Quick answer
Trees make carbohydrates through photosynthesis. Chlorophyll in leaves and other green tissues captures light energy, which drives reactions that use carbon dioxide and water to form energy-rich sugars. Oxygen is released as a by-product. The tree then uses those carbohydrates for respiration, growth, storage, defence and reproduction.
Key points
- Photosynthesis stores light energy in chemical bonds; it does not create matter from nothing.
- Most of a tree's dry mass ultimately comes from carbon dioxide in the air.
- Trees also need water and mineral nutrients, but soil is not their main source of carbon.
The basic photosynthesis equation
A simplified equation is: carbon dioxide plus water, using light energy, produces glucose and oxygen. Real photosynthesis is a multi-stage biochemical process rather than one reaction. Light-dependent reactions generate chemical energy, and the Calvin cycle incorporates carbon dioxide into organic molecules.
Glucose is a useful shorthand, but plants quickly convert photosynthetic products into sucrose, starch, cellulose, oils and many other compounds.
How carbon dioxide enters a leaf
Microscopic pores called stomata open and close in the leaf surface. Carbon dioxide diffuses inward while water vapour usually diffuses outward. Guard cells regulate the opening according to light, humidity, internal carbon dioxide, water status and chemical signals.
This creates a trade-off. Opening stomata supports photosynthesis but increases water loss. During drought or intense heat, a tree may close stomata, reducing both transpiration and carbon gain.
How water reaches the leaves
Fine roots absorb water from soil. Water enters xylem and is pulled upward as evaporation from moist cell walls inside leaves creates tension. Cohesion between water molecules and adhesion to conduit walls help maintain the water column.
Root damage, frozen soil, drought and xylem cavitation can interrupt supply. Leaves may wilt, scorch or be shed when demand exceeds transport capacity.
What happens to the sugar?
Sucrose moves through phloem from source tissues, usually mature leaves, to sinks such as developing leaves, roots, fruits, seeds and active cambium. The tree burns some carbohydrates through cellular respiration to release usable energy.
Other carbon becomes cellulose and lignin in wood, protective chemicals, nectar, root exudates or stored starch. Stored carbohydrates help a deciduous tree leaf out before its new leaves are fully productive.
Why leaves are shaped and arranged differently
Broad, thin leaves capture light efficiently but can lose water and suffer frost or wind damage. Needles reduce exposed area and often persist for several years. Leaf angle, crown architecture and seasonal leaf fall all influence the balance between light capture, cooling and water conservation.
Leaves within one crown are not identical. Sun leaves are often smaller and thicker; shade leaves may be broader and thinner. A canopy distributes foliage so that some light reaches lower layers rather than being absorbed only at the top.
Does more carbon dioxide always mean more growth?
Additional carbon dioxide can increase photosynthesis in some circumstances, but growth also requires water, nutrients, suitable temperature and healthy tissues. Heat, drought, pests, fire and nutrient limitation can cancel or reverse any fertilisation effect. Forest carbon storage also depends on mortality, decomposition and disturbance, not only leaf-level photosynthesis.
Frequently asked questions
Do trees get food from soil?
Trees obtain water and mineral nutrients from soil, but they build carbohydrates from atmospheric carbon dioxide through photosynthesis.
Do trees photosynthesise in winter?
Evergreens can photosynthesise when temperature, water and light permit. Leafless deciduous trees have little photosynthesis, although green bark and twigs may contribute small amounts.
Do red or purple leaves photosynthesise?
Usually yes. Other pigments can mask green chlorophyll, which remains active in the leaf.
Related Nature9 guides
- How to Identify Trees by Their Leaves
- How Trees Store Carbon and Affect the Climate
- Why Do Leaves Change Colour in Autumn?
- How Do Trees Breathe and Exchange Gases?
Sources and further reading
- IPCC AR6: Cities, settlements and key infrastructure
- FAO: Climate and ecosystem service benefits of forests and trees
- IPCC: Special Report on Climate Change and Land
Information is presented for general education. Tree health, structural safety, protected wildlife and regulated plant-health problems may require qualified local advice.