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Fall Foliage

October 11, 2015

red maple fallIt’s time to check in on the red maple tree (Acer rubrum) that I’m following through a round of the seasons as part of Lucy’s tree-following meme at Loose and Leafy.

October is fall foliage time in Maine – the season when the leaves of our deciduous trees turn glorious shades of scarlet, orange, and gold before fading to brown and falling off.

The United States National Arboretum web site has a wonderfully clear explanation of the science behind fall foliage. Color in leaves is created by pigment-producing chemicals. It seems that the chemicals producing yellow (xanthophylls) and orange pigments (carotenoids) are present in the leaves throughout the growing season, but those colors are covered up by the green pigment produced by chlorophyll in the leaves. As the days get shorter, however, the leaves stop producing chlorophyll, the green color fades, and the yellows and oranges begin to show. Because this process is an effect of the shortening hours of daylight, we can expect it to proceed from north (where the days get shorter more quickly after the autumnal equinox) to south and we can expect the yellows and oranges to begin showing through at about the same time each year.

This last bit of information surprised me because my sense is that the colors appear later some years than others and are also much more vivid in some years than others. This is because some parts of the process are dependent on weather conditions. According to the U.S. National Arboretum, “Abundant sunlight and low temperatures … cause the chlorophyll to be destroyed more rapidly.” In years when those conditions are met, the foliage color will reach the point where we are aware of it more quickly. In years like this year, when September was exceptionally warm, the fall foliage colors are slow to put on the big show that is known as “peak color.”

There is another way that weather influences fall foliage, and that is in the production of  additional pigment chemicals, anthocyanins, which produce the red and purple hues in autumn leaves. In most tree species, the anthocyanins are not present throughout the growing season but are produced in fall from sugars in the tree leaves. Warm sunny days and cool (but not freezing) nights are particularly conducive to the production of anthocyanins. When those scarlet hues mix with the yellows and oranges, we experience the foliage colors as more brilliant. Thus, years when weather conditions are good for the production of anthocyanins are experienced as more vivid foliage years. Soil moisture also plays a role here; in years when trees are stressed by drought, they are more likely to drop their leaves before the anthocyanins have much chance to do their magic.

The quality of fall foliage also depends on tree species. Different types of trees differ in when they begin to slow the production of cholorphyll, how quickly they do so, and how prone they are to produce anthocyanins. In New England, maple (Acer) trees are known for their brilliant fall colors. And, true to their name, red maples are particularly likely to produce red pigments in their autumn leaves. This is why it is a disappointment that my red maple by the driveway is showing lots of yellows and oranges in its canopy, but little red – especially since there are red maples across the road from my house that are a blaze of scarlet hues. Since all these trees have been experiencing the same weather conditions, micro-climates must be at work here. Do those trees across the road get more sunlight during the day? Are they exposed to lower nighttime temperatures? Are they growing in more moisture-retentive soil? I’m hoping that the leaves on the lower part of my tree, which are still mostly green, will produce more anthocyanins in the days to come and give me a memorable display of color.

Most people in Maine are not expecting outstanding foliage color this year. The folk wisdom is that our unusually dry summer will dull the colors. But our color is not yet at its peak, so there is still time for a beautiful season of fall foliage.

12 Comments leave one →
  1. October 11, 2015 9:30 pm

    Such interesting questions, Jean! I’m watching the Maple tree in our front yard in Illinois. No Autumn colors yet, but it won’t be long now. Wishing you scarlet leaves soon! ♡

    • October 16, 2015 9:06 pm

      Dawn, I find the science of fall foliage fascinating. I did get some nice reds one day. We have very cold weather (20s) coming over the weekend, so the leaves will mostly be on the ground in a few days.

  2. October 12, 2015 2:17 am

    I do love the brilliant colors of fall and I also watch the changes daily; it is such a truly amazing process.

    • October 16, 2015 9:06 pm

      Charlie, It’s a great time of year to be outdoors!

  3. October 12, 2015 5:22 pm

    the fiddlewood has some orange leaves
    and my Japanese maple is leafing out wine-dark.
    As we step into summer.

    • October 16, 2015 9:07 pm

      Diana, It is interesting the way the spring colors anticipate those of fall. When spring comes, my red maple will first get red flowers, and its leaves will have a red tint when they first emerge.

  4. October 12, 2015 11:10 pm

    That’s an excellent summary, Jean. Because it doesn’t get really cold here we get little in the way of fall color but I’ve noted that both my ornamental pear (Pyrus calleryana) and my coral bark Japanese maple (Acer palmatum ‘Sango Kaku’), usually my most reliable sources of our version of fall color, have been much slower than last year to show a color change. For all practical purposes, with even nighttime temperatures back in the upper 70s, it’s still summer here.

    • October 16, 2015 9:09 pm

      Kris, Fall foliage was one of the things I missed when I lived in southern California. We are rapidly closing in on winter here, with temps expected to dip down into the mid-twenties over the weekend.

  5. October 13, 2015 8:05 pm

    Wonderful info…I adore acers in fall!

    • October 16, 2015 9:10 pm

      Donna, They are beautiful. We are expecting very cold weather in the next few days, which will put an end to the color show.

  6. October 18, 2015 6:51 am

    Hello Jean, we’re having a very good autumn this year for colour – though little can match New England. We’ve had cold nights but the temperatures have risen during the day, sometimes having almost 15C difference between night and day. These yo-yoing temperatures have really bought out the autumn colours and we’ve noticed trees that – while last year went rather ungracefully – have turned wonderful autumn colours.

    • October 24, 2015 9:24 pm

      Sunil, We ended up having wonderful fall color here this year, too — even though it didn’t look promising at first. Some much needed rain at just the right moment helped the trees to produce more of those anthocyanins. We’ve now had several hard frosts and freezes, however, and the leaves are rapidly falling from the trees.

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