Under An Autumn Sunbeam: Fall Color Explained

Fall Color 2015 in Michigan

Under An Autumn Sunbeam, photo by Owen Weber

As we wait for the fall color season in Michigan to kick off, here’s a look back to last October and my annual rework of one of the most popular posts on Michigan in Pictures, the Science of Fall Color. If you already know the words you can sing along – have a great weekend folks!

The Science of Color in Autumn Leaves from the United States National Arboretum says (in part):

Many think that cool weather or frost cause the leaves to change color. While temperature may dictate the color and its intensity, it is only one of many environmental factors that play a part in painting deciduous woodlands in glorious fall colors.

…The process that starts the cascade of events that result in fall color is actually a growth process. In late summer or early autumn, the days begin to get shorter, and nights are longer. Like most plants, deciduous trees and shrubs are rather sensitive to length of the dark period each day. When nights reach a threshold value and are long enough, the cells near the juncture of the leaf and the stem divide rapidly, but they do not expand. This abscission layer is a corky layer of cells that slowly begins to block transport of materials such as carbohydrates from the leaf to the branch. It also blocks the flow of minerals from the roots into the leaves. Because the starting time of the whole process is dependent on night length, fall colors appear at about the same time each year in a given location, whether temperatures are cooler or warmer than normal.

During the growing season, chlorophyll is replaced constantly in the leaves. Chlorophyll breaks down with exposure to light in the same way that colored paper fades in sunlight. The leaves must manufacture new chlorophyll to replace chlorophyll that is lost in this way. In autumn, when the connection between the leaf and the rest of the plant begins to be blocked off, the production of chlorophyll slows and then stops. In a relatively short time period, the chlorophyll disappears completely.

This is when autumn colors are revealed. Chlorophyll normally masks the yellow pigments known as xanthophylls and the orange pigments called carotenoids — both then become visible when the green chlorophyll is gone. These colors are present in the leaf throughout the growing season. Red and purple pigments come from anthocyanins. In the fall anthocyanins are manufactured from the sugars that are trapped in the leaf. In most plants anthocyanins are typically not present during the growing season.

As autumn progresses, the cells in the abscission layer become more dry and corky. The connections between cells become weakened, and the leaves break off with time. Many trees and shrubs lose their leaves when they are still very colorful. Some plants retain a great deal of their foliage through much of the winter, but the leaves do not retain their color for long. Like chlorophyll, the other pigments eventually break down in light or when they are frozen. The only pigments that remain are tannins, which are brown.

Temperature, sunlight, and soil moisture greatly influence the quality of the fall foliage display. Abundant sunlight and low temperatures after the time the abscission layer forms cause the chlorophyll to be destroyed more rapidly. Cool temperatures, particularly at night, combined with abundant sunlight, promote the formation of more anthocyanins. Freezing conditions destroy the machinery responsible for manufacturing anthocyanins, so early frost means an early end to colorful foliage. Drought stress during the growing season can sometimes trigger the early formation of the abscission layer, and leaves may drop before they have a chance to develop fall coloration. A growing season with ample moisture that is followed by a rather dry, cool, sunny autumn that is marked by warm days and cool but frostless nights provides the best weather conditions for development of the brightest fall colors. Lack of wind and rain in the autumn prolongs the display; wind or heavy rain may cause the leaves to be lost before they develop their full color potential.

OK, sorry to share a novel with you. Might have to change the name of the blog to “Michigan in a Whole Bunch of Words with a Picture.”

Owen took this last October in Glen Arbor. View it bigger and see more in his Michigan slideshow.

PS: I have to think that it doesn’t look the same there this year due to the crazy storm they are still recovering from.

Tons more fall photos on Michigan in Pictures.

Knockout storm packs 100 MPH winds & huge hail

Road Closed in Glen Arbor

Road Closed in Glen Arbor, photo via the Glen Arbor Sun

One more day of storm reporting from my neck of the woods…

In Glen Arbor Hit By Knockout Storm, Jacob Wheeler of the Glen Arbor Sun lays out a diary of the destruction of the storm and wrote:

In the storm’s wake yesterday, Glen Arbor residents immediately recognized that the destruction they witnessed was unprecedented for our town. This was worse than the 1987 storm, people said. In fact, it was far worse. The storm was more powerful and more destructive than any other Glen Arbor storm ever recorded. And now we have stats to prove it.

I spoke late this afternoon to Jeff Lutz, meteorologist with the National Weather Service (NWS) in Gaylord. While Lutz clarified that yesterday’s storm was not a tornado (you can blame some of the hyperbole on this newspaper) he did confirm that the straight-line winds which accompanied the sudden thunderstorm reached speeds of 100 miles per hour. That’s strong enough to be called a tornado. More significantly, it blows away the previous wind velocity record for Leelanau County. According to the NWS, on Sept. 13, 2005, a barrage of wind traveling at 63 miles per hour hit Leland and Empire, but not Glen Arbor. But 63 is not 100. Not even close. Nope, yesterday’s storm was the strongest to ever hit Leelanau County, since records were kept starting in 1950.

You can click through to the Sun for more pics and storm coverage and get even more on their Facebook. Jacob also shared the National Weather Service’s Aug. 2 severe weather recap:

  • Multiple rounds of severe weather impacted northern Michigan on August 2nd, 2015. The first severe thunderstorm warning was issued 10:34 am with an additional 27 warnings being issued before the last warning of the day expired at 8:00 pm.
  • The largest hail reported was 4.25″, or the size of a softball, seven miles north of West Branch at 4:55 pm. The large hail was reported by trained spotters and members of the public. There were several reports of damage to vehicles and other property. At the time of the event, a Severe Thunderstorm Warning was in effect for all of Ogemaw County, issued at 4:33 pm. Additional reports of 1.00″ to 2.00″ hail were received from law enforcement, emergency managers, trained spotters and the public across multiple locations in northern Michigan.
  • The 4.25″ hail observed seven miles north of West Branch was the largest documented hail stone ever to impact northern Michigan since records began in 1950 and the largest since 1998 when a 3.50″ hail stone was recorded in Arenac County.
  • Hundreds of downed trees and power lines were reported on Sunday as 60-80 mph (locally 90-100 mph) straight-line winds accompanied the severe thunderstorms.

Lots more at that link including some pics of that hail.