Climate Change in Context: Falling for Fall Colours

Above: Dundas Peak provides a good vantage point to view the array of fall colours in the HSA. Photo: Brendan Church on Unsplash

This is the sixth article in a series about climate change impacts on various ecosystems. To read the other articles in this series, click here.

Fall in the Hamilton Study Area is as if a painter suddenly took control of the landscape. Seemingly overnight, vibrant shades of orange, yellows, and even purples spill throughout the forests. We are reminded of the true array of nature’s painter’s palette during the fall.

These colours showcase an important transition time in a tree’s annual life. Changing colours are produced by photosynthesis slowing to a stop, allowing for non-green colours to come through. With a quickly changing climate, it is likely that the fall colours are being influenced, as the tree’s dormancy changes.

Although the primary trigger for leaf colour change is daylight hours (which is not changing with climate change), temperature and moisture levels also play a role. Whether colours are more intense in warmer years or colder years is still rather unclear. It appears it is mostly species-specific (Archetti et al., 2013). More important is the difference between daytime and nighttime temperatures. When the days are bright and warm, the trees photosynthesize as if it was summer. When followed by a cool, crisp night, the leaves produce the colourful compounds more vividly due to the active daytime photosynthesis (Kyne & Diver, 2012). In a similar sense, moisture needs a perfect balance between the seasons for the most colourful falls. Wet growing seasons followed by drier autumns produce the most vibrant leaf pigments, but if the fall is suddenly too wet or too dry, the leaves will simply fall off before any signification colouration occurs (Kyne & Diver, 2012).

The changes projected for leaf colour change are interconnected through three main climate variables: temperature, moisture, and sunlight intensity.

When temperature is considered by itself, it is expected that colour intensity may be muted or peaking later in the season (Kyne & Diver, 2012). As the perfect daytime-nighttime temperature situation is pushed back further into the year, so is the peak colour intensity.

Precipitation alone suggests that leaf colour change may not even happen. The shifting rainfall patterns towards a drier summer and wetter fall may overwhelm the tree’s natural reactions and stress the tree. This will cause the tree to drop leaves sooner. In addition, we are seeing that storms are happening less often, but are more intense. During the fall, a tree is ready to shed its leaves quickly, and a strong storm may be the encouragement it needs to let them go.

When considering sunlight intensity, things start to get messy. The light intensity spurs and slows photosynthesis and colour production. If temperature does indeed push the peak colour intensity later in the year, this will create a disconnect between the daylight hours and temperature, which the tree has evolved to connect. Since there is less sunlight during the optimal temperatures, there will be less photosynthesis and less colour production. Even if temperature does not change enough to create a disconnect with daylight hours, the increasing fall rain and associated cloud cover blocks the sun from reaching the leaves. In this situation, the peak colour intensity may still be at about the same time of year but will be much less vibrant.

An indirect way that climate change is changing our fall colours is by changing which trees are in the forests in the HSA. Between the interplay of climate change-assisted insect outbreaks and natural tree species range migration, the trees are changing and, consequently, the colour palate as well. Already it is noted that with the Emerald Ash Borer greatly decreasing the abundance of healthy ash trees, the bright orange, yellow, and even purples are gone in the fall (Kyne & Diver, 2012). It is expected too that the maples and birches will be further pushed northwards and replaced with more oaks and hickories, limiting the oranges and reds we all enjoy (Kyne & Diver, 2012).

The ecosystem impacts of colour change variations are still to be explored. The increasing availability of satellite measurement technologies and long-term data management will help open this untouched realm of ecology.

Leaf colour change is what we see when the trees work to reabsorb nutrients and minerals that are stored in the leaves before winter. It is speculated that during years when trees produce more colour, particularly red, more nitrogen is stored within the overwintering parts of the tree (Schaberg et al., 2003). If leaves fall off the tree with muted colours, it could be that the tree has fewer nutrients for the winter. Over several winters, this could lead to die-off. Conversely, though, this would mean there are more nutrients in the soil from the decomposing leaves.

More importantly, and more certainly, is the changing fall colours will change how people relate to nature. The colours give people excitement in an otherwise grey descent into winter. The colours attract people from the city out into nature, even during a pandemic. If fall colour timing changes, likely there won’t be much change to our immersion in nature. But if the colour intensity changes, the excitement and wonder of nature will likely diminish.

It seems that the canvas of nature is shifting, asking for colour later in the year. The ultimate ecological impacts of this shift are still unclear. Perhaps it is simply a change in the artist’s pacing, perhaps it is more significant. Perhaps the mystery is part of the art. All we can do is enjoy the colours when we see them, just as we do an abstract painting, and wonder about the underlying meaning.

References

Archetti, M., Richardson, A.D., O’Keefe, J., Delpierre, N. (2013). Predicting climate change impacts on the amount and duration of autumn colours in a New England Forest. PLOS One, 8(3), e57373

Kyne, A., Diver, K. (2012). Climate change and autumn colors in New England’s forests. The Northeastern Geography­, 4, 34-53.

Schaberg, P.G., Van Den Berg, A.K., Murakami, P.F., Shane, J.B., Donnelly, J.R. (2003). Factors influencing red expression in autumn foliage of sugar maple trees. Tree Physiology, 23, 325-333.

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