Fall foliage on the Danforth campus of Washington University in St. Louis. Credit: Joe Angelis/University of Washington
At that time of year again. The days get shorter and the nights calm down. But when will the natural fall leaf fireworks display actually begin this year?
Many people believe that climate change delays the onset of fall foliage until later in the year. The general thinking is that warmer conditions projected under climate change will mean that trees can “hold on” to their green, energy-producing leaves for longer. But scientists don’t see this actually happening across North American forests, according to an expert at Washington University in St. Louis.
“Warmer temperatures in September and October reduce anthocyanin production in the leaves, which may mean that fall colors will become less bright red or purple,” said Susan S. Rayner, professor emeritus of biology in arts and sciences. “This effect has been well documented in some species, such as sugar maple, where experimental cooling of the branches increases anthocyanin concentration and color brilliance. If the first frost comes later than before, shiny foliage will appear later than before.”
“However, there are other factors that oppose this. Most important is that trees shed their leaves earlier if they have had a very productive spring and summer. This cancels out any delaying effects of the warm fall.”
“The end result is that leaves still start to die after the same amount of time on the tree as they have in years and even decades,” she said.
In fact, under some scenarios, we may see the leaves turn red and yellow earlier. Renner co-authored the journal’s 2020 study Sciences which showed that increased growing season productivity leads to early fall leaf senescence–the process by which plants decompose and reabsorb key nutrients that were diffused into the leaves–in temperate trees.
“If climate warming continues unabated, the situation is likely to change after about 2040, with aging progressing earlier than is happening now,” Renner said.
Here, Renner answers some additional questions regarding changing the color of fall leaves.
How do temperature and humidity conditions affect leaf color?
The yellow color in autumn leaves is due to the retention of carotene pigments (xanthophylls) in the formation of chloroplasts. The red and purple color is due to the accumulation of anthocyanins in the vacuoles, starting around September.
Rain does not affect these basic processes. However, cold temperatures enhance the production of anthocyanins, as shown experimentally in maple sugar. One can notice this in the sugar maple trees along the streets of St. Louis, which begin to turn colors at the top, where the local climate is cooler.
How can climate change alter color dynamics?
As climate warming has led to warmer dips, we in Canada and North America are seeing less bright fall colors. It is difficult to determine how bright the color is, but the effect has been demonstrated experimentally in sugar maple trees.
An interesting confounding factor is clean air. Thus, in Europe, the brightness of the atmosphere due to clean air since 1983 has led to an increase in plant photosynthesis in spring and summer – and premature aging of leaves (compared to 1950-1982). However, these data relate to the breakdown of chlorophyll, not the production of red or yellow colors.
Will all tree species respond in the same way?
of course not. Experimental work reveals significant species-specific differences. For example, trees with nitrogen-fixing symbionts never turn red or yellow. Both pigments help protect leaves from sun damage that are no longer usable for photosynthesis due to the massive decomposition of chloroplast proteins in aging leaves. Trees with constant access to nitrogen may not bother with costly anthocyanins, but instead drop their leaves while still relatively green.
Do scientists actually see changes in the timing or intensity of leaf color changes?
Warmer fall reduces the brilliance of the red and yellow leaf colors, but it is difficult to determine the brilliance in a long time series for many species. Also, many species simply do not turn red or yellow.
We have a paper under review that analyzes satellite data about the “greenness” of the forests of the Northern Hemisphere. We found that across the vast majority of forest areas, high temperatures in spring and summer resulted in premature aging (!!!) at a rate of 1.5 days per °C. Aging here refers to the deterioration of chlorophyll or vegetation.
Where in the world are autumn leaves most likely to be affected by climate change?
Species that change leaf color to red or yellow are more common in North America than in Europe. As the climate continues to warm and the frost nights decrease in October, the colors will be less bright, but the beginning or dates of the color change will not change much due to the counter factors I mentioned earlier. In the far future (after about 2040), with undiluted climate change, leaf aging in trees and shrubs in the northern hemisphere will occur earlier.
Why does North America fall red compared to Europe?
Submitted by Washington University in St. Louis
the quote: No, the color of autumn leaves does not change later due to climate change (2022, September 23) Retrieved on September 23, 2022 from https://phys.org/news/2022-09-autumn-climate.html
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from San Jose News Bulletin https://sjnewsbulletin.com/no-autumn-leaves-do-not-change-color-later-due-to-climate-change/
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