# Photosynthesis without leaves: the green layer under bark

> Where the green layer under bark sits, why wet bark shows it, how much of what it fixes is carbon the tree itself has respired, and when it matters most.

Published 2026-09-20, updated 2026-09-21. Tags: How trees work, Protection & care. By Blue Whale Trees, a native tree nursery in the United States. Web version: https://bluewhaletrees.com/articles/photosynthesis-without-leaves/

Leaves do most of a tree's photosynthesis, but they do not do all of it. Under the outer bark of twigs, branches and many trunks lies a band of green tissue that fixes carbon too, and it keeps working after the leaves are down. The job it does is mostly recycling: it recaptures carbon dioxide the tree's own living cells have already respired, before that gas escapes to the air. That is a smaller job than a leaf's, and the tree does it with tissue built for other reasons.

![Mottled sycamore trunk with gray, olive and tan plates of bark flaking off the smooth surface beneath](https://bluewhaletrees.com/img/articles/photosynthesis-without-leaves-1-1000.jpg?v=457a6f4b "American sycamore in Kennedy Park, Munhall, Pennsylvania, in August. Sycamore sheds its outer bark in plates, and the inner bark that comes into view is white to cream, not green.")
*Photo: Richard Stephen Haynes, [CC BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0) ([source](https://commons.wikimedia.org/wiki/File%3ASycamore_Tree_Bark.jpg))*

## The green layer sits under the outer bark

Bark is not one material. The outermost skin is phellem, the corky layer people mean when they say bark, and it is dead at maturity. It is laid down by the cork cambium, a sheet of dividing cells that makes cork on its outer side and a tissue called phelloderm on its inner side. Inside the phelloderm are the cortex and the secondary phloem. The chlorophyll sits in the phelloderm and the cortex, not in the dead cork over them.

That arrangement has been mapped cell by cell. In a study of 23 eucalypt species, the densest band of chloroplasts lay directly beneath the outer skin in thin branchlets and in trunks alike, including bark cut from a trunk 40 centimeters across.

> "In both small and large diameter stems, the greatest concentration of chloroplasts was in a narrow band (usually 100–300 μm thick) immediately beneath the epidermis or phellem." (Burrows and Connor, *Chloroplast Distribution in the Stems of 23 Eucalypt Species*, 2020)

None of that shows from outside. The authors scraped a few cells of cork off trunks whose surface was not green at all and found bright green tissue directly underneath. Across the wider literature, chloroplasts have been recorded from current-year shoots up to trunks 60 centimeters in diameter, and in thin stems they run deep, in through the wood rays and into the pith. A survey of 85 shrub and tree species at six sites in Australia and Mexico found photosynthetic bark under the outer layer on 94 percent of twigs and about 45 percent of main stems.

What shuts it down is thickness. Cork blocks light, and a rough, furrowed, plated bark that is centimeters thick leaves the living tissue below it in the dark. The trees doing the most of this are the ones with the thinnest outer skin, and the thinnest skin on any tree is on its youngest wood. None of this touches the heartwood at the center, which is dead by design, and which is why [a shelf fungus working inside an old trunk](https://bluewhaletrees.com/articles/fungus-is-not-a-death-sentence/index.html.md) is not the emergency it looks like.

## What sycamore and aspen bark really show

Sycamore is the tree people name first, because it drops its outer bark in plates and the surface beneath looks raw and new. What that exposes is not the green layer. On American sycamore the upper bark is light gray and flakes away to show inner bark that is white to cream-colored. The pale patchwork on a sycamore trunk is a shed skin next to a fresh one, not a window onto chlorophyll. Whatever green tissue a sycamore carries sits below that cream surface, as hidden as it is on a tree that never drops a plate.

Aspen is the better-measured example. Measurements taken at several heights up an aspen stem found bark photosynthetic capacity rising toward the top of the tree as the stem narrowed and the bark over it thinned. A tree is not evenly photosynthetic down its length, and its thin young upper branches are better at it than its butt.

![Grove of quaking aspen with smooth pale trunks tinged cream and green, standing in tall grass](https://bluewhaletrees.com/img/articles/photosynthesis-without-leaves-2-1000.jpg?v=6d4db380 "Quaking aspen with pale trunks by the inlet of Convict Lake, Sierra Nevada, California, in June. In aspen, bark photosynthetic capacity rises with height as the stem and its bark grow thinner.")
*Photo: Dcrjsr, [CC BY 4.0](https://creativecommons.org/licenses/by/4.0) ([source](https://commons.wikimedia.org/wiki/File%3AAspen_woods_Convict_Lake_inlet.jpg))*

Closer to home, [sassafras](https://bluewhaletrees.com/plants/sassafras/index.html.md) shows the same tissue on a scale you can hold. Its twigs are smooth, round and greenish-yellow to greenish-brown, with gray lenticels, and its buds are green. We grow sassafras at Blue Whale Trees as saplings, for full sun or light shade on well-drained soil on the acid side, and it resents being moved after its first year or two.

## Why wet bark looks green

Rain makes the layer show. Many trees, young thin-barked ones most of all, take on a green tint while their bark is wet and go back to gray as it dries. The chlorophyll was there the whole time; what changes is the cork over it.

Dry cork is full of air, and light scatters off every air-filled cell wall before it gets far, so what comes back is the flat gray or brown of the surface. Water fills those spaces and bends light much as the cell walls do, so the scattering drops. More light reaches the chlorophyll beneath and returns colored by it. The film of water on the surface also holds light in, the same reason a wet stone looks darker. Manetas measured the effect on the twigs of seven Mediterranean species: their usual grayish surface turned darker or even greenish in rain. Reflectance fell most in the bands that photosynthetic pigments absorb. Light transmission through an isolated kermes oak periderm rose when it was wet. Wet bark is bark at its most transparent, which is also when the layer beneath it is best lit.

Thin bark shows it best because there is less cork to see through, so the effect is strongest on young trunks, twigs and smooth-barked species. Older bark can turn green after rain for a different reason: a film of algae on the surface. On smooth-barked forest trees, biofilms of the green algae Desmococcus, Apatococcus and Trebouxia coat the bark. They dry to a dull crust and revive on rewetting. Lichens do the same with a cortex that turns transparent when wet. The tree's own green sits under the skin, the film sits on top, and a fingernail tells them apart.

## Most of it is recycling, not new carbon

There are two ways a stem can fix carbon, and they are not the same trade. A green stem with working stomata takes carbon dioxide from the air, the way a leaf does. A stem sealed under a periderm cannot, because that layer resists gas diffusion as well as light, so its chloroplasts work on carbon dioxide already inside the wood. Researchers call the first stem net photosynthesis and the second stem recycling photosynthesis, and a temperate tree with corky bark is doing the second one.

There is plenty inside a trunk to work with. The partial pressure of carbon dioxide inside wood is high, higher than inside a leaf. It is fed by the respiration of living stem cells and by gas dissolved in sap on its way up from the roots. Most woody stems carry chlorophyll and photosynthesize on that stem-internal carbon dioxide with whatever light gets through the bark.

The share recovered varies widely. Across non-succulent plants, stem photosynthesis has been put at 16 to 60 percent of leaf photosynthesis, and the measured contribution swings with species, with season and with where on the tree the measurement is taken. A few published figures give the range:

| Tree | What was measured | Share |
| --- | --- | --- |
| European beech | Stem photosynthesis against the stem's own respiration in April | 50 percent |
| Western white pine | Carbon dioxide refixed inside the stem against dark respiration | 76 percent |
| Scots pine | Photosynthesis as a share of the stem's net carbon dioxide flux | Up to 13 percent |
| Eucalyptus miniata | Carbon from bark photosynthesis in stem growth | 11 percent |
| Black poplar | Carbon from woody tissue photosynthesis in stem growth | Up to 24 percent |

The spread is wide because stems are not specialized for photosynthesis and their rates per unit of surface run well below a leaf's. For a tree under corky bark the payoff is a reduced loss rather than a gain: less carbon leaving the trunk, not more coming in.

## When the leaves are gone or the water is short

Bark photosynthesis earns its keep at the times a canopy cannot work, and three of those times are ordinary.

The first is the leafless season. A deciduous tree in early spring has bare twigs, thin bark and full light on all of it. In European beech, stem photosynthesis peaked at the start of the growing season in April, when it accounted for half of that month's stem respiration, then fell away and stayed low until winter. That is the same window in which [new leaves come out red](https://bluewhaletrees.com/articles/why-new-leaves-come-out-red/index.html.md) under a pigment sunscreen: the stem is working while the leaf is still being built.

The second is drought. When stomata close, leaf photosynthesis stops and the stem's does not, because a stem under a periderm was never taking in air to begin with and loses almost no water doing its work. Where this has been tested by covering stems to shut the light out, the effect was not subtle.

> "In response to drought, individuals with shaded stems suffered 100% mortality, compared with 16.67% in non-wrapped plants, suggesting that stem photosynthesis and the carbon it generates contribute to drought resistance." (Ávila-Lovera and others, *The benefits of woody plant stem photosynthesis*, 2024)

That was a desert tree with genuinely green stems, so the figure is a ceiling rather than a rule for an oak in a yard. In trees more generally, bark photosynthesis has been shown to hold up plant water potential and sap flow through drought and to delay leaf drop.

The third is a stripped crown. A tree that loses its leaves to caterpillars in June is not a tree with no photosynthesis at all, because the twigs and branches carry on. Light-exclusion experiments show that stem recycling photosynthesis affects stem growth and the production of new buds after defoliation. What a stem cannot do is stand in for a canopy, since its rate per unit of surface is a fraction of a leaf's. A trunk standing in water has a different problem again, and [floodplain trees survive high water](https://bluewhaletrees.com/articles/how-floodplain-trees-survive-high-water/index.html.md) with equipment of their own.

## Fruits, flowers and buds

A 2023 review of fruit photosynthesis lists the organs usually counted as carbon sinks that photosynthesize anyway: stems, roots, flowers, fruits and seeds. A young fruit is built more like a leaf than a ripe one is. Early in development, fruits generally carry high levels of chlorophyll, a high density of working stomata and a thin cuticle. Most species then take the machinery apart: the chloroplast grana break down through ripening, and the stomata stop working. The swelling volume of the fruit makes it harder for light and for air to reach the inside. Fleshy fruits mostly refix their own respired carbon dioxide. Dry fruits, especially while young, can also take carbon dioxide from the air.

What the fruit gets from it is local rather than shared. The evidence points to photosynthesis inside a fruit supplying the energy carriers that expensive biosynthesis runs on, as well as the carbon skeletons for sugars, flavonoids, tannins and oils. It points to the same process supplying oxygen, which keeps the inner tissues and the seed itself out of hypoxia.

Buds are where the evidence thins, and we would rather say so than round up. A bud scale is a modified leaf that forms a protective covering for a bud, so the outside of a winter bud is a wrapper, not a working surface. Some buds are green underneath it, and sassafras buds are green and about a quarter of an inch long. Green means chlorophyll, and chlorophyll is not the same thing as a measured rate of carbon fixation. None of the sources below measured photosynthesis in the buds of an eastern tree, so a photosynthesizing bud is plausible and unproven. The [dormant buds that sit under the bark for decades](https://bluewhaletrees.com/articles/epicormic-buds/index.html.md) are a separate matter again.

![Sassafras twig in flower, the stem green and the small flower clusters yellow-green](https://bluewhaletrees.com/img/articles/photosynthesis-without-leaves-3-1000.jpg?v=1d00b62f "Sassafras flowering in Madison, Wisconsin, in April. The twig is green and the flower clusters are yellow-green; sassafras twigs run greenish-yellow to greenish-brown with gray lenticels.")
*Photo: James Steakley, [CC BY-SA 3.0](https://creativecommons.org/licenses/by-sa/3.0) ([source](https://commons.wikimedia.org/wiki/File%3ASassafras_albidum_flower.jpg))*

## What this means for a trunk in your yard

Three practical things follow from the green layer.

- **Girdling still kills.** Girdling is the damaging, cutting, removing or clamping of cambium all the way around a trunk or branch. Rodents do it under snow and mulch, a string trimmer does it in one pass and a wire or tie left on a stake does it over years. The green layer under the bark is no defense, because what a girdle severs is the phloem that carries sugar down the trunk, and photosynthesis above a cut does not get past the cut. [Keeping voles and rabbits off the bark](https://bluewhaletrees.com/articles/protecting-trees-from-animals/index.html.md) is worth more than anything done afterward.
- **A wrap is a trade.** A white tree wrap on a young trunk reflects winter sun and evens out the bark temperature swings that [split thin bark](https://bluewhaletrees.com/articles/frost-cracks-and-sunscald/index.html.md). The instruction is to put it on in autumn before temperatures freeze and take it off in spring once nights stay above freezing. It also does on a lawn what researchers do on purpose in an experiment, which is put the bark in the dark. We found no measurement of what a winter of wrapping costs a landscape tree in carbon, and on a young thin-barked tree in an exposed spot the protection is still the bigger consideration. Take it off on schedule.
- **Paint is the one to skip.** Wound paint and tar do not help a damaged trunk.

> "If trunk damage does occur, do not apply a wound treatment such as paint or tar." (Illinois Extension, *Exploding trees? A closer look at frost cracking*)

Those treatments get in the way of the response the tree already has, which is to wall the damage off and grow over it rather than heal it. [How trees defend themselves](https://bluewhaletrees.com/articles/how-trees-defend-themselves/index.html.md) covers that response, and the [glossary](https://bluewhaletrees.com/articles/glossary/index.html.md) defines the terms used here.

## Common questions

### Do trees photosynthesize through their bark?

Most woody plants carry chlorophyll in a thin layer under the outer bark and fix carbon there, at a far lower rate than a leaf and mostly using carbon dioxide the tree has already respired.

### Why is there green under a tree's bark?

That green is the chloroplast-bearing tissue of the phelloderm and the cortex, and in surveyed species it sits under the outer bark of about 94 percent of twigs and about 45 percent of main stems.

### Does wrapping a tree trunk block photosynthesis?

It does, since excluding light from a stem is exactly how researchers switch stem photosynthesis off in experiments. On a young thin-barked tree the winter protection is still worth more, so wrap in autumn and unwrap in spring.

### Why does tree bark look green after rain?

Water fills the air spaces in the dead outer cork and makes it more transparent, so the green layer underneath shows through until the bark dries again. On rough old bark the green after rain is usually a film of algae on the surface rather than the tree's own tissue.

## Sources

- [Chloroplast Distribution in the Stems of 23 Eucalypt Species](https://pmc.ncbi.nlm.nih.gov/articles/PMC7767473/): Burrows and Connor, Plants 9 (2020): where the chloroplasts sit in a stem, how thin the outer bark has to be, and how many twigs and trunks carry photosynthetic bark.
- [Partitioning seasonal stem carbon dioxide efflux into stem respiration, bark photosynthesis, and transport-related flux in Scots pine](https://pmc.ncbi.nlm.nih.gov/articles/PMC11350082/): Dukat and others, Journal of Experimental Botany 75 (2024): what bark photosynthesis contributes to a mature tree's stem carbon balance, and the beech, pine and aspen figures used here.
- [The benefits of woody plant stem photosynthesis extend to hydraulic function and drought survival in Parkinsonia florida](https://pmc.ncbi.nlm.nih.gov/articles/PMC10918054/): Ávila-Lovera and others, Tree Physiology 44 (2024): net versus recycling stem photosynthesis, its share of stem growth, and what shading a stem does to a tree under drought.
- [Fruit Photosynthesis: More to Know about Where, How and Why](https://pmc.ncbi.nlm.nih.gov/articles/PMC10347175/): Garrido and others, Plants 12 (2023): chlorophyll, stomata and function in fruits, flowers and seeds.
- [Platanus occidentalis](https://plants.ces.ncsu.edu/plants/platanus-occidentalis/): NC State Extension Gardener Plant Toolbox: what American sycamore's exfoliating bark exposes.
- [Sassafras albidum](https://plants.ces.ncsu.edu/plants/sassafras-albidum/): NC State Extension Gardener Plant Toolbox: the color of sassafras twigs and buds.
- [Extension Gardener Handbook glossary](https://content.ces.ncsu.edu/extension-gardener-handbook/glossary): NC State Extension: definitions of cambium, cork cambium, bud scale and girdling.
- [Exploding trees? A closer look at frost cracking](https://extension.illinois.edu/blogs/good-growing/2026-01-30-exploding-trees-closer-look-frost-cracking): Illinois Extension: why thin-barked trunks split in winter, how a white wrap is used, and why a wound should not be painted.
- [Photosynthesizing in the rain: beneficial effects of twig wetting on corticular photosynthesis through changes in the periderm optical properties](https://doi.org/10.1078/0367-2530-00161): Manetas, Flora 199 (2004): twig color and reflectance in rain, and periderm light transmission wet against dry.
- [Resurrection kinetics of photosynthesis in desiccation-tolerant terrestrial green algae (Chlorophyta) on tree bark](https://doi.org/10.1111/j.1438-8677.2009.00249.x): Lüttge and Büdel, Plant Biology 12 (2010): the green algal biofilms on smooth bark and their recovery on rewetting.
- [Lichen Biology](https://www.fs.usda.gov/wildflowers/beauty/lichens/biology.shtml): US Forest Service: why a lichen's cortex turns transparent when wet and the algae show.
- [Why some things are darker when wet](https://doi.org/10.1364/AO.27.001278): Lekner and Dorf, Applied Optics 27 (1988): the optics of a film of water on a rough surface.