Outdoor activities with a healthy dose of curiosity, brought to you by Laure Latham
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Every autumn in temperate climates, tree leaves turn all shades of red, yellow, orange and brown as the season gets colder and mornings get frostier. While adults are perfectly happy snapping pictures, kids love to know the “why” of things and my (almost teen) girls are no different. Why can’t leaves just fall when they’re green? Why do some leaves turn red and others yellow?
When I told them about the sugar science of autumn leaves, they looked at me with sparkles in their eyes. Did someone say sugar? Are autumn leaves edible? Is it time to go for a walk and start eating trees yet? Hold your horses. In fact, a few autumn leaves are edible but they’re not exactly like candy. First, here is why autumn leaves put on the best color displays of the year in the great outdoors.
You don’t know it yet, but deep inside, leaves are already yellow, orange and red year-round. They’re just hiding it well to make food for trees using a process called photosynthesis. This how it works.
This process is called “photosynthesis” meaning “putting together with light” and is facilitated by a chemical called chlorophyll.
Leaves contain various chemical pigments that affect their color. The main ones are:
During the growing season (spring and summer), chlorophyll is replaced constantly in the leaves, completely masking other pigments that are already there. As a result, spring and summer leaves are green but let’s not forget that carotene and xantophyll are present, if not visible.
As autumn approaches, temperatures begin to drop (particularly at night) and daylight hours get shorter. Longer nights, shorter days and cooler temps signal to trees that it’s time to get ready for winter. To do this, thin layers of cells grow over the water tubes in the leaves and close them, blocking all circulation of minerals to and from the tree (that includes water) and blocking sugar (and carbohydrates) from getting to the tree.
Without water getting to the leaves, the chlorophyll breaks down and the color green fades away, revealing the other pigments that aren’t affected by the cooler temperatures. Hello, yellow and orange!
At the end of the season, the color brown prevails as tannins are the only pigments not affected by cooler temps when all other pigments have died.
Wait. What about red and purple?
When the water tubes get “closed down,” some sugar (glucose) gets trapped in the leaves. Red and purple pigments come from anthocyanins, pigments that are absent from plants during spring and summer. They get literally “manufactured” in the fall from the sugars trapped in the leaf. That’s why certain trees have leaves that turn red or purple in the fall. Think of maple trees.
Typically, forests in North America and East Asia turn fiery red while forests in Europe turn bright yellow, a result of local tree varieties.
In Japan, kids look forward to the fall season not because of the vibrant reds and golds of maple leaves, but because of delicious edible snacks called momiji tempura. Momiji is the name of Japanese maples and tempura is a way of deep frying food in Japan.
Deep fried maple snacks are a tasty treat and apparently, the crunch you hear when you chew them is similar to the sound of walking on crispy autumn leaves. How poetic.
Does this mean that you can harvest maple leaves on your fall hikes to make them into snacks if you live in North America or Canada?
Sorry but no. First of all, momiji leaves are preserved in salt barrels during a year before being deep-fried in a sweet batter. Second, Japanese maples have much smaller and thinner leaves than American maples and who knows if those are edible. If you’re lucky to live in Japan or travel there during the fall season, please let me know how they taste. I’d really love to know.
I hope that this sweet conclusion on a scientific subject will inspire you to go for a family walk in a park where you can spot different colors of leaves. Now that you know all about it, won’t it be a good story to tell on the trail?
Sugar science rocks.