On Halloween, the Red Maple (Acer rubrum) outside our kitchen window gave up the ghost, so to speak. The leaves on the north side of the tree had been changing colors for weeks, but the rest of the tree remained stubbornly green. Overnight, the entire tree was awash in reds and oranges.
It’s the change in day length (or really night length) and temperature that signals deciduous trees and shrubs that it’s time to get ready for winter. They have to drop their leaves to protect themselves from damage that would be caused due to the heavy weight of winter ice and snow storms. As nights get longer and temperatures drop, these woody plants gradually slow and eventually stop replenishing chlorophyll, the substance that is responsible for the green pigment in their leaves.
But what accounts for the array of colors that are revealed as the chlorophyll gradually disappears? The yellow, orange, red, purple, bronze and browns?
These colors reflect some of the same nutrients that are present in the plant-based foods we eat. Many of these chemicals were present throughout the growing season, but were masked by or blended with the green of the chlorophyll.
The yellows are carotenoids, mainly xanthophylls, nutrients that help to reduce inflammation, boost the immune system and reduce tumor growth. They are present in yellow summer squash, beets, carrots, corn, peppers, green leafy vegetables, and many others. Xanthophylls help plants to absorb energy from the sun while protecting tissues against the sun’s intense radiation.
Carotenes (another group of carotenoids) are responsible for the orange shades revealed in fall leaves. Beta-carotene is an anti-oxidant, visible in the orange color of foods like winter squash, carrots and sweet potatoes. They assist in photosynthesis, and help protect plant tissues from too much exposure to the sun’s rays.
Reds and purples are anthocyanins, nutrients found in foods like blueberries, blackberries, cherries, grapes (and red wine; yay!), purple cabbage, other purple-tinged greens like red leaf lettuce, some kale and swiss chard, as well as many others that show red or purple colors. Anthocyanins are antioxidants, with anti-inflammatory, anti-viral, and anti-cancer properties. Plants may also benefit from anthocyanin’s antioxident effect, and the dark colored pigments help protect from sun damage. They continue to manufacture these chemicals until the leaves fall.
Brown and tan colors show the presence of tannins. They tend to be bitter or astringent tasting, and as a result discourage browsing by herbivores (plant-eaters) so they may provide some protection to plants. Tannins are found in foods like grapes, wine, tea, and chocolate, though, so you can see that this protection isn’t foolproof.
There is overlap and blending of colors based on the mix of chemicals in the leaves. These bright colors may also signal to birds that there is fruit available for consumption.
These nutrients, along with others that are obtained from the soil, like calcium and potassium, break down and return to the soil as the fallen leaves gradually decompose. During this process, animals may still take advantage of the nutrients. Red-banded Hairstreak caterpillars, for example, feed on fallen leaves, especially those of sumacs, contributing to the process of decomposition, and the cycle of life for the next generation.
A few days later, and the leaves on our Red Maple have almost all fallen. We’ll watch them gradually break down, nourishing the soil, and the plants and animals that rely on them.