The leaf as a sugar factory
Almost all the material in a tree’s wood, bark, roots and leaves comes, directly or indirectly, from the air. Trees take carbon dioxide from the atmosphere and combine it with water to make sugar, using the energy of sunlight. The oxygen released is a by-product. This is photosynthesis, and the leaf is the main place where it happens.
It is easy to think a tree is made from soil. It is not. A tree’s dry mass is built mostly from carbon taken from carbon dioxide in the air, with water supplying hydrogen and oxygen. The soil provides water and a relatively small amount of mineral nutrients. This is why forests figure so prominently in the global carbon cycle.
What goes in and what comes out
In simple terms:
- In: light, carbon dioxide (through stomata) and water (from the roots).
- Out: sugars, which the tree uses or stores, and oxygen.
The process takes place in structures inside leaf cells called chloroplasts, which hold the green pigment chlorophyll. Chlorophyll absorbs mostly red and blue light and reflects green, which is why leaves look green. In the first stage, light energy is captured and used to split water. In the second stage, the captured energy is used to build sugar from carbon dioxide.
Breathing and water loss
Carbon dioxide enters through stomata, the tiny pores on the leaf surface, which are most often on the underside. This creates a dilemma. Whenever the pores open for carbon dioxide, water vapor escapes. The tree must balance both: open enough to feed itself, closed enough to avoid drying out. The water side of this trade-off, including how water is pulled up through the trunk, is a separate story of water transport.
On a hot, dry day many trees close their stomata partly, often in the afternoon, and photosynthesis slows even if there is plenty of light. Under prolonged drought stress, the effect can last for weeks and cut sugar production substantially.
What happens to the sugar
The sugar is not simply stored. A tree uses it in several ways:
- Respiration. Like animals, trees burn sugar for energy in all living cells, including roots, wood and leaves. A considerable share of what the leaves make is used up this way. The difference between what is made and what is respired is the net gain.
- Growth. Sugar is converted into cellulose and other materials to build new leaves, roots, and wood.
- Storage. Trees store reserves as starch and other compounds, often in roots and the sapwood, to support the next spring’s growth or to recover after damage.
- Defense and reproduction. Part is spent on resins and other chemicals, and on flowers, fruit and seeds.
- Sharing with partners. Some goes to fungi in the roots, through mycorrhizal fungi.
Sugars travel from leaves to the rest of the tree through the inner bark, in a tissue called phloem.
Sun leaves and shade leaves
Within a single tree, leaves are not all the same. Leaves in the bright upper crown, called sun leaves, tend to be smaller, thicker and sturdier, with more layers of photosynthetic cells. They can use strong light effectively. Leaves in the shaded lower crown, called shade leaves, are generally larger and thinner, with more chlorophyll per unit of weight, so they can capture the little light that reaches them. This is a sensible adjustment: it would be wasteful to build a thick, expensive leaf in dim conditions.
Even so, photosynthesis in a leaf has an upper limit. Past a certain light level, additional light adds little, because the chemical machinery is already working at capacity. In deep shade, a leaf may barely make enough sugar to cover its own respiration.
Shade changes the forest
Light is among the most important limits in a dense forest. A closed canopy intercepts most of the sunlight, and the light that reaches the ground is both dimmer and different in quality: it has been filtered through leaves, so it contains relatively less red and blue and more far-red light. Plants can detect this change and respond.
The result is a sorting of species:
- Shade-intolerant species, often called pioneers, grow quickly in full sun but need open conditions. They tend to fail in deep shade.
- Shade-tolerant species grow slowly but can survive for years with little light, ready to respond when a gap opens. Examples vary by region.
Many seedlings in the dim understory plants zone spend years waiting, growing slowly, until a tree falls and opens a canopy gap. A sudden increase in light can then trigger a burst of growth. Sunflecks, brief patches of direct sun moving across the floor, can supply a surprisingly large share of the light that shaded plants receive.
Seasons and evergreen leaves
Leaves are costly to build and run, so trees must earn back that cost in sugar. Deciduous trees in temperate climates make a thin, efficient leaf that works hard during the warm months, then drop it. Evergreens invest in tougher, longer-lived leaves or needles that photosynthesize at lower rates but can work whenever conditions allow, including on mild winter days. These are different solutions to the same problem and explain much of the contrast between deciduous forests and boreal conifer stands.
The timing of leaf-out matters too. A canopy that opens earlier in spring may have a longer period to photosynthesize, which is why researchers pay attention to spring phenology as temperatures change.
Limits on photosynthesis
Photosynthesis in forest trees is limited by several factors that can act at different times:
- Light, especially in the understory and lower crown.
- Water, through stomatal closure in dry conditions.
- Temperature, since the process slows when it is too cold or too hot.
- Nutrients, especially nitrogen, which is a key part of the photosynthetic machinery.
- Carbon dioxide concentration, which in some experiments increases photosynthesis, although other limits such as nutrients and water often keep trees from fully benefiting.
- Leaf damage, from insects, disease, frost or air pollution.
Why it matters beyond the tree
Photosynthesis is the starting point of forest carbon storage. The carbon fixed by leaves is partly returned to the air through respiration and decomposition, and partly stored in wood, roots and soil. The net result, over time, is what is meant by carbon sequestration in forests. How much a given forest stores depends on all the limits above, which is why simple claims about forests absorbing a fixed amount should be taken with caution.