Autumn Weather Brings Natural Splendor

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Autumn Weather Brings Natural Splendor

CHAMPAIGN, Ill.--Autumn's cooler weather brings a lot of changes to our scenery. The green leaves of spring and summer turn to rich and vibrant yellows, reds, and oranges. According to the International Society of Arboriculture (ISA), science is the key to understanding the beauty of autumn. Leaf pigments, the physics of light, weather conditions, plant species, and geography all play their part in creating the stunning colors of autumn.

Leaf Pigment and Light - Sunlight striking pigments in leaves creates the various colors we see throughout the year. In the autumn, leaf pigments play a crucial role in the new palette of colors we see. Chlorophylls, carotenoids, and anthocyanins are all pigments that help determine what color the leaf will display.

• Chlorophyll: This pigment gives leaves their green color. It is present in photosynthesis, the process by which sunlight is used to transform carbon dioxide and water into carbohydrates (sugars) that fuel a tree's growth. Photosynthesis is most active during the spring and summer when there is more sunlight and warmer temperatures.
• Carotenoids: As the daylight hours dwindle and temperatures begin to cool, photosynthesis slows down and less chlorophyll is produced. Carotenoids, usually masked by chlorophyll, begin to emerge, as trees begin to lose their green appearance, and leaves bare yellow and orange pigments.
• Anthocyanins: This pigment is only produced during the autumn months, unlike chlorophyll and carotenoids, which are present in a leaf throughout the year. These complex water soluble compounds in leaf cells react with excess stored plant sugars and exposure to sun light resulting in brilliant pink, red, and purple leaves. A mixture of red anthocyanin pigment and yellow carotenoids often results in the bright orange color of some leaves.

Weather and Color - Weather conditions that occur before and during the decline of chlorophyll production can affect the color that leaves may display. Carotenoids are always present so the yellow and gold colors are the least affected by weather.

Colors most affected by weather are the red tones created by anthocyanin. On warm and sunny days, lots of sugar is produced in leaves. Trees exposed to brighter sunlight generate the reaction between the anthocyanins and the excess sugar creating the bright red hue.

Sharp changes in climate can paint the most spectacular display of color. Cooler temperatures cause the veins in the leaves to gradually close preventing the sugars from moving out, which preserves the red tones. Warm sunny days followed by crisp and cool nights can be responsible for the lush tones of fall we see all around us.

Soil moisture can also affect autumn color. A particularly dry summer can delay the onset of color change by weeks. A warm, wet spring, favorable summer weather, and sunny fall days with cooler temperatures at night are the ideal conditions for producing the most radiant colors.

Species and Geography - Tree genetics help determine what color leaves will turn. Color depends on the levels of iron, magnesium, phosphorus, and sodium in the tree and the acidity of the chemicals in the leaves. Some tree species displaying yellow foliage are ash, birch, beech, elm, hickory, poplar, and aspen. Red leaves are most often seen on dogwood, sweetgum, sumac, and black tupelo trees. Some oaks and maples present orange leaves, while others range in color from red to yellow, depending on the specific species.

Deciduous forests and trees, with their many broad leaves that change color almost in unison, display the most noticeable fall color. Evergreen species also display fall colors, only more slowly and gradually. The appearance of autumn color starts as early as mid September in the more northern latitudes, and moves southward reaching its peak in October. Color continues to appear in the more southern regions and higher elevations in the west throughout November. Remember that cooler, higher elevations will display color before lower, warmer geographic areas. Even with these facts, the timing, location, and intensity of autumn colors are not completely predictable.

For more interesting facts on trees visit www.treesaregood.org.

The International Society of Arboriculture (ISA), headquartered in Champaign, Ill., is a nonprofit organization supporting tree care research and education around the world. As part of ISA's dedication to the care and preservation of shade and ornamental trees, it offers the only internationally-recognized certification program in the industry. For more information, or to find an ISA Certified Arborist, visit www.treesaregood.org.