How Weather Influences Fall Foliage (credit: Meteorologist Ash Bray)

Now that fall has begun in the Northern Hemisphere, we find ourselves turning our attention to the foliage riot of colors. However, the simple transition from summer’s lush, leafy greens to fall’s bold reds, yellows, and oranges can be impacted in complex ways via weather conditions – temperature, precipitation, and amount of sunlight – as well as climate.

As most of us know, leaves serve a functional purpose for trees, producing energy for the entire plant. Their broad shape makes them perfect for absorbing sunlight, which after absorption, interacts with carbon dioxide and water within the leaves, producing sugars and oxygen in a process known as photosynthesis. The plant molecule responsible for photosynthesis is called chlorophyll – giving leaves their trademark green color. But chlorophyll isn’t the only pigment that resides within leaves! Orange (carotenoids) and yellow (xanthophylls) pigments are also present. However, they remain hidden for most of the year due to chlorophyll’s masking capabilities. Throughout the spring and summer months, chlorophyll is continually depleted by sunlight, only replenished during the growing season. Once chlorophyll levels subside, other pigments are able to shine through.

The most brilliant leaf displays tend to follow a summer filled with warm, sunny days and cool, crisp nights. During this weather cycle, the sunny days allow for leaves to produce an overabundance of sugars, while the cool evenings allow for the narrowing of leaf veins, trapping the majority of the sugars within. An overabundance of sugars and light within the leaves leads to the production of vivid anthocyanin pigments – which produce purple, red, and crimson colors. Soil moisture also plays a crucial role in the timing and brilliance of leaf foliage. The healthiest displays are produced when the soil has been adequately moist throughout the year, teamed with the aforementioned late summer weather. A severe late spring or summer drought can delay the onset of colors. A warm period during the beginning of fall can also decrease the intensity of fall colors by triggering early leaf drop before the colors have had a chance to fully develop.
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In the future, these temperatures could potentially increase throughout all seasons due to climate change. However, the effect on fall foliage will depend on just how much the temperatures change. For example, New England is projected to have moderate temperatures – potentially leading to a later emergence of fall colors – while in southern New England, trees may be subjected to heat stress – leading to earlier coloration and faster leaf drop. Future precipitation trends in the Northeast aren’t clear, though there are indications that rain could potentially become more intense in episodic bursts, leaving enough time between these episodes for trees to experience drought stress. These warmer temperatures and droughts could also affect the brilliance of red leaves in the future due to nighttime temperatures – which help stimulate red pigments – potentially rising faster than day time temperatures. Local colors may also shift in the future as tree species migrate upwards in elevation and further north to stay in their preferred temperature range.

While the future remains uncertain, autumn is the one season that allows people to go out and connect with nature, possibly influencing people to pay more attention to climate and weather. In that way, fall foliage is a subtle, kind way to increase environmental awareness.

Curious to see how your local weather will affect the fall foliage in your area? The variability of the conditions involved ensures us that no two fall foliage displays will be identical year to year. However, scientists make predictions about the timing of fall colors, and they even created an app that allows you to view their predictions for your region.

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© 2018 Meteorologist Ash Bray