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A Pale Imitation

Climate ChangeIssue 3NatureNature Issue 3

One of the joys of living in Southwestern Ontario is the multi-coloured festival nature throws every year: Autumn, and the resulting brilliant fall colours on display. People flock outside for walks on the trails and to the national parks to see the colours. Since the fall colours pop out sometime around Thanksgiving, some families even make a tradition out of visiting a local park for a walk. Tourism is also big business, and tourists descend around Ontario simply to see the splendid colours on display in the forests.

Yet, we may soon see the day that the fall colours are no more. It’s hard to believe a naturally occurring phenomenon could disappear, but it’s quite possible the most splendid fall colours are to be left behind in the past. The reason? Global warming.

When people think of global warming, they don’t immediately think of losing the fall colours. Generally, a predominant attitude is that global warming will be far more extreme at first, rather than what is actually happening, and that is a long, slow, burn. Global warming is affecting one of the smaller joys in life: the beautiful fall colours. Since the temperature is rising every year, local climates are becoming warmer and warmer. This past summer Conservation Halton reported a Level II Low Water Condition for Halton Region. It’s been a severe drought, and droughts can directly affect the fall colours. To understand how a drought affects the fall colours, we need to know a little more about the science of senescence in trees, and how fall colours come into being.

The Science Behind Senescence

The initial cue for the leaves to change are shortened days, and then the secondary cue is cooler temperatures. To complete photosynthesis, plants need light, water, and soil. Both light and water are scarce resources during winter. Additionally, broad leafed plants are vulnerable to frost. To promote the survival of the plant, the leaves must die, seal off the branch, and fall to the ground. Once the weather cools and the period of light during the day shortens, it’s the beginning of the end for nutrient production. Trees shut down photosynthesis, and begin to ‘hibernate’ for the winter.

Trees produce an excess amount of glucose (also known as sugar, and a plant’s favourite food!) during the summer when they receive the most amount of light. Once the days begin to dwindle, glucose production is shut down, and the excess is turned into starch. The tree sends the starch to the roots for storage where it will stay until the spring warmth awakens new growth.

Once the supply of nutrients to the leaves stops, the internal water supply is cut off and prevented from escaping. This process is called abscission. Since water stops flowing, and photosynthesis stops producing, then the chlorophyll recedes from the leaves. Chlorophyll is the chemical that infuses leaves with a green colour. When the chlorophyll recedes, the remaining pigments of the leaves start to show through, causing the gradual change in colour. Carotenoids turn the leaves orange; xanthophyll turns the leaves yellow; and, anthocyanin turns the leaves red and purple.

Global Warming & Droughts

However, a severe drought can cause a tree to begin the process of senescence earlier. Higher temperatures mean drier weather, and less rain. Without enough water to grow in the summer, trees will begin to conserve their nutrients and the sugars needed for colour much sooner in the growing season. Furthermore, autumns are warming up, which means warmer temperatures but shorter days. Remember, a tree’s first cue to shut down photosynthesis is shorter days. Shorter days shortens the process of photosynthesis, and by shortening the process of photosynthesis, there is even less sugar reserves to trigger the anthocyanin chemicals that turn leaves that brilliant red. Therefore, in a season of drought and warmer but shorter days, tree are hit with a double whammy to their abilities to produce sugars that make autumn so glorious. The results can be that the colours start looking muddier and muted compared to previous years. The wonky temperatures also means that the timing of the fall colours can shift to later in the year than usual, but that’s yet to be documented extensively.

We don’t fully yet know how global warming will truly affect our forests in the long term, but in the meantime we can watch the fall colours for the signs of what is in the future for a healthy environment.

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Last modified: September 5, 2017

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