When I arrived at the Nature Institute early last September, the entire landscape was overwhelmingly green; the deep, rich, saturated green of late summer. Each tree line — at the edge of our small meadow, bordering the cow pastures — was a solid wall of green, undifferentiated to my eye, obstructing any view into the forest beyond.

There was, however, much more to these leaves than green. In the spring of 2023, during a fellowship at The Nature Institute, Ceinwen Smith studied these trees emerging from bud, observing the changing colors of the developing leaves, flowers, and fruits (see In Context #51). My work was to continue her project, observing these same trees as once again their colors transformed at the other end of their growing season. Carefully studying fall color change has given me a new understanding for leaves as processes, not merely objects to be gazed upon, and has opened intriguing questions to be further explored.

One of Ceinwen’s tools which I adopted was a paper chromatography technique to separate organic pigments, revealing aspects of color beyond those that first meet the eye. Paper chromatography makes use of the property familiar to us from watching the wick of a lamp or a dishtowel whose corner is in a puddle of water: fluid migrates out of the pool of liquid and into the dry areas. In this case, a portion of the leaf is crushed and transferred onto chromatography paper which is then set into a shallow dish of liquid solvents. The liquid rises up the paper, dissolving the pigments it encounters, and depositing those pigments at different rates, thus separating them out into distinct bands of color.

Ceinwen’s techniques, and variations thereof, quickly revealed that the green chlorophyll pigment was always accompanied by two yellow pigments — one more golden-orange and one more lemon-yellow. A literature search confirmed that every green, photosynthesizing leaf has not only chlorophyll but also a different class of substances, the carotenoids, which include both carotenes (beta-carotene is well-known among these) and xanthophyll. Chromatography separates these pigments into distinct bands of color: yellow and yellow-orange carotenoids and green chlorophyll (Figure 1).

Outside my window, the yellowing leaves of a graceful gray birch were glowing gold, illuminated by the afternoon sun. Sampling one of these leaves — part green and part yellow — showed an absence of the green pigment in the yellow region. The same carotenoid pigments were present as in the fully green leaves, but the chlorophyll was gone (Figure 2).

I began to see yellowing leaves everywhere: the broad leaves of corn and soybeans in the fields, the short rich grasses in the cow pastures, the fern-like asparagus stalks in the garden. In the scientific literature, I read how in each leaf chlorophyll withdraws and transforms into sugars and even starches. These substances are stored in the roots of those plants that overwinter (the birch, the asparagus), or in the seeds of annuals (the kernels of corn, the soybeans), in each case providing nourishment for the next growing season. As the chlorophyll withdraws, the ever-present carotenoids are revealed as shades of yellow.

Now the golden colors all about me took on new meaning. Life was not merely dying away, fading like a bunch of cut flowers on my table or the wilting lettuce in my fridge, but rather an important phase was occurring in each and every plant. This final stage of the growing season, senescence as it is named, finds processes taking place in plants that will provide them with a strong start in the spring, whether that is an emerging asparagus stalk in the garden, a flowering catkin on the birch, or the soybean sending out its first root and shoot.

It is truly remarkable that each and every plant appears to “prepare” itself for what will only come months in the future, after a long winter of dormancy here in our part of the world. Something is at work — different in kind from that of creatures like us with central nervous systems and conscious thought — that has a relationship to what will be in the future and not only with what is taking place in the present moment.

Back to the leaves. At the same time, an entirely different process was taking place. A few trees at first — red maples, hornbeams — were developing strong reds in their green leaves. These red pigments — a different class of substances named anthocyanins — resolved under chromatography into a cool purple-red and a warmer fuchsia-red (Figure 3).

Unlike the yellow carotenoids, revealed by the withdrawing green, these anthocyanins were not previously present in the green leaves of the red maples, the sugar maples, the oaks, or the hornbeams. These red pigments were something entirely new, created in the leaf, just as the growing season was coming to an end.

Here the existing literature provides no simple answer. How satisfied we feel when we learn of a functional explanation — a purpose — for a natural process. Chlorophyll is broken down and re-purposed by the plants for a future use. But why would a leaf, weeks or even days away from falling to the ground, use precious energy to synthesize a new compound? In the words of one biologist, “The puzzle is why anthocyanins should be produced in leaves that will soon be discarded. Plants, like all organisms, are usually very conservative with their resources. It doesn’t make sense that a maple would waste energy creating molecules soon to be shed with their leaves. From a Darwinian evolution point of view, the production of colorful pigments should serve some survival function.”1 Indeed, studies continue to test different functional benefit hypotheses of anthocyanin production.2

We might also allow ourselves to investigate reddening differently. Does each tree reveal an aspect of itself through its own particular manner of reddening? If so, then I would have to practice new ways of perceiving trees, develop a new language of color in order to understand what each tree is showing to us. I started with careful daily observations.

A sugar maple growing near The Nature Institute by the side of the road was one of two trees I visited daily. Every morning, I looked for signs of color change, sometimes lying on the huge rock at its base to look up through the network of branches with their layer upon layer of leaves. The first sign of color change was in the stalk of certain leaves, a faint reddening along the top ridge of the stalk, particularly in those parts with the most sun exposure. Then, slowly, the green leaf blades began to lighten, becoming slightly yellow- green. Over time, distinct yellow clumps of leaves appeared here and there, often at the ends of the lower branches but also scattered throughout the tree (Figure 4). All at once the tree was yellowing everywhere.

The yellow leaves – or yellow parts of leaves – appeared thinner, lighter, more transparent than the green ones. It was into these delicate yellows that blushes of red emerged, giving the leaves their unique orange glow (Figure 5).

Other sugar maples in the area reddened much further, and more deeply. The stately trees lining the boulevard of a nearby village offered a majestic display: leaves still green near the trunks of the trees, but with shades of luminous yellow and blazing orange and fiery red further out. My tree, however, blushed with a subtle orange and then faded into ever paler shades of brown. It was one of the first trees to drop its brown leaves and stand bare-branched along the road.

Other trees were reddening in very different ways. The early red in the red maples came into each leaf, and the tree as a whole, in a characteristic way from the periphery in towards the center. The hue and placement of this peripheral red is shown in Figure 6 first in a single leaf, and then in layers of branches in Figure 7.

Just at this time, Ella Lapointe was offering her autumn painting course at the Nature Institute. This was a remarkable experience for me as I learned, through painting, to develop new eyes for the autumn colors. Ella took us through carefully structured exercises which, like learning to hear musical intervals, taught a method of coming to know the colors. The exercise “taking a color for a walk” began with one small square of color on the page: perhaps a crimson red. The practice was then to mix, on the palate, a variation of that color, moderating it with a little yellow, then perhaps with a touch of blue, and placing a small square of each new tone beside the previous one on the page, trying to proceed in small steps, not giant leaps, until the colors began to echo the earthy tones of autumn. In the inverse of how the chromatography process separated out the leaf color into its constituent pigments, here I practiced composing colors out of separate pigments. I slowly grew able to see the colors more truthfully.

Having gained some capacity to see and mix colors, Ella showed us that we could trace the outline of the leaf, so as not to be distracted by the challenge of matching its form, and instead direct all of our attention to matching the colors of the leaves that were now rapidly developing multiple hues. Figure 8 shows a leaf from a red maple tree on the right and, on the left, my painting of it. How many shades were now visible to me through this painting exercise! The early peripheral reds had in places darkened into nearly purple tones. The green had withdrawn completely, allowing the yellows to be seen. Where red had developed in the yellow areas of the leaf, a whole range of orange tones were now visible.

Elsewhere on The Nature Institute grounds a scarlet oak was reddening in a completely different manner. This tree leans out into our small meadow from its place at the forest’s edge and unlike the sugar maple’s early shedding of leaves, it stood into snowy winter still holding its dry leaves — as did other varieties of oaks in our landscape.

Just as that scarlet oak was late in letting its leaves fall, so too it was late to develop color. The first red appeared in leaves close to the trunk — in the understory of the leaves, if you will. Here, the red developed throughout the leaf blade in a mottled fashion. This red-coming-into- green yielded a deep, rich red — entirely different than the blushing orange-into-yellow of the sugar maple. Over time, the scarlet oak leaves also developed more colors and, one afternoon, I painted two versions of a single leaf from that scarlet oak (Figure 9). Through painting, the leaf revealed more of itself to me than I had previously thought to see — yellow, orange, and brown tones, each appearing differently into the leaf, alongside the initial red tones, always deep and earthy.

Even as I selected and painted individual leaves, I continued to observe the changing colors of the trees as a whole: no leaf could exist without its tree. Just as a variety of tones appeared throughout the leaf, concentrated in different spots, so the tree as a whole developed a variety of tones strongly localized on certain branches. After painting the full scarlet oak one afternoon, I selected from the trees those leaves that exemplified the colors and their distribution along the different branches (Figure 10).

Widening the view again, the tree was also visible as a part of the landscape, which was itself transforming each day. That same scarlet oak, seen in its place at the edge of the meadow (Figures 11 and 12), became suffused through and through with a rich, deep, red before growing brown.

This spring, I am watching these trees emerging from bud into leaf and flower, with a particular eye for the changing colors at this start of the growing season. What is the connection between spring colors and fall colors? This fall, I will look more carefully at other reddening trees which I only glanced at in my first season. The hornbeam, in particular, showed a strong connection between reddening and sun exposure: how exactly does that unfold? Do other trees show that same connection? The trees have much more to say and I am gradually learning the language they speak.

NOTES

1. Saupe, Steven G. (2020). “Autumn Leaf Color: The Majesty & Mystery,” Minnesota Maple News. College of St. Benedict/St. John’s University, Collegeville, MN.

2. For one such study see: Taylor S. Feild, David W. Lee, and N. Michele Holbrook (2001). “Why Leaves Turn Red in Autumn. The Role of Anthocyanins in Senescing Leaves of Red-Osier Dogwood.” Plant Physiol. (October) 127(2): 566–574.

Autumn Colors: Continuing a Study

Marisha Plotnik