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Treebeard's Stumper Answer
3 November 2000

Beautiful Death

We missed fall colors in Washington D.C., though we saw a few early trees that had already turned bright yellow and red. Our local California trees are mostly evergreen, though we also have deciduous trees that are now starting to turn color before they drop their leaves for winter. The stumper is not why these trees drop their leaves while other trees don't. That's just one of nature's many successful strategies. But why do deciduous trees make a show of their autumn leaves? The leaves are dying. How does it benefit the tree that they are also beautiful?

Looking south into the low autumn sun towards the remote confluence of Mono Creek and the upper Santa Ynez River. The golden trees are mostly Fremont Cottonwood (Populus fremontii). This area is a protected Critical Habitat for endangered Least Bell's Vireo (Vireo bellii pusillus) birds. It should be protected. Hiking into this magical forest reminds me of the trip through Mirkwood in The Hobbit. Barka Slough at the mouth of San Antonio Creek on Vandenberg AFB is the only place like it.
These Vine Maple (Acer circinatum) leaves along the North Umpqua River in Oregon have more intense fire colors than the Bigleaf Maple (Acer macrophyllum) in our local Santa Barbara canyons.
These yellow Bigleaf Maple (Acer macrophyllum) leaves are not so pretty up close, as insects and fungi take their toll. Deciduous autumn leaves are dying, so why are they also so beautiful?
These photos are slightly enhanced to bring out the fall colors.
For some reason, my Olympus 320 digital camera doesn't pick
up autumn leaf colors very well. Is this another stumper?

Before trees let go of their autumn leaves, they break down and recycle the proteins and green pigments for other purposes. This exposes other yellow, orange, and red pigments that were already there, but hidden by green chlorophyll. You can find yellow leaves on evergreens too, since trees do this process on all old or damaged leaves. The beautiful fall colors are mostly not new colors the trees create. They are part of what made the trees beautiful all along. That leaves the question why nature is beautiful at all, and that's the real stumper!


Graybear's first answer addresses my closing question:

My answer is: "Why does coffee smell so good in the morning? It doesn't benefit the coffee beans! What is the advantage of each snowflake being different? Does a beautiful sunset benefit the cirrus clouds?" I know the answer to the scientific question, 'How', but I figured the answer to the 'Why' question was just part of God's gift of creation.
Then he sent this second answer (with tongue in cheek):
Wait! I think I got it! The tree benefits because, if it has the prettiest fall colors, no one will want to cut it down for firewood!

Yeah, you see, the trees that had ugly fall colors got chopped up for firewood and are now extinct...just look at artists renditions of what trees looked like before man invented fire! UGLY! Perhaps that's why pine trees and other conifers grow so fast, too, they don't change colors AND they burn so easily. Hehehehehehehehehe!

I laughed out loud when I read this. Then I went "Hmm... maybe so!" I know my Mom carefully protects the prettiest Vine Maples around their house in Oregon, and she isn't so fussy with the less-colorful Bigleaf Maples, so maybe there is some selection happening! But that doesn't account for deciduous oaks that usually turn a dingy yellow-brown. :-)

Humans influence fall colors in other ways too. Flying over the Rockies on our school Washington D.C. trip, just north of Santa Fe, New Mexico, we could look down and see brilliant patches of golden Aspen trees, like a field of California Poppies, but seen from 40,000 feet! The distribution of these trees no doubt depends on human caused (and prevented) fires. Maples line the roads in Oregon because they typically grow in clearings in the woods, and roads qualify.

My point is that deciduous trees aren't actually producing any new pigments for their fall colors, so they need no special explanation. Green chlorophyll is an expensive molecule for trees that contains magnesium and nitrogen compounds. So it makes sense for the tree to dissassemble the molecules and save what they can to use again before allowing the leaves to fall away as compost.

It's interesting that the colors we see are the colors that are not being used by the leaves. Chlorophylls absorb blue and red light the best, but they are not so good with green and yellow light which is mostly reflected That's why plants are green. Other photosynthetic pigments like carotenoids have different sensitivities. That's why different trees have different fall colors, and also part of how we can identify trees from a distance based on color and form. Just like bird watchers can identify a bird on the wing, so I've become a very good botanist at 55 mph on Highway 154 on my drive to school!

Absorption spectra of chlorophyll and carotenoid pigments.
(From An Introduction to Photosynthesis by Wim Vermaas.)

Deciduous plants drop their leaves when it becomes too expensive to maintain them. On the East Coast, that's winter. But here in Central California, we have a classic Mediterranean Climate with mild winters and a six month summer drought. Our dry summer is the most challenging time for most native plants, and a number of our plants like Buckeye (Aesculus californica), Chapparal Current (Ribes malvaceum), and Deerweed (Lotus scoparius) are true summer-deciduous plants that develop "fall colors" and drop their leaves in early summer.

It's striking that our winter-deciduous trees like willows, maples, and some oaks drop their leaves in the fall and stand bare for the winter at the same time that the hills and fields are starting to turn green with next year's wildflowers. Why do these trees go dormant just when most other plants are starting to grow? It's a hint that many of these deciduous trees grow along streams and rivers, for example the Fremont Cottonwood (Populus fremontii), los alamos in Spanish, that grow along Alamo Pintado Creek ("painted cottonwood", a reference to their fall colors) near Dunn Middle School. See my Fall or Spring? (21 Nov 97) stumper for more discussion. I've made a fairly complete list of local deciduous trees and shrubs from my Woody Plants of the Central Santa Ynez Mountains database project.

Several local shrubs such as Poison Oak (Toxicodendron diversilobum), Elderberry (Sambucus mexicana), and Foothill Ash (Fraxinus dipetala), are winter-deciduous plants that drop their leaves early in summer during dry years. Scrub Oaks (Quercus berberidifolia and Q. dumosa) are semi-deciduous shrubs that drop their leaves at the same time as new leaves appear. Mormon Tea (Ephedra) and most Cacti (Opuntia etc.) drop their true leaves soon after they appear and depend on their photosynthetic stems. California Bay Laurel (Umbellularia californica) and Madrone (Arbutus menziesii) are evergreen trees that develop brilliant yellow and red colors in their old leaves that they drop one by one during summer. Nature has many successful strategies! This shows the complex origin of our California flora from successive northern and southern invasions as our climate has shifted between ice age and warming over the course of recent geological time.

New Poison Oak foliage is often reddish... are the fall colors.

Poison Oak leaves are often bright bronze-red when they first appear in spring, as in the photo. They also show red colors before they drop in mid-summer or fall. Maybe this is an example of how green chlorophyll can mask other pigments that are already there. There are also street trees with permanent reddish bronze or copper foliage, which shows that red pigments can also mask green chlorophyll.

I might be wrong about this stumper, at least as regards bright red pigments. Orange and yellow colors in tree leaves are due to carotene and xanthophyll pigments. But several sources say that the fiery red colors of Vine Maple and Sugar Maple are due to the late-season production of special anthocyanin pigments that are produced from sugars that remain in the leaves after sudden cold weather makes the sap too viscous to travel to the roots for storage. Are there different red pigments? It's reported that Einstein said "Everything should be made as simple as possible, but not simpler." Maybe a stumper does remain here.

There's lots of info on fall leaf colors on the Web. Here are some links for further research:

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