Trees breathe in a sense, though not through their leaves alone. Every living cell, from the root tip to the smallest twig, burns sugar for energy around the clock. Respiration is the quiet reverse of photosynthesis: it takes in oxygen and releases carbon dioxide. A surprising amount of a tree’s breath never leaves the trunk. It gets trapped, carried upward in sap, or even reused.
What Respiration Does Inside a Tree Cell
Respiration takes sugar made during photosynthesis and breaks it apart with oxygen to release energy. That energy is stored in a small molecule called ATP, which powers growth, repair, and the movement of nutrients. The leftovers are carbon dioxide and water, so respiration gives back carbon the tree once captured. All of this happens inside mitochondria, the tiny power units found in nearly every living plant cell.
Over a healthy day photosynthesis outpaces respiration, so the tree gains carbon. At night only respiration runs, which makes a tree a net source of CO2 after dark.
Respiration runs all day, not just at night. It is hidden by the much larger photosynthesis, and a leaf measured in daylight shows a net release of oxygen. Roots and inner wood never photosynthesize at all, and they respire constantly.
Trees also have a backup route for the last step of respiration. This backup route is less efficient, making only about one third as much energy as the main one.1University of Canterbury. School of Biological Sciences. The temperature dependence of plant alternative oxidase and its impact on respiration rates in nature All plants use it, as do some fungi and animals.2University of Canterbury. School of Biological Sciences. The temperature dependence of plant alternative oxidase and its impact on respiration rates in nature It looks wasteful, but in poplar, ramping up this route helped leaves resist a fungal leaf disease.3PubMed. Jasmonic Acid- and Ethylene-Induced Mitochondrial Alternative Oxidase Stimulates Marssonina brunnea Defense in Poplar
Respiration in Leaves by Day and Night
Leaves respire more slowly in daylight than in the dark. In temperate forest trees, light cut leaf respiration by a little over half.4iForest – Biogeosciences and Forestry. Day and night respiration of three tree species in a temperate forest of northeastern China The drop came fast as light first rose, then leveled off in brighter light.5iForest – Biogeosciences and Forestry. Day and night respiration of three tree species in a temperate forest of northeastern China In holm oak, daytime respiration ran at about 69% of the night rate, and dry soil barely changed this.6Functional Plant Biology. Light inhibition of foliar respiration in response to soil water availability and seasonal changes in temperature in Mediterranean holm oak (Quercus ilex) forest
Daytime respiration is hard to see. In daylight a leaf photosynthesizes far faster than it respires, so you can’t read respiration off a normal gas measurement. Researchers use special methods to estimate it, and the daytime numbers are estimates, not exact readings. A leaf’s nighttime rate is the cleanest number, and it overstates what the leaf burns during the day. That is why day-to-night comparisons are given as percentages of the night rate.
Light cutting respiration by half means the leaf’s respiratory CO2 fell, not that it stopped. Part of the drop likely reflects the leaf’s chemistry shifting in light: photosynthesis supplies energy and building blocks right inside the leaf, so mitochondria have less to do.
Position and leaf type matter too. In Amazon trees, top-canopy leaves respired faster than shaded understory leaves.7Frontiers in Forests and Global Change. Canopy Position Influences the Degree of Light Suppression of Leaf Respiration in Abundant Tree Genera in the Amazon Forest Yet light suppressed respiration most in the understory, about 68%, versus about 37% in the canopy.8Frontiers in Forests and Global Change. Canopy Position Influences the Degree of Light Suppression of Leaf Respiration in Abundant Tree Genera in the Amazon Forest In a wet tropical forest, overstory leaves respired about three times faster than understory ones, and height explained more than species did.9Tree Physiology. Variation in foliar respiration and wood CO2 efflux rates among species and canopy layers in a wet tropical forest
Leaf type shows a similar pattern. A deciduous species respired faster in the dark than an evergreen, but rates in light were similar.10PubMed Central. Dark Leaf Respiration in Light and Darkness of an Evergreen and a Deciduous Plant Species Light cut respiration by 62% in the deciduous leaves and 51% in the evergreen.11PubMed Central. Dark Leaf Respiration in Light and Darkness of an Evergreen and a Deciduous Plant Species
Sun leaves burn more sugar because they pack in more working machinery to capture bright light, and that machinery needs constant upkeep energy. Shade leaves run lean, saving sugar in dim light. Thin, short-lived deciduous leaves likewise work harder than tough evergreen ones.
How the Trunk and Roots Breathe
A trunk’s heartwood is dead and doesn’t respire. In pine and spruce, sapwood held over 80% of the stem’s living cells,12Canadian Journal of Forest Research. Growth and maintenance respiration in stems of Pinus contorta and Picea engelmannii so trunk respiration rose in step with the amount of living tissue.13Canadian Journal of Forest Research. Growth and maintenance respiration in stems of Pinus contorta and Picea engelmannii As woody plants grow, they pile up inert wood, and each kilo of stem and root respires less.14PubMed Central. Scaling of shoot and root respiration of woody and herbaceous plants
Much of the trunk’s breath stays inside. On a given day, 15 to 55% of the CO2 made in a trunk can stay inside the tree,15PubMed. Origin, fate and significance of CO2 in tree stems because inner bark and wood block gas from escaping.16PubMed. Origin, fate and significance of CO2 in tree stems Trunk air is often 3 to 10% CO2, sometimes over 20%, versus about 0.04% in the open air.17PubMed. Origin, fate and significance of CO2 in tree stems Some of it dissolves in sap and travels up toward the leaves.18PubMed. Origin, fate and significance of CO2 in tree stems In a tropical tree, CO2 leaking from the trunk fell up to 35% by day as sap flowed.19Hoehnea. Effects of xylem water transport on CO2 efflux of woody tissue in a tropical tree, Amazonas State, Brazil
Counting only the CO2 leaving the bark misses gas carried away in sap, and sap can bring in CO2 from roots below. Accurate estimates need sap flow and stem temperature as well.
Green bark recycles some of this gas. Green tissue under the bark of young twigs can use trapped CO2 and light to make sugar.20PubMed. Ecology and ecophysiology of tree stems: corticular and wood photosynthesis This can recapture 60 to 90% of the carbon young twigs and branches would lose.21PubMed. Ecology and ecophysiology of tree stems: corticular and wood photosynthesis Bark photosynthesis also cuts CO2 leaking from the stem beneath.22Oecologia. Carbon isotope discrimination in photosynthetic bark
You can see this yourself. Scratch a young twig of birch, aspen, or beech and a green layer shows underneath. Thick, old bark blocks light, so the recycling happens mostly in young stems.
Roots respire in the soil. Fast-growing tree species have thinner, longer fine roots that respire faster.23PubMed. Linking root traits to potential growth rate in six temperate tree species Mangrove fine roots, though, respired slowly compared with temperate trees,24PubMed. Fine root respiration in the mangrove Rhizophora mangle over variation in forest stature and nutrient availability and adding fertilizer didn’t change how fast they respired.25PubMed. Fine root respiration in the mangrove Rhizophora mangle over variation in forest stature and nutrient availability
Upkeep Costs Versus Building Costs
Scientists split respiration into two jobs. Maintenance respiration is the upkeep cost of keeping cells alive. Growth respiration is the building cost of making new tissue.
Building is expensive. In young hinoki cypress, building new tissue used up nearly as much carbon again as the tissue itself contained.26Annals of Botany. Long-term Respiratory Cost of Maintenance and Growth of Field-grown Young Hinoki Cypress (Chamaecyparis obtusa) That’s because making new wood, leaves, or roots means assembling sugars into large molecules such as cellulose, lignin, proteins, and oils, and each assembly step uses energy. Tissues rich in oils, proteins, or lignin generally cost more than ones made mostly of simple sugars and cellulose.
As the stand aged and growth slowed, upkeep took a larger share and building a smaller one.27Annals of Botany. Long-term Respiratory Cost of Maintenance and Growth of Field-grown Young Hinoki Cypress (Chamaecyparis obtusa) A big old tree has more living tissue to maintain but grows proportionally less.
Growth timing sets when building costs peak. In Norway spruce, trunk widening peaked around mid-June, but building cost peaked later, in late June or July.28PubMed. Seasonally varying relationship between stem respiration, increment and carbon allocation of Norway spruce trees The tree keeps spending energy thickening new cell walls after widening slows.29PubMed. Seasonally varying relationship between stem respiration, increment and carbon allocation of Norway spruce trees In ponderosa pine, wood respiration ran slightly higher in mid-summer, when growth happens.30Global Change Biology. Seasonal and annual respiration of a ponderosa pine ecosystem
How Much of Its Sugar a Tree Burns
Many climate models assume forests keep about half of their sugar and burn the other half.31Global Change Biology. Forest carbon use efficiency: is respiration a constant fraction of gross primary production? Real forests vary widely, and older forests keep less.32Global Change Biology. Forest carbon use efficiency: is respiration a constant fraction of gross primary production? In loblolly pine plantations, trees kept about 42% of their sugar.33Global Change Biology. Respiratory carbon use and carbon storage in mid‐rotation loblolly pine (<i>Pinus taeda</i>L.) plantations: the effect of site resources on the stand carbon balance Of every 100 units of sugar made, about 42 became wood, leaves, and roots, and the rest was burned for energy and released as CO2. Watering and fertilizing doubled how much those pines made, yet the 42% share stayed the same.34Global Change Biology. Respiratory carbon use and carbon storage in mid‐rotation loblolly pine (<i>Pinus taeda</i>L.) plantations: the effect of site resources on the stand carbon balance
Some carbon also goes to partner fungi. Many tree roots partner with fungi that receive sugar in exchange for nutrients. These fungi got up to about 22% of a tree’s production in field studies.35PubMed. Carbon allocation to ectomycorrhizal fungi correlates with belowground allocation in culture studies Trees send them a larger share when soils are poor and growth is slow.36PubMed. Carbon allocation to ectomycorrhizal fungi correlates with belowground allocation in culture studies
How Heat, Drought, and Flooding Change Respiration
Respiration speeds up as temperature rises, roughly doubling or more for every 10 °C in tropical leaves.37PubMed. Thermal acclimation of leaf respiration of tropical trees and lianas: response to experimental canopy warming, and consequences for tropical forest carbon balance Given time, though, trees acclimate. In ten North American species warmed by 3.4 °C for years, leaf respiration rose only 5%, not the expected 23%.38PubMed. Boreal and temperate trees show strong acclimation of respiration to warming Acclimation canceled about 80% of the expected rise.39PubMed. Boreal and temperate trees show strong acclimation of respiration to warming Across trees from boreal to tropical regions, warming cut respiration measured at a fixed temperature by 14%.40PubMed Central. Temperature responses of photosynthesis and respiration in evergreen trees from boreal to tropical latitudes
Acclimation means the leaf’s own machinery adjusts over weeks to years, so the same leaf respires less at a given temperature than before. The 14% figure uses one fixed temperature, which strips out the direct speeding-up effect of heat. It shows that warmed trees have turned down their baseline respiration. Leaves in the warmed study still respired about 5% more, so warming still costs some extra sugar.
A heat wave hits harder than slow warming because acclimation takes weeks to seasons. A sudden spike raises respiration before trees can adjust, and burning extra sugar in heat can drain reserves.
Drought shifts the work underground. In drought-stressed spruce seedlings, photosynthesis and leaf growth fell while root respiration rose,41PubMed. Drought increases carbon allocation to root respiration while maintaining storage in Qinghai spruce seedlings so the seedlings sent more of their carbon to roots.42PubMed. Drought increases carbon allocation to root respiration while maintaining storage in Qinghai spruce seedlings Trunk respiration held steady until trees were severely dried out.43PubMed. Stem metabolism under drought stress – a paradox of increasing respiratory substrates and decreasing respiratory rates
Flooding starves roots of oxygen. Waterlogged soil shuts down normal root respiration,44PubMed Central. Keep Calm and Survive: Adaptation Strategies to Energy Crisis in Fruit Trees under Root Hypoxia so roots switch to fermentation, making energy without oxygen.45PubMed Central. Keep Calm and Survive: Adaptation Strategies to Energy Crisis in Fruit Trees under Root Hypoxia Producing ethanol leaks carbon the tree can’t use.46PubMed Central. Keep Calm and Survive: Adaptation Strategies to Energy Crisis in Fruit Trees under Root Hypoxia Flood-tolerant trees steer more of their fermentation toward a less harmful product.47PubMed. Differences in C metabolism of ash species and provenances as a consequence of root oxygen deprivation by waterlogging Extreme heat from fire can also impair respiration and build up ethanol, contributing to tree death.48CSIRO Publishing (International Journal of Wildland Fire). Methods to assess fire-induced tree mortality: review of fire behaviour proxy and real fire experiments
For a yard tree, flooding shows up as wilting or yellowing leaves even though the soil is soaked. The roots are suffocating, not drowning in water itself. Some species tolerate days of flooding while others suffer quickly. Improving drainage helps a tree that sits in wet soil.
Tree Respiration and the Climate
Most carbon forests return to the air comes out through the soil.49Forest Ecology and Management. Partitioning of soil respiration into its autotrophic and heterotrophic components by means of tree-girdling in old boreal spruce forest Soil CO2 comes both from decomposers and from roots and fungi fed by fresh sugar.50Forest Ecology and Management. Partitioning of soil respiration into its autotrophic and heterotrophic components by means of tree-girdling in old boreal spruce forest Cutting sugar flow to roots in an old spruce forest cut soil respiration by up to 53%.51Forest Ecology and Management. Partitioning of soil respiration into its autotrophic and heterotrophic components by means of tree-girdling in old boreal spruce forest Roots and partners make up about 53% of soil respiration in northern forests versus about 44% in temperate ones.52Forest Ecology and Management. Partitioning of soil respiration into its autotrophic and heterotrophic components by means of tree-girdling in old boreal spruce forest In ponderosa pine, soil respiration was lowest in winter and highest in late spring.53Global Change Biology. Seasonal and annual respiration of a ponderosa pine ecosystem
Plants grown in high CO2 don’t generally respire less per gram.54PubMed Central. Plant Respiration and Elevated Atmospheric CO2 Concentration: Cellular Responses and Global Significance Whole forest canopies didn’t raise respiration as much as they gained mass, and more respiration shifted to roots.55PubMed Central. Plant Respiration and Elevated Atmospheric CO2 Concentration: Cellular Responses and Global Significance Sweetgum trees in high CO2 raised trunk respiration because leaves supplied more sugar.56New Phytologist. Stem respiration increases in CO2‐enriched sweetgum trees And because trees acclimate to warming, respiration may add less CO2 to the air than once predicted.57PubMed. Boreal and temperate trees show strong acclimation of respiration to warming
For planting and protecting forests, this means trees store carbon only in the share not burned. Healthy, well-watered young forests grow the most per year, while old forests store huge amounts even if they add less each year.