“Autumn is a second spring when every leaf is a flower.” — Albert Camus
Why do leaves turn different colors and then fall off in autumn?
Fall changes to deciduous trees and shrubs often are thought to be due to lower temperatures. Shortening days are a much more significant influence on the change, however.
Extreme weather conditions can interrupt the process. That happened in October 2019, when a mid-October hard freeze caused a sudden change from green to brown overnight. Our freeze this week was much milder and hopefully we will get color change in 2020.
During summer, cells containing chlorophyll enable trees to produce carbohydrates from light, carbon dioxide and various other nutrients taken up by the roots of the tree. As daylight hours shorten, light available for photosynthesis diminishes. As light wanes, photosynthesis is less efficient.
The tree responds by not producing chlorophyll, thus shutting down photosynthesis. The chlorophyll produced earlier in the season deteriorates, removing the dominant green color from the leaves. The yellow and orange pigments (carotenes and xanthophylls) that have been there all summer are now visible.
Red colors are another story. Trees that turn red actually produce new pigments (anthocyanins) in autumn. These are the same pigments found in blueberries.
The final color change in autumn is to brown. The yellow and orange pigments eventually are broken down, leaving only brown pigments (tannins) in the leaf.
Since the leaves no longer produce carbohydrates, the tree prepares to “let go” of the leaves. Not all deciduous trees go through this process in autumn, but the vast majority do.
The zone on the twig where the leaf attaches is called the abscission layer. This layer allows transport of nutrients and water from the roots to the leaves and transfer of carbohydrates from the leaves to the roots for storage. That layer is formed in spring. In autumn, the tree produces a layer of dry, corky cells at the abscission layer that basically cut off the link between the woody material and the leaf. Since the leaf is cut off, it falls off.
You may notice that different parts of a tree will be in different stages of these processes. That is because there can be microclimates within different parts of the same tree canopy
Weather during the growing season can influence the quality and duration of the fall colors. Spring drought can cause leaves to shut down and fall off earlier than usual. Sunny and dry conditions in the summer will favor better leaf color.
While it is important to make sure trees are adequately watered during the warm months, long rainy periods will mitigate against brilliant fall displays. In autumn, windy days will shorten the display time. Early hard freezes can cause water in leaves to freeze; the cells are destroyed and the leaves turn brown. They may not fall, but instead linger into winter on the tree.
The late spring and early fall storms that have characterized 2019 and 2020 have been stressful to woody plants. Interrupting the natural growth cycles, causing broken limbs and requiring trees to refoliate (after spring storms) twice instead of once have stressed our woody plants.
Make sure you follow the best care practices for your trees, including winter watering. Loss of our fall color is disappointing, but loss of a mature tree is far more problematic.
El Paso County Extension is operating remotely due to COVID-19. Submit gardening questions by email at email@example.com.