How a tree's photosynthesis changes through the year, from leaf-out to leaf fall

A deciduous tree in the East makes nearly all of its year’s sugar between leaf-out and leaf fall. In an oak forest in southern Ontario, photosynthesis began between mid-April and early May and ended between late October and mid-November. The forest’s net gain peaked in early June and its gross photosynthesis in July. Each leaf’s photosynthetic capacity tops out within days of the June solstice, then slides as the days shorten while the leaf stays green. Evergreens and the green layer under bark keep some photosynthesis going in the other months, and understory shrubs leaf out weeks ahead of the canopy to catch the spring sun.

The year in five stages:

A new leaf spends sugar before it makes any

Spring growth runs on sugar stored the year before. A white oak followed through one spring began growing new roots on March 3 and new wood on March 26, and its flowers, leaves and shoots started on April 10. The study’s summary puts the order plainly:

“Examination of the relationship between photosynthesis and growth of a dominant white oak […] tree has shown that most growth processes were either completed or well underway before the establishment of significant positive rates of net photosynthesis.” (Dougherty, Teskey, Phelps and Hinckley, Net Photosynthesis and Early Growth Trends of a Dominant White Oak, 1979)

The oak’s new leaves began to show a net gain once they reached about 15 percent of their final size. Until the leaves are out, a whole forest gives off more carbon than it takes in.

In five species at Harvard Forest, the new leaves were paid for from the branches, while the roots’ stores were not used for spring growth. Red oak branches gave up 70 percent of their stores over winter and spring and refilled them by autumn. White oak and northern red oak are the oaks of these two studies. When oaks leaf out covers the timing of bud break, and why new leaves come out red the red flush.

The peak comes in June and July

In a southern Appalachian forest in North Carolina, the strongest, steadiest net gain came in the weeks after full leaf-out, late May to mid-June. In the Ontario forest, respiration peaked in July along with gross photosynthesis, in the warmest air and soil of the year.

A leaf’s own capacity follows the day length more than the heat. William Bauerle and colleagues followed it in 23 tree species, among them Nuttall oak and black gum. It peaked within days of the longest day of the year, while air temperature peaked about a month later.

Heat and dry soil close the pores

A leaf takes in carbon dioxide through stomata, pores in its skin that also let water out. In drought or high heat the stomata close to save water, and photosynthesis slows, or stops, while they are shut. A tree short of water may close them during the day. Most plants photosynthesize fastest between 65 and 85°F. Leaves deep in the crown are built for dim light, as sun leaves and shade leaves sets out.

Late summer: the leaves stay green as the rate falls

From late June a leaf’s capacity declines with the shortening days, though the leaf looks no different. The trees in Bauerle’s study had ample water and nutrients, so the decline was not stress, and their leaves began to fall about five weeks after photosynthesis had stopped:

“Although leaves remain green, photosynthetic capacity peaks just after summer solstice and declines with decreasing photoperiod, before air temperatures peak.” (Bauerle and others, Photoperiodic regulation of the seasonal pattern of photosynthetic capacity and the implications for carbon cycling, 2012)

In the North Carolina forest, daily net uptake from early August to mid-September fell a third below its early-summer pace in two cooler years and by more than half in two warm ones.

A 2026 study of eight oak species found their wood growth finished by the end of July, although about 36 percent of the year’s photosynthesis at its eastern sites came later. Their wood forms only while the air is cool and humid enough. Meanwhile the tree’s stores keep filling, from their low in April to their high in October. What a leaf does in its last weeks, and why some leaves turn red, is in why leaves turn red and yellow in fall.

What evergreens and early shrubs do while the canopy rests

Rosebay rhododendron can keep a leaf for seven or eight years under a canopy, and it rolls its leaves up in cold and in drought. In the North Carolina forest, which has dense rhododendron thickets under its canopy, warm late winters raised the spring’s photosynthesis, partly through the evergreens’ own uptake.

Evergreen conifers hold their needles through winter, but in a cold climate they stop photosynthesizing for the winter months. Where winters are mild they keep photosynthesizing at a reduced rate. In a Maine forest dominated by red spruce, the needles were more than halfway back to their summer green, which tracks the restart of photosynthesis, before the new buds broke.

Deciduous shrubs of the understory race the canopy instead, since less than 2 percent of open sunlight reaches them once the trees above are in full leaf. These leaf out early:

• Spicebush: leafs out weeks before the canopy trees. Zones 4 to 9, 6 to 12 feet.
• Red buckeye: leafs out and flowers before the canopy trees are in full leaf, and often drops its leaves by late summer. Zones 4 to 9, 10 to 25 feet.
• Painted buckeye: its leaves open in March, among the first in the woods, and usually drop by September. Zones 6 to 8, 10 to 20 feet.
• Ohio buckeye: among the first trees to leaf out, and a late frost sometimes kills its young leaves. Zones 3 to 7, 20 to 40 feet.

Bare twigs keep a little photosynthesis going too, through the green layer under the bark that photosynthesis without leaves describes.

What this means for a tree you planted

Blue Whale Trees ships live plants only while they are dormant, in spring and fall, so a deciduous sapling opens its first leaves in your ground. Water through its first dry spells keeps those leaves working, since a leaf short of water closes its stomata and slows or stops making sugar. Summer water also feeds the next year, because a tree forms next year’s leaf buds in the current summer, and a dry summer can leave fewer and smaller buds.

The watering calculator puts a bare-root sapling past its twelfth week on one watering a week in the growing season until it is established, about a year and a half per inch of trunk caliper and at least a year. After that it needs water only in a drought. The rest of the first five years is in the care calendar for a new tree.

Common questions

When do trees photosynthesize the most?

In June and July, once the leaves are full size and the days are longest. A deciduous forest’s net gain peaks in early June and its gross photosynthesis in July.

Do trees photosynthesize in winter?

A leafless deciduous tree makes almost nothing in winter, apart from the green layer under its bark, which mostly recycles carbon dioxide the tree has already respired. It lives on its stores until spring.

Do evergreens photosynthesize in winter?

In a cold climate evergreen conifers keep their needles but stop photosynthesizing for the winter months. Where winters are mild they keep photosynthesizing at a reduced rate.

Why do trees stop growing in summer?

Oaks make wood only while the air is cool and humid enough, and they finish by the end of July although about a third of their photosynthesis is still to come. Their stores keep filling until October, ready for next spring’s leaves.

Sources

• Warmer temperatures reduce net carbon uptake, but do not affect water use, in a mature southern Appalachian forest: A.C. Oishi, C.F. Miniat, K.A. Novick and colleagues, Agricultural and Forest Meteorology 252 (2018): the Coweeta forest’s photosynthesis near zero while dormant and rising with leaf expansion, its strongest net uptake in the weeks after full leaf-out, the late-season fall in cooler and warm years, and evergreen uptake under rosebay rhododendron thickets in warm late winters.
• The Impact of Seasonal and Annual Climate Variations on the Carbon Uptake Capacity of a Deciduous Forest Within the Great Lakes Region of Canada: E.R. Beamesderfer, M.A. Arain, M. Khomik and J.J. Brodeur, Journal of Geophysical Research Biogeosciences 125 (2020): the start and end of photosynthesis in a southern Ontario oak forest, its peak net gain in early June, gross photosynthesis and respiration peaking in July, and the forest a carbon source until leaf-out.
• Photoperiodic regulation of the seasonal pattern of photosynthetic capacity and the implications for carbon cycling: W.L. Bauerle, R. Oren, D.A. Way and colleagues, Proceedings of the National Academy of Sciences 109 (2012): leaf capacity in 23 tree species peaking near the solstice and declining from late June in green, well-watered leaves, a month before air temperature peaked, and leaf fall five weeks after photosynthesis stopped.
• Seasonal variation in the canopy color of temperate evergreen conifer forests: B. Seyednasrollah and colleagues, New Phytologist 229 (2021): conifer forests dormant through cold winters and working at a reduced rate where winters are warm, and the red spruce forest at Howland, Maine, greening more than halfway before budburst.
• Phenoseasonal subcanopy light dynamics and the effects of light on the physiological ecology of a common understory shrub, Lindera benzoin: J.E. Hudson, D.F. Levia, S.A. Hudson, H.P. Bais and D.R. Legates, PLOS One 12 (2017): spicebush leafing out weeks before the canopy trees, and less than 2 percent of open light reaching the understory in the leafed months.
• Whole-tree nonstructural carbohydrate storage and seasonal dynamics in five temperate species: M.E. Furze, B.A. Huggett, D.M. Aubrecht, C.D. Stolz, M.S. Carbone and A.D. Richardson, New Phytologist 221 (2019): stores drawn on for respiration in the dormant season, lowest in April and highest in October, red oak branches down 70 percent and refilled by autumn, and root reserves left alone in spring.
• Net Photosynthesis and Early Growth Trends of a Dominant White Oak: P.M. Dougherty, R.O. Teskey, J.E. Phelps and T.M. Hinckley, Plant Physiology 64 (1979): a white oak’s roots, wood, then leaves and shoots starting between March 3 and April 10, growth well under way before net photosynthesis, leaves in net gain at 15 percent of final size, and root and wood growth pausing while the leaves expanded.
• Decoupled carbon assimilation and growth responses to aridity in temperate deciduous oaks: M.P. Rao and colleagues, Science Advances 12 (2026): eight oak species finishing their wood growth by the end of July while about 36 percent of the year’s photosynthesis at the eastern sites came after July, and wood forming only in cool, humid weather.
• 3. Botany: NC State Extension, Extension Gardener Handbook: carbon dioxide entering and water leaving through the stomata, stomata closing in drought and heat, photosynthesis slowing or stopping while they are shut, and the 65 to 85°F range of fastest photosynthesis.
• Rhododendron maximum, Fire Effects Information System: USDA Forest Service: rosebay rhododendron’s leaves rolling up in cold and drought and lasting seven or eight years under a canopy.
• How to Water Trees and Plants: The Morton Arboretum: next year’s leaf buds formed in the current summer, and fewer and smaller buds after a dry one.