Following the past couple of weeks that saw us at the beach, we’re moving in-land and re-visit one of my favorite subjects of trees. However, this time we’re looking at a specific aspect of trees’ leaves that pulls tourists into New England during the Autumn season in droves: changing of leaf color during Fall.
Entire forests provide spectacular colors that travel from North to South in a band of vibrance that can be seen from space. As we’re nearing the Autumn season, it’s time to start planning some photography trips to capture this display of Nature’s beauty.
Are there seasonal changes that you’re looking forward to in your area?
Chlorophyll and Leaf Color
A green leaf is green because of the presence of a pigment known as chlorophyll, which is inside an organelle called a chloroplast. When they are abundant in the leaf’s cells, as they are during the growing season, the chlorophyll’s green color dominates and masks out the colors of any other pigments that may be present in the leaf. Thus the leaves of summer are characteristically green.
Chlorophyll has a vital function: it captures solar rays and uses the resulting energy in the manufacture of the plant’s food — simple sugars which are produced from water and carbon dioxide. These sugars are the basis of the plant’s nourishment — the sole source of the carbohydrates needed for growth and development. In their food-manufacturing process, the chlorophylls breaks down and thus are being continually “used up”. During the growing season, however, the plant replenishes the chlorophyll so that the supply remains high and the leaves stay green.
In late summer, as daylight hours shorten and temperatures cool, the veins that carry fluids into and out of the leaf are gradually closed off as a layer of special cork cells forms at the base of each leaf. As this cork layer develops, water and mineral intake into the leaf is reduced, slowly at first, and then more rapidly. It is during this time that the chlorophyll begins to decrease.
Often the veins will still be green after the tissues between them have almost completely changed color.
Much chlorophyll is in Photosystem II (Light Harvesting Complex II or LHC II), the most abundant membrane protein on earth. LHC II is where light is captured in photosynthesis. It is located in the thylakoid membrane of the chloroplast and it is composed of an apoprotein along with several ligands, the most important of which are chlorophylls a and b. In the fall, this complex is broken down. Chlorophyll degradation is thought to occur first. Recent research suggests that the beginning of chlorophyll degradation is catalyzed by chlorophyll b reductase, which reduces chlorophyll b to 7‑hydroxymethyl chlorophyll a, which is then reduced to chlorophyll a. This is believed to destabilize the complex, at which point breakdown of the apoprotein occurs. An important enzyme in the breakdown of the apoprotein is FtsH6, which belongs to the FtsH family of proteases.
Chlorophylls degrade into colorless tetrapyrroles known as nonfluorescent chlorophyll catabolites (NCCs). As the chlorophylls degrade, the hidden pigments of yellow xanthophylls and orange beta-carotene are revealed. These pigments are present throughout the year, but the red pigments, the anthocyanins, are synthesized de novo once roughly half of chlorophyll has been degraded. The amino acids released from degradation of light harvesting complexes are stored all winter in the tree’s roots, branches, stems, and trunk until next spring when they are recycled to re‑leaf the tree.
Pigments that Contribute to Other Colors
Carotenoids are present in leaves the whole year round, but their orange-yellow colors are usually masked by green chlorophyll. As autumn approaches, certain influences both inside and outside the plant cause the chlorophylls to be replaced at a slower rate than they are being used up. During this period, with the total supply of chlorophylls gradually dwindling, the “masking” effect slowly fades away. Then other pigments that have been present (along with the chlorophylls) in the cells all during the leaf’s life begin to show through. These are carotenoids and they provide colorations of yellow, brown, orange, and the many hues in between.
The carotenoids occur, along with the chlorophyll pigments, in tiny structures called plastids within the cells of leaves. Sometimes they are in such abundance in the leaf that they give a plant a yellow-green color, even during the summer. Usually, however, they become prominent for the first time in autumn, when the leaves begin to lose their chlorophyll.
Carotenoids are common in many living things, giving characteristic color to carrots, corn, canaries, and daffodils, as well as egg yolks, rutabagas, buttercups, and bananas.
Their brilliant yellows and oranges tint the leaves of such hardwood species as hickories, ash, maple, yellow poplar, aspen, birch, black cherry, sycamore, cottonwood, sassafras, and alder. Carotenoids are the dominant pigment in coloration of about 15-30% of tree species.
The reds, the purples, and their blended combinations that decorate autumn foliage come from another group of pigments in the cells called anthocyanins. Unlike the carotenoids, these pigments are not present in the leaf throughout the growing season, but are actively produced towards the end of summer. They develop in late summer in the sap of the cells of the leaf, and this development is the result of complex interactions of many influences — both inside and outside the plant. Their formation depends on the breakdown of sugars in the presence of bright light as the level of phosphate in the leaf is reduced.
During the summer growing season, phosphate is at a high level. It has a vital role in the breakdown of the sugars manufactured by chlorophyll. But in the fall, phosphate, along with the other chemicals and nutrients, moves out of the leaf into the stem of the plant. When this happens, the sugar-breakdown process changes, leading to the production of anthocyanin pigments. The brighter the light during this period, the greater the production of anthocyanins and the more brilliant the resulting color display. When the days of autumn are bright and cool, and the nights are chilly but not freezing, the brightest colorations usually develop.
Anthocyanins temporarily color the edges of some of the very young leaves as they unfold from the buds in early spring. They also give the familiar color to such common fruits as cranberries, red apples, blueberries, cherries, strawberries, and plums.
Anthocyanins are present in about 10% of tree species in temperate regions, although in certain areas — most famously New England — up to 70% of tree species may produce the pigment. In autumn forests they appear vivid in the maples, oaks, sourwood, sweetgums, dogwoods, tupelos, cherry trees and persimmons. These same pigments often combine with the carotenoids’ colors to create the deeper orange, fiery reds, and bronzes typical of many hardwood species.
The brown color of leaves is not the result of a pigment, but rather cell walls, which may be evident when no coloring pigment is visible.
This image was captured with a Canon EOS 1D MkIII with an EF 24-105mm f/4L lens attached. Exposure settings were at 1/320 second at f/5.6 with 100 ISO.