Friday, November 18, 2011

Why Do Trees Turn Red in Fall?

Color Comes to AVC

It's that time of year when "Fall" (the season) also means "fall" (what the leaves are starting to do).

AVC looks especially nice right now.

It also is looking good in Arizona, as my brother confirms with a picture from near Flagstaff.  Here's what it looks like now in Northern Arizona.  This photo was taken by Fred Hood, whose wild cat website is  No ocelots or tigers in this picture, just a lovely autumn scene.

What causes trees to change color?  If you recently got an A in botany, go ahead and skip this: nothing here is original research or in any way deviant from the usual story.  But if you're not sure, here's the basic idea.

Fact A.  In North America, on average, we have sunny summers and cold winters.  Just like wearing flip flops and shorts on the 4th of July, trees that want to specialize in summer need appropriate leaves.  But our beach gear is not so good during a winter snow storm, especially in a habitat like interior Maine's.  So trees can either be like the pines, with year-round needles that don't soak up sunlight so well but which can endure a blizzard, or they can be like a sycamore, with a parasol instead of a knitting needle for a leaf shape.  An ice storm could weigh the tree down with too much heavy snow and ice in the off season, breaking branches or bringing down the whole tree, plus of course there's the year's accumulation of insect damage and parasites.  It makes sense just to get rid of the entire leaf, spend the winter with bare branches, and grow a fresh one when it's warm and sunny again.

(We'll ignore for the moment the tricky part about why manzanita has red bark instead of red leaves.)

Fact B.  On campus, we have both types of trees, the evergreen pines and the deciduous broad-leafed types.  Therefore by mid-winter, some trees will be bare, and some will still have leaves, which among other things, is a good thing for our campus owls.  With the right perspective, one can usually see both tree types in the same view.  (The dark shadow below is a pine outside the Learning Center, as one looks out towards the F entrance of Parking Lot 10.)

Fact C.  The red part of the color has been there all along.  Now I turn to David Sibley, the famous nature artist.  In his book on the trees of North America, here's what he says.

"The brilliant colors of the autumn forest are among the most striking and most viewed spectacles of nature.  Nearly all boradleaf trees develop some amount of yellow or red color before they drop their leaves in the fall.  Yellow pigments are already present in the leaves, masked by green chlorophyll, and are simply revealed when the tree begins to withdraw resources from the leaf and the chlorophyll breaks down."

Red, Sibley says, is synthesized by the trees just before the leaves drop.

He goes on to note that by "combining just a few pigments --- green, brown, yellow, and red --- trees produce the entire range of fall colors."

One can plant for this, in local backyards.  If you want a taste of New England, a book called Trees of the California Landscape (Charles Hatch, UC Press) lists ornamentals one can plant, itemized by color.  It has over one hundred species in its review, and in fact, one could alternate, with a red tree and then an orange one and then a yellow one.  Sibley again: "Each species or genus often shows a particular patten of color.  Aspens are famous for their dramatic show of golden-yellow color. [ . . . ] Sweetgums are distinctive for having scattered purple, red, yellow, and green leaves all simultaneously on a single tree, even on the same branch."  (Some people may know this plant by another common name, liquid amber.  It is also called red gum.)

Maples are especially famous for their brilliant colors.  When growing wiild in the woodlands of the eastern USA, why don't they all turn the same color, and at the same time?  It turns out that it may be a sexual difference.  We all know that a male lion has a mane (at least in the African types), while females do not.  It may be the same in maples --- not manes, but variations in color and timing.  This possibility is explored in a post by David Sibley on his blog, which one should see for the full (and illustrated) discussion.

And that just (ahem) "leaves" us with this final thought.  Does it even matter why?  Do we need to know the answer in a technical sense?  Maybe the great Creator in the sky just likes variety, and too, has a soft spot in His / Her heart for beauty.  Whatever the cause, we have just concluded a week that was brim-up with beauty galore.

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