1. What Happens to Chlorophyll, the Green Pigment, in Autumn Leaves?
The green in autumn leaves does not fade because a new color is painted on top. The green comes from chlorophyll, a pigment. As autumn approaches, this pigment breaks down and becomes less abundant [2][3]. Why does the tree break it down? It is to recover materials from the leaf and move them to the trunk and branches, saving them for new leaves in the spring [2][3]. Inside the leaf, chlorophyll is attached to proteins that build the photosynthesis machinery. To recover the proteins, the pigment must be detached and destroyed first [2]. Furthermore, detached chlorophyll can damage cells if it is exposed to light, so it is quickly converted into a harmless form [3][4].
The broken-down chlorophyll becomes small, colorless molecules stored inside the leaf cells [3][4]. The tree recovers the proteins, but the waste products from the pigment breakdown fall to the ground with the leaf [2]. In an experiment with fullmoon maple seedlings, nitrogen levels in summer leaves were about 30 mg per 1 g of dry weight. In fallen leaves, this dropped to about 10–15 mg [5]. Think of it like packing up a shop: valuable items are moved out first, and clutter is cleared. However, the tree does not "think" about this; it is a set biological process. What remains after the green disappears?
2. Where Does the Yellow Color of Autumn Leaves Come From?
When the green fades, leaves of trees like ginkgo and hackberry turn yellow [2]. This yellow color is not created in autumn. It comes from carotenoids, yellowish pigments that have been present in the leaves since spring [2][3]. During summer, the large amount of green chlorophyll simply hides the yellow [2][3]. A 2000 review summarizes that autumn coloring is mainly caused by the decrease of chlorophyll and the partial retention of carotenoids [3]. The Japan Meteorological Agency defines the "yellowing day" for ginkgo as the first day when most leaves are yellow and green is hardly visible, viewed as a whole tree [6]. It is not about how much yellow increased, but whether the green has vanished. So, where does the red come from?
3. How Do Leaves Make Red Pigment in Autumn?
Red and yellow are not just different intensities of the same change. Yellow is a pre-existing color that becomes visible. Red is a new color made in autumn. The red pigment is called anthocyanin. In trees that turn red, this pigment is almost absent in summer leaves and is produced in autumn [3][2]. The raw material for this pigment is sugar made by photosynthesis [7][8]. Sunlight is also required [2]. In fullmoon maples, outer leaves in the sun change color first, while inner leaves stay green longer. Eventually, the inner leaves become a yellowish red with less red pigment [5]. Even on the same tree, color progression depends on sun exposure.
Whether a tree turns red or yellow also depends on its species. The Japan Meteorological Agency uses *Iroha* maple for observation, which turns red. In Hokkaido, one substitute species is the *Itaya* maple, which turns yellow [6]. The Japanese Society of Plant Physiologists explains that in yellow-turning trees, one of the genes for making red pigment may be missing or suppressed [9]. Some trees, like the konara oak, turn brown [2]. But why would a leaf that is about to fall bother to make red pigment?
4. Why Do Trees Make Red Pigment for Leaves That Will Fall?
Why do trees create red pigment for leaves they are about to drop? There is no final answer yet. A 2009 review states that autumn coloring is not just a side effect of aging; there are several hypotheses, but the adaptive meaning remains a mystery [10]. One theory is the "sunscreen" hypothesis. It suggests red pigment blocks strong light, preventing leaf damage [2]. However, the Japanese Society of Plant Physiologists notes that evidence for this protective function in living plants is not yet sufficient [9]. Research on fullmoon maples proposed that red pigment might prevent aging from accelerating due to excessive sugar accumulation [7].
A second theory is the "signal to insects" hypothesis. Published in 2001 by evolutionary biologists Hamilton and Brown, it suggests that bright colors are a hard-to-fake signal that the tree is investing in defense against insects [11]. According to this idea, aphids avoid trees with bright colors. The researchers showed that trees with more aphid species tend to have stronger autumn colors [11]. Which theory is correct? It is still undecided [10].
Note: Remember that the theory about bright colors signaling defense to insects is just one idea. When observing, always stay safe.
5. Why Do Mountains Change Color From the Top Down?
Mountains change color from the top down because lower temperatures trigger coloring earlier. The Japan Meteorological Agency explains that coloring starts sooner when temperatures are low and later when they are high [12]. In Japan, temperature is said to drop by about 0.5–0.6°C for every 100 m (about 330 ft) increase in elevation [13]. So, a place 500 m (about 1,640 ft) higher is roughly 2.5–3°C cooler. A common rule of thumb for coloring is the daily minimum temperature. A guide from Fukuoka Prefecture suggests coloring begins when the minimum temperature drops to 8°C or below, and progresses further at 5–6°C or below [2]. This varies by tree and location.
A study using data from over 740 citizen observation sites in Japan found that the start of peak coloring had a linear relationship with a "warmth index" (based on monthly average temperatures), whether moving north or up mountains [14]. The reaction to temperature was similar for both latitude and elevation. However, the effect of elevation weakened as the coloring season progressed [14].
6. Why Does Autumn Leaf Color Differ From Year to Year?
The timing of coloring varies by year. From 1965, the Japan Meteorological Agency issued annual forecasts for the best viewing time in the Kanto region in September [15]. The 2005 forecast predicted a later season because September temperatures were higher than average, and October and November were also forecast to be warm [12]. Temperature is the main factor. Over the long term, dates are getting later. Observations since 1953 show that maple coloring dates have delayed by about 3.1 days per decade [16]. Long-term temperature rise is thought to be the cause [16]. A 2023 study of forests in the Northern Hemisphere found that warming before the summer solstice accelerated the start of leaf aging, while warming after the solstice lengthened the aging period [17]. Summer heat and autumn heat can work in opposite directions.
Vibrancy is linked to large differences between day and night temperatures and good sunlight [2]. Even in the same year and place, trees differ. A study of 16 sugar maples found that trees with less nitrogen in their leaves turned red earlier and more widely [18]. The year’s colors are a combination of temperature, sunlight, and tree condition.
7. How Can You Observe Autumn Leaf Color Yourself?
If you want to investigate, pick one tree in a nearby park or school. Compare the color of leaves on the sunny side and the shady side every week. Record the date, weather, and minimum temperature. You can find minimum temperatures on the Japan Meteorological Agency’s "Past Weather Data Search" [20]. Compare these with the start of color change, using the 8°C and 5–6°C guidelines as rough references [2]. The Agency’s "Phenological Observation Information" lists coloring dates for maples and ginkgo by location [19]. Choose a nearby spot and compare data over several years.
You can also read the *Man'yoshu* poems about "momiji" in annotated books. The single poem said to use the characters for "red leaves" is No. 2201 in Volume 10. However, some editions use the characters for "yellow leaves" for this poem [1]. What color did the poet see?
Safety note: Observe during daylight with an adult. Stay in safe places like parks, schools, or near home. Do not go near roads, rivers, cliffs, or mountain trails. Do not climb trees or break branches. Look only at fallen leaves. Do not touch leaves from unknown trees.