthesis, have allowed researchers to estimate photosynthetic activity across entire continents. These remote sensing methods are becoming increasingly practical as sensors get cheaper and the statistical methods for interpreting their data improve. For the first time, it’s becoming possible to map not just where forests exist, but how hard they’re working photosynthetically at any given moment.

Dense Forests vs. Managed Plantations

Not all forests photosynthesize equally. The total carbon uptake and oxygen production of a stand of trees depends heavily on how dense the vegetation is and how large the individual trees have grown. Comparisons between dense conservation forests and managed production forests in tropical regions show that conservation forests with tighter canopies and more large trees absorb substantially more carbon dioxide and produce more oxygen per hectare than more open, younger production forests.¹⁷ Large, mature trees with deep canopies contribute the most to these totals, which is an argument for preserving old-growth forests even when younger, fast-growing plantations might seem like a more efficient carbon solution on paper.

That said, young, vigorously growing trees are adding wood mass quickly and can have high rates of carbon accumulation per individual tree. The distinction is between a tree’s rate of carbon gain and the total standing stock of carbon a forest holds. A young plantation sequesters carbon quickly as it grows, but a mature forest holds far more carbon in its existing biomass and soil. Both matter for the climate, and they’re answering different questions: how fast is carbon being removed from the atmosphere right now versus how much carbon is being kept out of the atmosphere long-term.