Every autumn, we revel in the beauty of the fall colours in The Land Between. The transition from green to bursting hues of red, orange, yellow and purple are a result of a chemical process that is taking place when the seasons change here in Canada. Contrary to popular belief, the annual change in the leaves is not caused by cooler temperatures or frost, but is triggered by changes in light (McGuire, 1998).

Before we discuss exactly why leaves change colour, it is important to understand why leaves are green. In the spring and summer, leaves act like small factories to produce food for the tree and allow it to grow. Just like humans, trees need food and water to survive. Chlorophyll, a molecule that gives leaves their basic green colour, takes sunlight, carbon dioxide and water and turns it into sugar (McGuire, 1998). The process of creating nutrients that feed the tree is called photosynthesis (Kane, 2002). Essentially, chlorophyll is so strong it absorbs the red and blue parts of the visible spectrum, which is why the leaves are green, as it is the only colour left to be reflected (Cappucci, 2017). This explains why leaves are green and so is grass and other green plants.  

However, when the weather changes from warm and sunny to cold nights with shorter days, the leaves no longer have enough energy to perform photosynthesis and the trees know it is time to hibernate for the winter. This is when fall arrives and photosynthesis comes to a halt and chlorophyll dwindles (Cappucci, 2017). Since chlorophyll is not the only chemical residing inside a leaf, the rest of the chemicals become visible in fall, when the plant’s food engine shuts down (Cappucci, 2017). This allows the other pigments that have been in the leaves all through the spring and summer to finally have their chance to emerge.

Why don’t all leaves turn red?

Now that we know why leaves are green and why they change colour, you are probably curious as to why the leaves are changing different colours. It is important to note that the range in colours is not related to photosynthesis (Anderson, 2000). The different kinds of trees have different colored leaves because there are different functions of each tree. Some trees have more sugars or different sugars than others (Engber, 2013). The sugar maple for example keeps large amounts of sugar in their leaves, which make them dark red. Other trees, like poplars, have very little amounts of sugar which makes their leaves yellow.  

As we know, leaves are loaded with chlorophyll which makes them green. What most people do not know is that all green plants also carry a set of chemicals called carotenoids which is responsible for the colour of carrots (Engber, 2013). However, these colours are invisible until fall, when the chlorophyll breaks down and the leaves near the end of their life cycle (Engber, 2013). Most trees have also evolved to produce a different set of chemicals called anthocyanins (Engber, 2013). These chemicals have a reddish tint and are responsible for the colour of a blueberry (Anderson, 2000). Anthocyanin also works like sunscreen to protect cells from ultraviolet light (Anderson, 2000). This happens during stressful periods when there’s a lot of light but cool temperatures which is common in our Canadian fall (Anderson, 2000). This process protects the increasingly fragile leaves when the tree draws out the last of the nutrients.

Why do leaves fall?

Essentially the leaves begin to fall off the tree when they can not do their job anymore. This happens by a layer of cells that starts to grow between the leaf stalk and the twig. The process starts with the vessels that carry water to the leaf and sugars to the rest of the tree get closed off. The cells serve to slowly cut the leaf from the tree without leaving an open wound. As the leaves fall, the tree enters dormancy and saves its energy for spring.

You may be wondering now why some trees shed their leaves and some do not? I encourage you to get outside and take a look around you. What other changes are happening as the weather turns from warm to cold? Take the time to observe and let yourself connect to the beauty all around you. If you are interested in exploring more of what nature has to offer, please stay tuned for weekly blog posts!

“Autumn is a second spring when every leaf is a flower.” – Albert Camus

 

Written by: Catherine Judge, Conservation Communications Specialist 

References

Cappucci, M. (2017). The leaves are starting to show signs of autumn. Here’s why they change: As the sun’s rays become less intense, the season for leaf-peeping and brilliant foliage is upon us. In Washington Post – Blogs. WP Company LLC d/b/a The Washington Post.

Engber, D. (2013). Question: why do leaves turn different colors?(Brief article). Popular Science (New York, N.Y.), 283(5), 73–.

Kane, K. M. (2002). Why Do Leaves Change Colors? Parenting, 16(8), 205. https://link.gale.com/apps/doc/A96399992/CIC?u=utoronto_main&sid=bookmark-CIC&xid=2b876377

McGuire, E. (1998). Foliage afire: why leaves change colors. New York State Conservationist, 53(2), 2+. https://link.gale.com/apps/doc/A21281251/CIC?u=utoronto_main&sid=bookmark-CIC&xid=c05c3cd3