Category: Flora

Hi, I’m Holly Strand.

As early as 4th grade, children learn that leaves are green because of the high concentration of chlorophyll relative to other pigments. And that autumn leaves turn color because the breakdown of chorophyll molecules unmasks the yellow, orange and red pigments that remain.

This simple and satisfying explanation has existed for awhile. However, research in the past 15 years has uncovered a much more complicated story. Leaf physiologists jumped from the question, “What makes autumn leaves so colorful?” And started to ask the evolutionary question: “What’s the purpose behind this autumn color fest?” “What’s in it for the tree itself?”

The group of pigments that produce yellow or orange are called carotenoids. The carotenoids are in the leaf throughout the growing season but only become apparent with the breakdown of chlorophyll into colorless metabolites. However, the red pigments—or anthocyanins– are produced in the leaf AFTER much of the chlorophyll is lost. This active production of new anthocyanins led scientists to believe that the pigment must be performing a critical function.

Just what function is still not known but many hypotheses exist. One is that anthocyanins act as a sunscreen shielding leaf tissues against the harmful effects of sunlight. The risk of sun damage is particularly high in fall because of the lack of chlorophyll protection and because there is increased light due to a thinning canopy. Without sun damage, the leaves can continue to absorb nutrients right up to the end.

Another possibility is that anthocyanins function as an antioxidant shield. Plant chloroplasts produce fewer free radicals when shielded from green wavelengths of light. Anthocyanins absorb the green wavelengths. Without anthocyanin, a surplus of superoxides could cause damage to the plant cell structure.

Lastly, there is the coevolution hypotheses. Aphids and other insects feed, carry viruses and bacteria. So red coloration was an adaptation developed by trees designed to lighten their insect load. The red leaves signal to insects that the tree is not a suitable host. The coevolution theory is supported in part because trees with autumn colors tend to be the ones with a history of interaction with aphids. Also, experiments have shown that most aphids strongly prefer green leaves over red leaves. In some cases anthocyanin may be positively correlated with a form of chemical defense.

However, when considering this last theory, remember that species perceive color differently. To the human eye, a tree may be a brilliant autumn shade of red. To the aphid, that same tree might be an indistinguishable gray. Further the insect may be avoiding red leaves because of differences in texture, taste, scent, texture or something else that is important to an aphid.

Whatever the reason is, I hope you are enjoying Utah’s colorful reds and yellows.

For links to current research and a Forest Service update on fall color around the state, go to www.wildaboututah.org.

For Wild About Utah, I’m Holly Strand.

Credits:

Images: Courtesy and Copyright 2003 Michael Kuhns, Extension.usu.edu
Text: Holly Strand

Sources & Additional Reading

USDA Forest Service Fall Colors web site for the Intermountain Region https://www.fs.usda.gov/detail/r4/recreation/?cid=FSBDEV3_016189

Archetti, M., Doring, T. F., Hagen, S. B., Hughes, N. M., Leather, S.
R., Lee, D. W., Lev-Yadun, S., Manetas, Y., Ougham, H. J., Schaberg,
P. G., Thomas, Howard (2009). Unravelling the evolution of autumn
colours: an interdisciplinary approach. Trends in Ecology and
Evolution, 24, (3), 166-173. https://digitalcommons.fiu.edu/cas_bio/38/
[FIU moved to behind a paywall-URL updated 260111-https://fiu-flvc.primo.exlibrisgroup.com/discovery/fulldisplay?docid=cdi_proquest_miscellaneous_66985692&context=PC&vid=01FALSC_FIU:FIU&lang=en&search_scope=MyInst_and_CI&adaptor=Primo%20Central&tab=All40&query=any,contains,Unravelling%20the%20evolution%20of%20autumn%20colours:]

Chittka L, Döring TF (2007) Are Autumn Foliage Colors Red Signals to
Aphids? PLoS Biol 5(8): 187. https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.0050187

Lee, David and Kevin S. Gould Why Leaves Turn Red. American
Scientist, Volume 90. 524-531 https://harvardforest.fas.harvard.edu/sites/harvardforest.fas.harvard.edu/files/leaves/2002_11_leaf_article.pdf

Gunnell, JayDee, Reese, Julene, Ask a Specialist: What Causes the Fall Leaves to Change Color?, USU Cooperative Extension, https://extension.usu.edu/htm/news-multimedia/articleID=18662

Holly: Hi, I’m Holly Strand from Stokes Nature Center in beautiful Logan Canyon.

The canyons and valleys of Utah will soon be awash with brilliant fall hues. Cascades of red, orange and gold will blanket the hillsides as the weather turns cooler and morning frost dusts the mountain ridges.

The Canyon Maple is one of Utah’s main sources of autumn color. Like most maple species, its leaves are carved into deep lobes. The leaves are medium-to-bright green now, but soon the entire tree will glow with spectacular color.
Canyon maple is found throughout Utah at medium elevations between 4,500 to 7,500 feet. It tends to grow on lower slopes and canyon bottoms in the mountains in association with Douglas-fir and junipers.

Its scientific name, Acer grandidentatum [AY-ser gran-dih-den-TAY-tum], means “Big Tooth”, referring to the tree’s distinctive lobed leaves with large, toothed margins. In fact, bigtooth maple is another common name for this species.

Some think that he canyon maple is related to the sugar maple of the northeastern and midwestern United States,” says forestry professor Mike Kuhns of Utah State University’s Department of Wildland Resources. It’s possible that long ago, the Rocky Mountains rose up and isolated a sugar maple population that eventually evolved into a unique species.

The canyon maple rivals its eastern relatives in fall color but does it produce sap suitable for tasty, syrup-covered waffles and pancakes? Back in 1970s, a group of scientists set out to determine just that. The trees were tapped and yielded plenty of sticky liquid. The color was very light, resembling light honey and the flavor was delicate and fruity, almost like pineapple. However, it was notably less sweet than the northeastern sugar maple. Of 30 panelists from Utah who participated in a taste test, 57 percent preferred eastern sugar maple syrup but the remaining 43 percent preferred canyon maple syrup.

The researchers concluded that while canyon maple sap was not practical for large-scale syrup production it might be enjoyable for individuals to try on a small scale on private land within its habitat range.

Syrup aside, the medium-sized tree thrives in Utah’s residential landscapes, parks and urban areas, as well as in the wild. Its year-round beauty, hardiness and manageable size make it a perennial favorite in the Beehive State.

Thanks to the USU College of Natural Resources for supporting research and development of this Wild About Utah topic.

For Wild About Utah and Stokes Nature Center, I’m Holly Strand.
Credits:

Image: Courtesy and Copyright 2003 Michael Kuhns, Extension.usu.edu

Text: Holly Strand & Mary-Ann Muffoletto, Stokes Nature Center

Sources & Additional Reading:

Barker, Phillip A.; Salunkhe, D. K. 1974. Maple syrup from bigtooth maple. Journal of Forestry. 72(8): 491-492. [9065] https://www.ingentaconnect.com/content/saf/jof/1974/00000072/00000008/art00016

Kuhns, Michael. 2003. Canyon Maple: A Tree For the Interior West,” USU Forestry Extension, https://extension.usu.edu/forestry/HomeTown/Select_CanyonMaple.htm [2009, September 16].

Tollefson, Jennifer E. 2006. Acer grandidentatum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: https://www.fs.fed.us/database/feis/ [2009, September 16]

Acer grandidentatum – Bigtooth Maple, Water-wise Plants for Utah Landscapes, https://www.waterwiseplants.utah.gov/default.asp?p=PlantInfo&Plant=17