The Science of Fall

Every year, the summer inevitably comes to an end, closing out what is arguably the best time of year for cottage country. However, the ending of summer brings the brilliant fall foliage colour. This natural phenomenon brings thousands north to experience the beauty of red, yellow and orange leaves. However, have you ever wondered why the leaves change colour? Well, we all know that come winter, the deciduous trees (aka leafy trees) are bare of leaves until spring. So the beautiful colour change is essentially the death of the leaves. It is a small but amazing part of the in a natural life cycle. But why do some trees turn yellow and others red? Why not Purple or even blue?

To understand that question we need to understand that colour comes from specific compounds. In general we see a certain colour because a specific compound within a thing that reflects light in a certain wavelength. This triggers our brain to recognize it as “red” or “blue” (fig 1). We could go on about the physics of colour theory for a very long time, but we are tree people, not physicists. So we will just stick with the understanding that certain compounds make certain colours. 

Fig. 1. Basic physics of how compounds in the leaves reflect light differently. The reflected light is captured by our eyes to signal the recognition of a certain color.

In a leaf, there are 3 specific colour compounds that play a significant role in the fall foliage change:

CHLOROPHYLL

Leaves are green because of chlorophyll. Chlorophyll is a compound making up the chloroplast in the leaf that performs photosynthesis.  

CAROTENOIDS

Carotenoids are yellow/orange coloured antioxidants that aid in trapping light for chlorophyll to use for photosynthesis. Fun Fact: this compound is also the compound responsible for the orange colour of Carrots (hence the name “carot”enoids).! 

ANTHOCYANINS

Anthocyanins are red coloured compounds that are produced as a byproduct of excess sugar during the fall months. 

Now that we know the players, what are their roles in the foliage change? 

As the temperatures drop and the days get shorter, there is less light for chlorophyll to photosynthesize, so the little green compounds begin to break down. This allows for the carotenoids to shine through, starting the change from green to yellow and orange. 

With fewer chlorophyll to perform photosynthesis, the tree starts its transition into winter dormancy by essentially killing off the leaves. Veins within the leaves begin to close to preserve the sugars in the trunk. This causes sugar in the leaves to accumulate and then break down during the dying process. Anthocyanin, the byproduct of excess sugar, is now taking the show by storm creating red leaves. This is why we see the iconic reds of fall emerge later than the yellows and oranges. 

If you have ever seen the fall foliage change, you know that different trees will be different colours, and some trees will change before others. This is because the sugar content within a leaf will vary between species, but also between individual trees. For example, a Sugar Maple will have brilliant red leaves come late fall because they naturally have a lot more sugar compared to other tree species. The sugar content of a tree will also change depending on the age or health of a tree. If a tree is struggling to maintain its health come late summer, it will force itself into leaf death to preserve more sugar in its trunk for survival, causing early colour change.

Fig. 3. Warm days with cold nights results in more brilliant color expenditure vs. a warm fall with drought where the colors are more muted.

The temperature and weather patterns will also impact the fall foliage change. If you are lucky enough to experience the colour change year after year, you will notice every year is a little different. One year there will be brilliantly bright colours, while the very next year, the leaves will be more muted. Warm days with cold nights will cause increased sugar transportation throughout the tree due to the drastic temperature changes. This change in temperature will cause vein closures to trap more sugars during leaf death (fig 4). The excess sugars will result in more anthocyanins than normal, revealing bright red colours. The harsh transition from warm days to cold nights will also act as a trigger for the breakdown of chlorophyll, leading to rich yellow and oranges. On the contrary, if we experience a warmer than normal fall or even drought with warm nights, there is often a more muted tone to the season. This is because there is less sugar transportation and chlorophyll breakdown as a result of a weak season trigger, leading to a lower concentration of carotenoids and anthocyanin compared to years with a harsh season trigger. 

So there you have it, the science behind why the trees change colour in the fall. Long story short, the change in season triggers the leaves to start dying, causing the internal colour compounds to change concentration, changing the sea of green leaves into a mosaic of red, orange and yellow. Some actually believe the fall foliage change goes much deeper than simply a biochemical change. It has been proposed that the colour expenditure a tree has during fall has a significant evolutionary impact as well. For instance, insects are known to target the most vibrant of trees since their colour suggests the tree is healthy, and thus, a good food source. So, as a result of high colour expenditure, the tree is killed by an insect. This means that trees with genetically less colour expenditure during fall are more likely to survive and pass on its genes. Whether it is genetics, biochemistry or a natural phenomenon, everyone should experience the fall foliage change at its peak at least once in their lives!