About birds and trees and flowers and water-craft; a certain free margin, and even vagueness—perhaps ignorance, credulity—helps your enjoyment of these things, and of the sentiment of feather’d, wooded, river, or marine Nature generally.  I repeat it—don’t want to know too exactly, or the reasons why. -Walt Whitman

. . . The gods are growing old; The stars are singing Golden hair to gray Green leaf to yellow leaf,—or chlorophyll to xanthophyll, to be more scientific. -Edwin Arlington Robinson

by David Lee

Every autumn the trees, shrubs and herbs in the San Luis Valley prepare for the coming winter.  Among the evergreens, as the pinyon pines and junipers, the changes are not very obvious, as leaves stay green the entire winter.  However, in most of the plants the preparation for winter includes the breakdown of the valuable tissues in their leaves, which is visible to us as changes in color. I have been interested in these color changes for many years, starting in the tropics (where trees lose and gain leaves in no apparent coordination, and color changes are generally not dramatic) and then moving to the dramatic color production in New England forests.  So, there is some science needed to understand the color changes, but hopefully not to destroy the poetic sensibilities of the reader, more like Robinson than Whitman, quoted above.

Plants & autumn colors in the valley  

During autumn in the areas around Crestone, the predominant colors are yellow to gold.  These are predominantly seen in the riparian zones that spill out of the mountain valleys, but also areas on slopes at higher elevations.  The cold and sunny days of autumn stimulate that color production.  The color starts in the high elevation stands of aspen, steadily moving down in altitude.  Later, those colors are produced in the valleys coming out of the mountains, and finally into the riparian zones at the edges of the valley.  The trees that produce these colors are the narrow leaf cottonwood (the principal yellows of Crestone), the mountain birch, the alder, the many willows, and, especially, the aspen.  Oranges and reds are produced mainly by shrubs and small trees, including the black cherry, the wild roses, and skunkbrush.  The large patches of skunkbrush and wild roses seen on the lower slopes of the Sangres, in Penitente Canyon and along Carnero Creek, are also noticeable when driving north towards Poncha Pass. In contrast to the brilliant reds of New England forests, predominantly produced by the red and sugar maples, our two small maples turn yellow.  Leaving the valley and driving east (along the Arkansas River or over La Veta Pass), the low stands of the gambel oak turn a brownish red, just as most eastern oaks turn some shade of red.  Some stands of this oak also grow at Valley View Hot Springs.  At Valley View, and in the mountain valleys along the creeks, stands of choke cherry turn from yellow, to pink, to bright red. Even a short hike along one of our mountain trails will reveal the color changes in more detail.  In addition to the trees and shrubs, leaves of wildflowers change color as well, as the leaves of wild strawberry and wild geranium turn an eye-catching red.

The color palette

Think of a leaf as a sheet of white water color paper.  Leaves and paper are similar because they are both largely composed of cellulose fibers.  Without pigments, as in a variegated leaf, the leaf is also white.  When pigments are added to water color paper, resulting colors are due the subtractive effects of pigments absorbing different colors; the color produced is that which is not subtracted by the pigments.  Red color means that all other colors are absorbed by the pigments.  Leaf color is produced in exactly the same way, and there are three kinds of pigments that can subtract different colors.  The most important plant pigment is chlorophyll, actually two similar molecules, a and b.  Both absorb reds and blues, and produce a green reflected or scattered and transmitted color.  Chlorophyll absorbs light energy that, in a complex and amazing process, splits water into oxygen gas (a by-product), frees up hydrogen and produces energy forms that allow the leaves to turn carbon dioxide into sugar.  Almost all of the carbon in our bodies, especially for us in a region far from the ocean, comes from the photosynthesis of leaves, either by eating meat from an animal living on leaves, or from seeds (grains) or vegetables and fruits, the result of plant photosynthesis, or even leaves directly.

Two other pigments are produced in leaves to complete the color palette.  Carotenes are yellow-orange pigments universally produced in green leaves.  They absorb the blue wavelengths, and allow the yellow-oranges (and rarely reds) to reflect. Carotenes help channel light energy to chlorophyll, and also protect the chloroplasts (the cell bodies where photosynthesis occurs) from too much light energy.  Carotenes are also important in the diets of all animals, as they are the raw material (mainly split in half) to make our visual pigments.  Anthocyanins are red-violet pigments (absorbing blue-green colors) produced in flowers and fruits, and also in leaves as they develop and die.  Although anthocyanins mainly function to make plant structures visible to animals (think of attracting pollinators and dispersing seeds) they also function as a sun-blocking agent to protect plant tissues.

These pigments, singly and in combination, produce the colors we see in leaves during a walk or drive in the valley during autumn.  The reds, greens and yellows are obvious.  However, adding pigments together produces a variety of other colors, just as mixing crayons do.  Red and yellow crayons produce orange (or anthocyanin and carotene).  Green and yellow produce lime-green (chlorophyll and carotenes).  Least expected is mixing green and red, producing a brown color, even approaching dark brown or black depending upon the density of coloring.  In leaves, a mixture of anthocyanin and chlorophyll, depending upon the concentration of each, produces colors of bronze, brown, muddy-red, or even black in a few horticultural varieties. The muddy red of the gambel oak is the combination of anthocyanin and chlorophyll.

Although the vast majority of aspens turn yellow, occasionally a small area or even a narrow line of red or orange (anthocyanins and carotenes) appear.  Jim Erdman, a botanist and naturalist who used to live in the Baca and was involved in local conservation issues, pointed this out to me and also mentioned an obscure report on the association of this color with the underlying geology.  Minerals producing soils with a lack of nutrients, or high concentrations of toxic ones, could stress the plants, and anthocyanin production is often a response to stress in plants.  We like to scan the large areas of aspen, as at higher elevations, looking for those bits of red color.  If we look to the ground along stream margins, we will find the brilliant red roots of aspens, birches and alders.

Although the autumn displays are beautiful and entice visitors to spend time in the Rockies during the autumn season, the color is much less spectacular than the broadleaf forests of New England and the Great Smokes.  The differences are due to the predominance of evergreen conifer trees in our forests.  The only conifer whose needles turn yellow and drop, the larch, is absent from our forests, more common in the north, and also in northeastern United States.  Among the broadleaf trees, the color changes are peculiar to certain species, as the maples in eastern U.S., largely absent from our forests.  Even in the east, color production has changed with the loss by disease of such common yellow senescing trees as the elm and American chestnut early in the 20th century, and the future loss of the spectacular sugar maple in the 21st century—displaced to the north by higher temperatures from global warming.

Why autumn color?

Autumn colors are the by-product of the process of the aging and death of leaves, more technically called senescence.  This is not a random act or even a consequence of growing old, but is a highly regulated process that extracts the maximum amount of nutrients in the leaves and moves them back into the trunks and roots of the tree prior to winter.  These nutrients can thus be used by the tree the following spring, when new leaves unfold and become green.  The crucial nutrient is nitrogen.  Nitrogen in the atmosphere is fixed into usable forms by soil bacteria, and bacteria living in the roots of many legumes and alders.  In plants, the fixed nitrogen is further processed into forms that can be incorporated into proteins, DNA and RNA, molecules that defend plants from animals, and chlorophyll.  The majority of nitrogen in a plant is stored in the foliage, and about half of the nitrogen there is in its chlorophyll.  In senescence, when colors appear, the chlorophyll is broken down and the residue is stored in the central sac, or vacuole, of each cell. The partly digested chlorophyll still can absorb light and can produce damaging by-products (reactive oxygen species, or ROS, also believed to contribute to aging in humans), so this happens quickly.  The nitrogen atoms in those by-products are not re-absorbed by the plant, but the proteins involved in photosynthesis are broken down into amino acids and are transported back into the stems and roots and stored during winter.  The nitrogen in the broken down chlorophyll is deposited in the soil as the leaves drop and decay, available to the roots of trees and other plants the following spring.  Another element that may also be resorbed as the leaves break down is phosphorus, also essential to plant growth.

When leaves change from green to yellow, most of the carotenes are kept in the cells (no nitrogen to capture there) and their color is unmasked by the loss of chlorophyll.  Leaves of a few trees with an abundance of nitrogen, as some legume trees, may not turn yellow, but turn from green to brown and fall off.  That is actually quite unusual.  To me, the most interesting color change is from green to red, like the skunkbrush.  Such plants begin to make the red anthocyanin pigments as the leaves break down the chlorophyll.  They are not unmasked, as the old botany books teach, but are newly made during the senescence process.  That doesn’t seem to make sense.  Why does a leaf, when about ready to die and fall off the plant, go to the trouble of making these pigments, a costly process?  There has been some scientific controversy about this riddle during the past fifteen years.

The benefits of autumn color

If there is a benefit to the appearance of bright leaf color during the autumn, it will most likely benefit the plant in the following growing season and not during the period at hand, when the leaves are about to fall.  Benefitting the plant in evolutionary terms means that it will grow more, produce more seeds, and more seedlings will germinate and grow to perpetuate the variety or species.  That growth will begin in the following spring time.  Two arguments have been developed to explain the evolutionary benefits of leaf color.  The first is an ecological argument, in which the color is a signal advertising the poor nutrition or toxic qualities of the leaf.  If insect pests, particularly aphids, are repelled from visiting the colored leaves, they will not lay eggs during the autumn, which could hatch the following spring and damage the tree.  Such a tree will produce fewer seeds and leave fewer offspring in the future.  There is some evidence that aphids can be repelled by the leaf color, but most of the details have not been well tested.  Biologists often think of color on the plant in terms of perception and changes in behavior, so this is an attractive hypotheses to those who do that kind of work.  This argument could work with yellow and red leaves.

The second argument, and one that I favor, is a physiological one—and only works for red leaves.  In it the red pigments, anthocyanins, are produced during the time when the chlorophyll is actively broken down; they function as a sun screen, to reduce the amount of light in the leaf tissues.  With fewer toxic by-products, the protein breakdown should occur more efficiently and more nitrogen be exported to the storage tissues.  That would mean that more nitrogen would be available for next spring’s growth, more seeds and offspring.  There is now quite a bit of evidence for the sun-screen effects of anthocyanins in different plant tissues at different times.  For instance, tender young expanding leaves often produce anthocyanins in the spring time, as roses (and many of the plants I have mentioned with autumn color produce pinks in the springtime).  There is evidence that red leaves have less residual nitrogen than green ones, in situations where both types of leaves are produced.  We can deduce that the excess nitrogen goes back to the plant, but that has never been tested, nor do we have a clear mechanism of how the leaves are protected—what the Achilles heel in the leaf requiring protection is.

Another means of protection of anthocyanins is as a free-radical scavenger, or anti-oxidant.  You may have heard about the anti-aging properties of blueberries (the first announcement of such effects on mice led to a run on anthocyanin-containing fruits as blueberries and cranberries).  The anti-oxidant anthocyanin in blueberries, cyanidin-3-glucoside, is also the most common one in leaves.  Its chemical name has been reduced to 3CG in the health store trade, and is available in highly concentrated forms.  It is not clear how such anti-oxidant activity could actually work in the leaf cells, however.

At any rate, I hope that this discussion of autumn color in the plants that you will see near your home or along the roads in Crestone and the San Luis Valley will heighten your enjoyment of the annual event, more like the amazement and enjoyment of Edwin Arlington Robinson, rather than destroying the vagueness that Walt Whitman valued!

David Lee is a new resident of the Baca, but a property owner since 2001.  He is a retired professor at Florida International University, in Miami.  He studied the ecology of plants in tropical forests, which led to his research on color changes in autumn leaves in temperate forests, especially in New England.