Ah, the bittersweet moments near the end of the summer. The nights are starting to cool, but the lake is still an incredible 76.5 degrees, at least here in Burlington, and the first of fall colors are poking into the green canopy. In this series, we celebrate the rainbow of fall (though a bit out of the ROYGBIV order) and explore the reasons behind the different colors we see in plants.

More on fall colors

Color, light, and plants

We live on green patches in an otherwise blue planet. While green is certainly the foliar backdrop of the planet’s living skin, the land is bedazzled with every color of the rainbow, and then some. And then some because the rainbow only contains a small selection of colors; many colors are mixes of other colors; brown is a mix of red, yellow, and blue while pink is a blend of red and violet). The colors we see on leaves – greens, reds, yellows, purples – frequently represent adaptations that plants have for living well in place. Here we discover the reasons behind the many colors of plants, particularly in the fall.

Fall color change in a black locust leaflet (Burlington)

Lets first start with light itself. The sun emits a broad spectrum of wavelengths, though not in equal concentrations, that continuously bombards our planet. Emissions peak in the visible spectrum, so we receive more blue light, which is towards the center of the visible spectrum, than violet. The higher energy shorter wavelengths of the cool end of the spectrum (indigos, and violets, see image below) are more likely to strike a particle of dirt, droplet of water, spore of fungus, etc. and be scattered in the atmosphere. We then see the sky as blue. When the sun is lower in the sky, light has to travel through more of the atmosphere before reaching us. A higher concentration of longer wavelength light (reds, oranges) is scattered, particularly when there are more particles in the air, and we get beautiful sunsets.

Green

The colors that we see on leaves are the colors that are reflected back to us. You can see in the image below the amount of light absorbed by two pigments (chlorophyll a and b) in chloroplasts, the photosynthetic structures in leaves. Absorption of visible light peaks at opposite ends of the spectrum (red and blue), but both pigments are less receptive to light green and yellow zone (though still 90% of these colors are still absorbed). It’s not that leaves are green per se, but that the components that reflect more greens than reds and blues so we see the leaf as green. If the leaf absorbed green as well, it would appear black and risk overheating in full sun (here’s some new research that suggests omitting some green light makes leaves less efficient but more stable: link). As for our greens, there seems to be two distinct classes of green-ness of our forest canopies: the rich dark greens of conifer needles and the light greens of hardwoods.

The absorption spectrum of both the chlorophyll a and the chlorophyll b pigments. The use of both together enhances the size of the absorption of light for producing energy (from Wikipedia)
Function of green

Whenever you see green on a plant, it indicates the presence of chlorophyll and therefore photosynthesis. This is true whether you’re looking at a leaf, a scratched twig, or an unripe fruit. Photosynthetic fruits help pay the energetic costs of fruit production. But keeping fruits green also “hides” away the developing seeds from the leering eyes of predators. When fruits ripen, they often turn a color that contrasts sharply with the green foliage. This is called flagging and signals to potential seed disperses that the fruits are now ripe for the picking.

Buckthorn shrub laden with fruits (Shelburne Bay, Shelburne)
Pale green

How dark the shade of green corresponds to how dense the chloroplasts are in the leaf. The upper canopy of a tree is in full sunlight. Sun leaves, those leaves that grow up at the top, are waxy and deeply cut, which help reduce water loss, and darker green than the shade leaves. Chloroplasts are concentrated in rows of photosynthetic cells called mesophyll (more on the anatomy of leaves here). More sunlight stimulates the development of more rows of mesophyll. Shade leaves and leaves on shade-tolerant plants of the understory (like striped maple) are pale green and ultrathin, containing just a single layer of palisade mesophyll. This is highlighted as the leaves during  a pale yellow in the fall (becoming nearly translucent) as the chlorophyll breaks down. These leaves are also much quicker to decompose.

Pale yellow of striped maple leaves in the fall (Centennial Woods, Burlington)
Dark green

One of the big advantages of being an evergreen conifer is that you can photosynthesize later in the fall and earlier in the spring than deciduous trees (there’s very little photosynthesis that happens in the winter). I couldn’t find any research that explains exactly why needles tend to be dark green, but it does appear that conifers actually have lower concentrations of chlorophyll that hardwoods. It might be that needles absorb a broader range of the spectrum because of other pigments in the needles. Or it’s just that needles are thicker and thicker leaves tend to be darker green. Whatever the cause, one advantage of being darker green is that a needle can heat up above ambient temperatures. On windless bluebird days in winter, a conifer’s needles may heat up to 20°C (36°F) above ambient temperatures!!

Red spruce peaking up on the dry summit of Bear Hill (Brookfield)

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