While the cooler temperatures of fall are not always welcome, the fall colours do provide a beautiful backdrop for fall walks and other activities. To understand why leaves change color we have to talk about why leaves are green in the first place and why deciduous trees, those that lose their leaves over the winter months, are demonstrating the efficiency of nature.
Leaves are green during the spring and summer months because they are filled with chlorophyll. Chlorophyll is the chemical that is able to absorb energy from the sun to feed the plant. When the sunlight hits the chlorophyll molecule it excites the electrons. Once the electron is excited it gets passed off to another chemical. I kind of like to think of it like the electrons are kids and the sunlight is the sugar that someone might feed a kid right before they send the kid back to their guardians. There is more going on but that is all we need to talk about why the leaves change colour. Plants use energy and resources to make chlorophyll, but it’s worth it when the sun hitting those green leaves has lots of energy. But when the angle of the sun is lower during the fall and winter it no longer has the ability to excite enough of the electrons in chlorophyll to make it worth while. In other words, the plant no longer gets enough benefit to make the work worth it. When this happens the plant stops making chlorophyll which is what makes leaves green. Without the green pigment, some of the other chemicals like beta-carotene become visible in the leaves. So, these other colours that appear in the fall are present all the time we just can’t see them because there is so much green from the massive amounts of chlorophyll. They do the same thing though, they trap the suns energy with excited electrons. They just don’t capture as much energy.
So all of this boils down to costs versus benefits. As long as it is worth it to the plant to make chlorophyll or even any of the other pigments then the plant keeps its leaves and keeps making food. But, when the plant stops getting enough energy it stops producing the chemicals in the leaves and shuts off the flow of resources to the leaves so that they fall from the tree.
We often hear the phrase “work smarter, not harder;”