It’s my favorite time of year: the leaves are beginning to change here in New England. Every year, I become obsessed with this seasonal fireworks display. I love putting together palettes that help me capture the bold yellows, oranges, and reds that briefly illuminate the trees before the bareness of winter.
But why does this happen? What is actually happening in the plants that causes the leaves to change color, and how can we use this knowledge to accurately paint leaf color – in every season?
The Science of Leaf Color
Leaf Pigments
Leaves produce three types of pigment.
- Chlorophyll (green) helps the plant absorb light and perform photosynthesis, the process that converts light into sugar – this is how plants “eat light.” Chlorophyll comes in two types, a and b, but I’ll treat them as one for simplicity in this post.
- Carotenoids (yellow), a group containing xanthopyll and carotenes, are also involved in photosynthesis, and have sun-protectant properties.
- Anthocyanins (red) have antioxidant and sun-protectant properties.
I’ve identified the colors these pigments appear to us, but note that these are the spectrums of light they’re reflecting. They’re absorbing the opposite – red light (for chlorophyll), blue/violet light (for carotenoids), or green/blue light (for anthocyanins).
Seasonal Changes
The relative amount of each pigment differs between species and over the growing season. Plants can use production of the different pigments to adapt to its environment. Let’s take a look at the lifespan of a deciduous tree, one that creates new leaves each spring and loses them in fall.
Spring
New leaves start with one type of pigment: chlorophyll, which is needed immediately to begin collecting food for the plant.
The “spring green” light green color is simply due to a lower concentration of pigment than more mature leaves, according to the American Institute for Physics article Spring Green: Why Do New Leaves Have Lighter Color? New leaves are also thinner, so the light color may be due in part to transparency.
Summer
As the season kicks into full swing, the plant is busily producing all of the needed pigments to efficiently perform photosynthesis and protect its leaves while it does. Chlorophyll is essential, and it’s also expensive; it’s constantly breaking down and needing to be replaced, so the plant is continuously producing more chlorophyll.
Darker summer leaf color is due to a higher concentration of overall pigment as well as changing balances in the percentage of chlorophyll, carotenoids, and anthocyanin. Carotenoids are there all season, but they aren’t easy to see while chlorophyll is also present because the green tends to overpower the yellow. Anthocyanin can add a brownish color to the leaf when paired with chlorophyll, but there usually isn’t all that much during the growing season.
As the days begin to the shorten after the equinox, the plant gradually winds down chlorophyll production. This reduces the percentage of green pigment, so more of the yellow and red shows through.
Autumn
At the end of the leaf’s lifespan, during a process called “senescence,” the remaining chlorophyll breaks down without being replaced. But rather than immediately discard the dying leaf, the tree will tend to hang onto its leaves until the chlorophyll is fully gone to reabsorb as much nitrogen as possible to store for winter, according to the Harvard Forest website The Science of Leaf Color Change.
Autumn Yellow
Yellow fall foliage occurs when the disappearance of green chlorophyll reveals the yellow carotenoids that have been there all along.
Autumn Red
Interestingly, in some plants, red anthocyanin is actively synthesized during senescence! As the plant is breaking down its chlorophyll and packing it in for the season, it’s also making new anthocyanin in its dying leaves. Why??
The reason is not fully understood, and it’s especially confusing because some trees do this and others do not. The prevailing theory is that the addition of anthocyanin helps trees protect vulnerable, chlorophyll-losing leaves from harsh sunlight while they reabsorb the nitrogen from broken-down chlorophyll, allowing them to more efficiently recoup their resources and prepare for next season. Some supporting facts:
- Red fallen leaves contain less nitrogen than yellow fallen leaves, suggesting the red-ness allowed the plant to remove more nitrogen before dropping the leaf.
- Plants that form symbiotic relationships with nitrogen-fixing bacteria don’t turn red in fall. They only turn yellow. This suggests that if you don’t need the extra nitrogen, the cost of producing anthocyanin isn’t worth it. This is true of legumes (e.g. locust), poplars, cottonwoods, alders, and mulberries.
- Red autumn color is most seen in harsh four-season climates with a lot of autumn sun, which describes eastern and midwestern North America. In the milder, cloudier climates of Northern Europe, red autumn color is rare. I remember feeling very blessed when leaf-peeping in Nova Scotia and running into a couple from England who were extremely wowed by red leaf color and blue jays. I endeavor to keep that sense of wonder about these marvels even though I live in this climate.
Autumn Brown
Some leaves appear brown if green and red pigment are present at high levels at the same time. But there’s another reason leaves can appear brown: tannins. Tannins aren’t pigments, but polyphenolic compounds (not that I know what that means). Tannins are brown in color and are important in the astringent, piquant taste of tea, chocolate, and wine. Tough to break down, tannins are found in high concentrations in some tough, leathery leaves, such as oak leaves.
Autumn Color Patterns
How leaf color occurs varies between species. In some trees, leaf color changes all at once; in others, different leaves will show a variety of different colors; and in others, the same leaf might show different patches of color.
Implications for painting
Spring
It is often assumed that spring greens should be painted with a lot of yellow. But according to this science, new leaves mostly contain green pigment; there’s just less pigment altogether compared to later in the year. This explains why spring greens look so cool compared to fall yellow-greens.
When self-mixing spring greens, I like to mix Phthalo Green Blue Shade with a cool lemon yellow. In terms of commercial light green mixes, I find Schmincke’s cool May Green (PG7, PY151) more accurate than Holbein or DV Leaf Greens which tend on the more yellow/orange side and may be more appropriate for fall.
As spring progresses, add more pigment, especially more of the green.
Summer
The science confirms what Jeanne Dobie figured out from experience: natural-looking leaf color is created with lots of green pigment, a moderate dose of yellow, and a tiny bit of red!
As the summer progresses, add more pigment in general to darken the color, and eventually begin ramping up the proportions of yellow and red to demonstrate the more muted color that begins to take over when chlorophyll production discontinues.
Autumn
Depending on the type of tree, you might be discontinuing green altogether, or dropping it in with other colors. Some leaves will be pure yellow without any green mixed in. And some leaves will be RED! RED! RED!
Observe specific trees carefully, as different plants have different patterns of chlorophyll loss, with some showing color changes all at once, on different leaves at different times, or even in different patches within the same leaf.
Conclusion
Here’s a sort of vague conceptual diagram to represent the relative proportion of various colors of a hypothetical deciduous tree throughout the seasons.
I chose these “primary colors” of leaf pigment:
- Very bright green, to represent chlorophyll: Phthalo Green Blue Shade (PG7)
- Yellow, to represent carotenoids: Azo Yellow (PY151)
- Red, to represent anthocyanins: Pyrrol Red (PR254)
I don’t think these colors are necessarily the only/essential mixers for leaf colors. The actual color of leaves may be influenced by lots of factors, including the species of plant, the environment/context, lighting conditions, and your own perception as a human interpreter of color. You may reasonably choose to represent the nuances of leaf color using many different mixing techniques. But I also think it’s a fun idea to use the science of leaf pigment to guide paint choices, and if you agree, then green/yellow/red is the way to go!