Jessica Short, Program Assistant
Forget “Why is the sky blue?” That’s a sucker’s question*. This time of year, we’re focusing on the other weird thing going on in colors overhead: Why are the leaves green? Whoa, wait, why are the leaves suddenly NOT green?
One of the best ways to begin understanding the Whys of any living thing is to think about its place in the food web. What does it eat, and what’s eating it? How does it get energy, and where does that energy go? In the animal kingdom, this eating is generally done with a mouth. Worms eat decaying material in the dirt, early birds eat worms; humans eat chicken McNuggets, mosquitos eat humans; and all of those little energy transfers are started when something opens a mouth. But outside the animal kingdom, among plants and bacteria and fungi and other things that don’t have mouths, the intake of energy and nutrients get interesting.
Trees are plants. (I know, getting technical. Stick with me here.) Plants get their energy several ways –through the roots, through leaves, and through photosynthesis. Photosynthesis! That’s right, the plant-specific trick to turn sunlight into energy. Photosynthesis is a process that happens at the cellular level of leaves, turning the solar energy of sunlight into glucose, a form of sugar, which trees use for metabolic energy.
Photosynthesis only happens in chlorophyll, and chlorophyll are always green. Every leaf on a plant is a tiny solar power collector, jam-packed with bright green chlorophyll that soak up sunlight and turn it into tree nutrients – and make the leaves vivid shades of green while doing so.
In climates like Michigan, trees go dormant in the winter. This energy-saving strategy helps plants survive months of freezing temperatures which would damage delicate plant tissue like leaves. It also allows trees to minimize their energy needs in months where daylight hours are few, skies are overcast, and light-blocking snow frequently blankets every surface.
When it’s time to settle in for the winter, trees cut off the nutrient exchange between the leaves and the rest of the plant, and the chlorophyll gradually fades out. But there are other types of pigmented cells** in every leaf, cells that contain marvelous oranges and yellows – just very low concentrations of these colors that are usually drowned out by overwhelming summertime greens. These other pigments take longer to fade than chlorophyll does, so while the green disappears relatively quickly, the oranges and yellows linger until the leaves drop off the tree.
Red pigments add additional variation. Reds aren’t present in all tree species, and where they are found, some trees don’t produce them until autumn, while some have red-tinged foliage all summer. Fall production of red pigments is heavily influenced by the weather. Brighter, bolder reds are often the result of warm sunny days and near-freezing nights, drought, or trees that are stressed by low nutrient levels.
One other thing to note is that the timing of when trees change color and drop their leaves varies based on several climate and environmental factors that can vary from year to year. Daylight length, how sunny the weather is, daytime highs and nighttime low temperatures, whether it’s more or less rainy than average, and whether trees have been stressed by poor growing conditions all contribute to how early the color change happens, and how late the leaves hold on. In a stressful year, some trees such as sugar maples and sumac may start to change color as soon as early August.
It also depends on species. The wooded areas of Clinton County contain many oak, hickory, beech, maples, and conifers. In general, oaks are the last to show their fall colors, and will often stubbornly cling to their brown leaves well into the winter, while conifers remain green.
Between the oranges, yellows, and reds left behind, there’s plenty of color variation to create the glowing, flame-like foliage of a bold fall morning. Eventually, each leaf falls. Eventually, each pigment fades, leaving behind only browns. Eventually, it will snow.
But we don’t have to talk about that quite yet. Enjoy kicking up some fall leaves, in every hue except green.
*Rayleigh scattering (link)
** Red pigment is called anthocyanin, yellow pigment is xanthophyll, and orange is carotenoid. Yes, carotenoid is named after carrots. There will not be a quiz on this, but this footnote is included for all those who love the poetry of science words.