Sun leaves and shade leaves: why one tree grows two kinds of leaf, and what it means for a seedling moved into shade

A tree builds each leaf for the light it grows in: small, thick sun leaves at the top and outer edge of the crown, and large, thin shade leaves inside it or under taller trees. Each kind pays its way only in its own light, because extra thickness is wasted in dim light and a thin leaf in full sun takes in more light than it can use. A grown leaf can barely change its build, so a seedling moved into new light lives on its old leaves until new ones grow.

How the two differ, and why

A sun leaf has a thicker cuticle, the outer coat that holds water in, and longer palisade cells, the column-shaped cells under its upper surface. They are often stacked in more layers. A shade leaf has more chlorophyll for its weight. On an oak, a leaf from the sunny outside of the crown is also more deeply lobed, and one from deep inside is often a deeper green.

A leaf’s photosynthesis rises with light only up to a point, and a shade leaf reaches that point in dim light. A sun leaf also costs more to build and to run, so it needs brighter light just to break even. It loses more water through the pores it holds open for carbon dioxide.

A seedling moved into sun

In a shade leaf moved into full sun, the surplus light causes photoinhibition, and severe damage can bleach and kill its tissue. Sun and heat also draw water faster than a new planting’s roots can replace it, so scorch shows first at the tips and edges. In a study published in 2022, Tomasz Wyka and colleagues moved seedlings of a European maple from 1 percent of full sun into the open in August. Every leaf bleached and fell within a month.

Pawpaw bears its best crops in full sun, where it keeps its leaves to the ground, but its seedlings need shade:

“Shading for the first year, and sometimes the second, is normally required outside, and it is for this reason that Pawpaws are almost always found in nature as an understory tree.” (Kentucky State University, Pawpaw Planting Guide)

A seedling moved into shade, and the next leaves

A seedling moved from full sun into shade keeps sun leaves that cost more to run than dim light repays, so it grows slowly at first. Leaves that grow after a move develop for the new light. Blue Whale Trees ships live plants while they are dormant, as our shipping page says, so a deciduous seedling opens its first leaves in its new spot. A tree moved in full leaf has only its old leaves until the next flush, and their scorch is not the verdict:

“Scorch damage alone is insufficient to kill an otherwise healthy plant.” (The Morton Arboretum, Leaf Scorch)

Judge the tree by next spring’s leaves, and water it through its first summers with the watering calculator.

Matching a seedling to its spot

Every plant page rates sun on three bars, Full, Part and Shade, and some carry a Tolerates shade tag. The site check counts six or more hours of direct sun as full sun, and three to six hours or dappled light all day as part shade. It calls mountain camellia, a plant for part shade and shade, a struggle in full sun, where its leaves can scorch. The check weighs the range a species takes, so a pawpaw passes in full sun, though it wants shade in its first two summers.

• Check your species and spot in the site check.
• Find species for a shady spot in the tree finder.

Common questions

Why are my new tree’s leaves brown at the edges?

It is likely scorch: new roots cannot yet replace the water sun and wind draw from the leaves, so the edges dry first. Water deeply in dry spells and judge the tree by next spring’s leaves.

Can a sun-loving tree grow in shade?

In part shade it grows thin and leans toward the light, and in shade our site check expects it to decline within a few years. Choose a species rated for shade there.

Why is my tree dropping leaves after a move?

Leaves built for the old light can bleach or scorch in the new one, and the tree may shed them early. Its next leaves fit the new spot.

Sources

• Photosynthesis in sun and shade: Plants in Action, Australian Society of Plant Scientists: smaller leaves at the top of a crown and larger ones inside, the light-saturation point, the higher building and running cost of sun leaves, the water lost through their open pores, and bleaching and tissue death from excess light.
• Leaf anatomy, light interception and gas exchange: Plants in Action, Australian Society of Plant Scientists: longer palisade cells in more layers, thickness that pays only where light reaches the lower cells, the cuticle as a seal against water loss, and shade leaves larger and thinner with more chlorophyll for their weight.
• shade leaves: USA National Phenology Network: sun leaves on the top and outer edge of a crown and shade leaves within it, shade leaves larger, thinner, less deeply lobed and deeper green, and the thicker cuticle of sun leaves.
• Anatomical acclimation of mature leaves to increased irradiance in sycamore maple: Tomasz Wyka, Piotr Robakowski and Roma Żytkowiak, Photosynthesis Research 154 (2022): seedlings moved from 1 percent of full sun into the open in August that bleached and lost every leaf within a month, early leaf fall after a move from brighter shade, and new leaves that develop for new light.
• Advances in understanding canopy development in forest trees: USDA Forest Service copy of a chapter by W. Keith Moser and colleagues (Burleigh Dodds Science Publishing, 2020): the very limited capacity of grown leaves to change their anatomy, and flatter leaves in the shaded parts of a canopy.
• Pawpaw Planting Guide: Kentucky State University: shade for pawpaw seedlings in the first year or two, the flat leaves of a pawpaw in shade, and its dense, drooping foliage and best crops in full sun.
• Leaf Scorch: The Morton Arboretum: water loss the roots cannot replace, scorch at the tips and edges first, newly transplanted trees more prone to it, and scorch alone not killing a healthy plant.