Each fall color comes from a different pigment, and the green leaf of summer is simply the one that hides the rest. That means much of autumn’s color is less something new appearing than green disappearing. The reds are the exception, because some trees spend energy making them fresh just before the leaves drop.1PubMed Central. Anthocyanin Biosynthesis Associated with Natural Variation in Autumn Leaf Coloration in Quercus aliena Accessions It’s also why scientists still argue over why trees bother to turn red at all.2PubMed Central. Are Autumn Foliage Colors Red Signals to Aphids?
Where the Yellows and Oranges Come From
Leaves contain yellow and orange pigments all through the growing season.3PubMed Central. What Are the Colors of Fall and How Do They Happen? You don’t see them in summer because the much stronger green of chlorophyll, the pigment that captures sunlight for food, masks them.4PubMed Central. Chemistry of Autumn Colors: Quantitative Spectrophotometric Analysis of Anthocyanins and Carotenoids and Qualitative Analysis of Anthocyanins by Ultra-performance Liquid Chromatography–Tandem Mass Spectrometry The most common carotenoids are beta-carotene, the same pigment that makes carrots orange, and lutein.5PubMed Central. What Are the Colors of Fall and How Do They Happen? Lutein looks yellow when there is little of it and orange-red when there is a lot.6PubMed Central. What Are the Colors of Fall and How Do They Happen?
In autumn both the green and the yellow pigments break down.7PubMed Central. Chemistry of Autumn Colors: Quantitative Spectrophotometric Analysis of Anthocyanins and Carotenoids and Qualitative Analysis of Anthocyanins by Ultra-performance Liquid Chromatography–Tandem Mass Spectrometry The green goes faster, so yellow takes over.8PubMed Central. Chemistry of Autumn Colors: Quantitative Spectrophotometric Analysis of Anthocyanins and Carotenoids and Qualitative Analysis of Anthocyanins by Ultra-performance Liquid Chromatography–Tandem Mass Spectrometry Some yellow pigments linger longer than others as the leaf ages.9Plant Physiology. A cellular timetable of autumn senescence In ginkgo, yellow pigment levels even rise while the leaves are turning, then fall as the leaf goes fully yellow.10Horticultural Plant Journal. Physiological and Transcriptomic Changes During Autumn Coloration and Senescence in Ginkgo biloba Leaves
The tree keeps these pigments for a reason. These pigments act like sunscreen, shedding excess light energy as heat.11PubMed Central. What Are the Colors of Fall and How Do They Happen? Once chlorophyll is gone, that protects a fading leaf from sun damage.12PubMed Central. What Are the Colors of Fall and How Do They Happen?
Green often fades first at the edges or between the veins, leaving patchy yellow. Because the pigment was already there, yellow trees like birches and ginkgos look much the same every year, unlike the year-to-year swings in reds.
Where the Reds and Purples Come From
Red and purple come from pigments most trees don’t have in their summer leaves.13The Source. Why is the North American fall so red, compared with Europe? Red-leaved trees switch on the whole chain of genes that builds them just as the leaves begin to turn, as shown in oaks.14PubMed Central. Anthocyanin Biosynthesis Associated with Natural Variation in Autumn Leaf Coloration in Quercus aliena Accessions In sugar maple, only red leaves held high levels of the red pigment.15Trees. Association of red coloration with senescence of sugar maple leaves in autumn Red and yellow maple leaves both had less green than green leaves, so both were further along in shutting down.16Trees. Association of red coloration with senescence of sugar maple leaves in autumn
The acidity of the sap inside the leaf shapes the shade.17Minnesota Department of Natural Resources. The science behind fall colors More acidic sap gives red, while less acidic sap shifts toward purple or blue.18Minnesota Department of Natural Resources. The science behind fall colors That’s one reason a red maple and a purple-leaved sweetgum can look so different, and why the same tree can look redder or more purple from one year to the next. Burgundy shades often come from red pigment mixed with leftover green or brown.
The leading idea is that red pigment shields the leaf from strong light.19PubMed. Trees growing in Eastern North America experience higher autumn solar irradiation than their European relatives, but is nitrogen limitation another factor explaining anthocyanin-red autumn leaves?: A comment on Peña-Novas and Marchetti 2021 Red leaves can then keep pulling nutrients back into the tree longer than yellow or brown ones.20PubMed. Trees growing in Eastern North America experience higher autumn solar irradiation than their European relatives, but is nitrogen limitation another factor explaining anthocyanin-red autumn leaves?: A comment on Peña-Novas and Marchetti 2021 The shielding works best under bright sun combined with cold.21PubMed. The occurrence of red and yellow autumn leaves explained by regional differences in insolation and temperature
Where the Browns Come From
Browns come from waste compounds called tannins that are also hidden by green all summer.22Lee Reich Garden. Autumn’s Leaves They give the season its muted tans and browns, especially in some oaks.23Lee Reich Garden. Autumn’s Leaves Tannins also add a warm bronze tone to beech yellows.24Lee Reich Garden. Autumn’s Leaves Oaks hold on to their light-collecting machinery longer than maples do, and lean on yellow pigment late in the season.25PubMed. Different strategies for photoprotection during autumn senescence in maple and oak
An oak that turns russet or tan every autumn is showing its usual tannin color, which is normal for the species and no sign of trouble. A maple that skips its color and goes crisp brown is more likely stressed, with drought a common cause, since the leaves die before they can build or reveal their colors. Stressed leaves often brown early, in late summer, and may drop quickly, while healthy browning arrives with the rest of the season.
What Tells a Tree to Start Changing
Days shorten from late June to late December, and trees use that as a cue to prepare for winter.26EarthDate. What Are the Colors of Fall and How Do They Happen? They slow, then stop, making chlorophyll.27EarthDate. What Are the Colors of Fall and How Do They Happen?
Temperature counts too. Across many tree records, shortening days started the change about 6 times in 10, and colder nights about 4 in 10.28Agricultural and Forest Meteorology. A new process-based model for predicting autumn phenology: How is leaf senescence controlled by photoperiod and temperature coupling? Day length tended to rule where summer days are shorter, and cold nights where summer days are longer.29Agricultural and Forest Meteorology. A new process-based model for predicting autumn phenology: How is leaf senescence controlled by photoperiod and temperature coupling? Across Europe, the start of leaf aging drifted later from 2001 to 2019, by about half a day a year in most areas.30Agricultural and Forest Meteorology. Photoperiod alone does not explain the variations of leaf senescence onset across Europe Day length alone can’t be the whole story, since it hasn’t changed but the timing has. Day length is identical every year on a given date, and temperature nudges the timing earlier or later.
The drop itself depends on a breakaway layer the leaf builds at its stem, which lets it fall.31PubMed. Synchrony in fall leaf drop: chlorophyll degradation, color change, and abscission layer formation in three temperate deciduous tree species In Japanese maple, this layer followed a fixed schedule whether the leaf was in sun or shade.32Trees. Timings of leaf abscission, nitrogen resorption, and autumn coloration across contrasting canopy positions in Acer japonicum Red color, though, depended strongly on light, so sunny leaves reddened more.33Trees. Timings of leaf abscission, nitrogen resorption, and autumn coloration across contrasting canopy positions in Acer japonicum Green fading and nutrient recovery kept pace with the breakaway layer forming.34Trees. Timings of leaf abscission, nitrogen resorption, and autumn coloration across contrasting canopy positions in Acer japonicum
Why Some Autumns Are Brighter Than Others
Red pigment does its protective job best under bright light and low temperatures.35PubMed. The occurrence of red and yellow autumn leaves explained by regional differences in insolation and temperature A run of sunny days and cool, not freezing, nights tends to bring stronger reds.
Drought works the other way. In dry years trees change color earlier and drop leaves faster, because water-stressed trees shut down early to save water.36Martha Stewart. Fall Foliage Could Look Different This Year—Here’s What to Expect Drought also lowers the sugar trees make in summer, which means duller reds and oranges.37Martha Stewart. Fall Foliage Could Look Different This Year—Here’s What to Expect Drought-stressed leaves are more likely to go brown with only a brief window of color.38Bartlett Tree Experts. What Are the Colors of Fall and How Do They Happen?
An early frost can knock leaves down before they finish coloring.39St. Olaf Natural Lands. More Than Just a Pretty View: The Life Cycle of a Fall Leaf Heavy rain and summer heat stress tend to bring earlier color and earlier leaf drop.40Agricultural and Forest Meteorology. Predicting autumn phenology: How deciduous tree species respond to weather stressors Ordinary warm spells and mild drought, though, delay fall timing for most species.41Agricultural and Forest Meteorology. Predicting autumn phenology: How deciduous tree species respond to weather stressors
For a tree in your yard, water deeply during late-summer dry spells and mulch the root zone to hold moisture. A stressed tree may color early or poorly, and good care can help it recover.
Why North America Is Redder Than Europe
Autumn is much redder in North America and East Asia than in northern Europe.42the Guardian. Why are autumn leaves mostly yellow in Europe and red in North America? Related species tend to split, with North American ones making red pigment and their European relatives not.43The Source. Why is the North American fall so red, compared with Europe?
One explanation is sunlight. Eastern North America and Asia get stronger autumn sunlight than Europe.44PubMed. The occurrence of red and yellow autumn leaves explained by regional differences in insolation and temperature Red’s sun-shielding benefit would be worth more there.
Another idea is that bright red warns insects that a tree is well defended.45PubMed Central. Autumn tree colours as a handicap signal Europe lost more tree lineages over time.46the Guardian. Why are autumn leaves mostly yellow in Europe and red in North America? By the time many red-leaved European trees vanished, the leaf-eaters that favored red may also have gone.47the Guardian. Why are autumn leaves mostly yellow in Europe and red in North America? Current knowledge, though, does not support red leaves being an effective aphid deterrent.48PubMed Central. Are Autumn Foliage Colors Red Signals to Aphids?
Soil may play a part as well. In red maple, links between color and soil acidity may really reflect phosphorus shortage, which tends to bring more red pigment.49PubMed Central. Early Autumn Senescence in Red Maple (Acer rubrum L.) Is Associated with High Leaf Anthocyanin Content
When Color Peaks and How It Is Shifting
In one Ohio town, the growing season ran about 20 days longer than a century earlier.50PubMed Central. A century of climate warming results in growing season extension: Delayed autumn leaf phenology in north central North America In New England, warming is expected to push leaf color and leaf fall later for most species.51PubMed Central. Predicting Climate Change Impacts on the Amount and Duration of Autumn Colors in a New England Forest Peak foliage in Acadia National Park in Maine is projected to fall between October 30 and November 2 by 2060.52Landscape Ecology. The effects of climate change on the timing of peak fall foliage in Acadia National Park
The length of the show may change too. Some trees, like sugar maple and black cherry, are expected to keep color longer.53PubMed Central. Predicting Climate Change Impacts on the Amount and Duration of Autumn Colors in a New England Forest White ash is expected to color for a shorter time.54PubMed Central. Predicting Climate Change Impacts on the Amount and Duration of Autumn Colors in a New England Forest More summer heat stress is expected to shorten color seasons for most species.55Agricultural and Forest Meteorology. Predicting autumn phenology: How deciduous tree species respond to weather stressors
Satellite data can now track and forecast foliage from low color to peak to past peak in real time.56Agricultural and Forest Meteorology. Prototype for monitoring and forecasting fall foliage coloration in real time from satellite data That makes trip planning easier. Peak color moves across a region over several weeks, in much of the Northern Hemisphere generally traveling south and from higher to lower ground, so the same weekend can be past peak in one place and early in another. Higher elevations and northern areas turn first. If you are early, drive up in elevation to find color, and if you are late, drive down. Peak at any single spot lasts only about a week or two, so a fixed date is a gamble; check regional foliage reports a week or two ahead, and again before you go.
Why Some Trees Hold Dead Leaves All Winter
Some deciduous trees, notably oaks and beeches, keep dry brown leaves through winter.57Ecosphere. Not all temperate deciduous trees are leafless in winter: The curious case of marcescence Proposed reasons include deterring deer, protecting buds from frost or drying, and better nutrient recovery.58Ecosphere. Not all temperate deciduous trees are leafless in winter: The curious case of marcescence It may also be a leftover trait with no purpose.59Ecosphere. Not all temperate deciduous trees are leafless in winter: The curious case of marcescence
The leaves seem to matter for the soil. Oak and beech leaves that weathered in sun before falling broke down much faster, losing 37% of their mass versus 23%.60Soil Biology and Biochemistry. Marcescence in temperate regions mediates tree litter decomposition through abiotic factors Leaves left hanging may therefore feed the soil faster once they drop.
Young oaks and beeches often keep their dead leaves, and this is normal for those species, not a sign of disease. The leaves usually drop in spring as the new buds swell and push them off, so an oak or beech holding leaves in March is usually on its normal schedule. A maple that normally drops its leaves but keeps them after a sudden early freeze is different; frost may have killed the leaves before they could build their breakaway layer. If a tree like that holds many leaves all winter, check the previous autumn’s weather first.
Evergreens keep needles through winter by switching on lasting sun protection, using the same yellow pigment family as fall leaves.61PubMed. Photosynthesis of overwintering evergreen plants This lets them shed excess light as heat when cold stops food-making.62New Phytologist. Sustained energy dissipation in winter evergreens