It’s as if a magic brush painted the northern New England landscape with every conceivable shade of vibrant red, orange and yellow this past week. The major player in this phenomenon is chlorophyll, the pigment that gives leaves their green coloration during spring and summer. Chlorophyll is able to absorb from sunlight the energy that is used in transforming carbon dioxide and water to carbohydrates, such as sugars and starch, inside cell-like structures called chloroplasts, a process referred to as photosynthesis. But in the fall, because of changes in the length of daylight and changes in temperature, the leaves stop their food-making process. Chlorophyll breaks down and the green color of leaves disappears, revealing colors that have been masked by the chlorophyll all summer (as well as reds manufactured in the fall). Imagine a world without chlorophyll, where the bright golds, purples, yellows, oranges and reds of autumn leaves would be the natural colors seen in spring, summer and fall.
Peak foliage has arrived in northern New England and will soon be evident further south. It’s fairly common knowledge that there are three main pigments that affect fall leaf colors: chlorophyll (green), carotenoids (yellow) and anthocyanins (red). Both chlorophyll and carotenoids are present in leaves throughout the growing season. Most anthocyanins are produced in the autumn, in response to bright light and excess plant sugars within leaf cells.
The amount and brilliance of the colors that develop in any particular autumn season are related to weather conditions that occur before and during the time the chlorophyll in the leaves is dwindling. Temperature and moisture are the main influences.
A succession of warm, sunny days and cool, crisp but not freezing nights seems to bring about the most spectacular color displays. During these days, lots of sugars are produced in the leaf but the cool nights and the gradual closing of veins going into the leaf prevent these sugars from moving out. These conditions – lots of sugar and light – spur production of the brilliant anthocyanin pigments, which produce reds, purples, and crimson. Because carotenoids are always present in leaves, the yellow and gold colors remain fairly constant from year to year.
The amount of moisture in the soil also affects autumn colors. Like the weather, soil moisture varies greatly from year to year. The countless combinations of these two highly variable factors assure that no two autumns can be exactly alike. A late spring or a severe summer drought can delay the onset of fall color by a few weeks. A warm period during fall will also lower the intensity of autumn colors. A warm wet spring, favorable summer weather, and warm sunny fall days with cool nights should produce the most brilliant autumn colors. (U.S. Forest Service, USDA)
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At this time of year, our eyes are immediately drawn to the brilliant orange, red and yellow pigments of Sugar Maple (Acer saccharum) leaves. However, there’s much to be said about the less flamboyant splashes of color adorning some of the species of trees that provide New England’s spectacular fall foliage. One such subtley-colored fall tree that often grows in upland forests along with Sugar Maples is White Ash (Fraxinus americana). One of the first trees to change color in autumn, White Ash can turn shades of yellow, orange and red, but deep red, maroon and purple are typically the grand finale of this species. Often its leaves progress from green to yellow and eventually maroon. While it might not be the first tree that catches your eye, make a point of looking for its colorful, compound leaves – you won’t be disappointed.
Eastern Larch, Larix laricina, is also known as Tamarack, the Algonquian name for the species which means “wood used for snowshoes.” This tree strongly prefers moist to wet sites in acidic soils and is a common sight in northern New England bogs. Eastern Larch is the only species of conifer in New England that drops all of its leaves/needles every year. The needles are borne on short shoots in groups of 10–20 and prior to falling off, they turn a beautiful golden color.
Go, sit upon the lofty hill,
And turn your eyes around,
Where waving woods and waters wild
Do hymn an autumn sound.
The summer sun is faint on them —
The summer flowers depart —
Sit still — as all transform’d to stone,
Except your musing heart.
Elizabeth Barrett Browning, 1883