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'Field Notes:' What Determines When Leaves Fall?

Fall leaves.
Fall leaves.

Every autumn, deciduous trees and shrubs shed their leaves. In some years, leaves are shed earlier than in others. Why does this happen? What determines when leaves fall?

Leaves are organs that enable plants to capture sunlight and turn it into useable energy in the form of sugars and other carbohydrates. In much of North America, however, winter is too cold for these functions, so most plants become dormant.

Leaves serve no purpose when trees are dormant, and they may be a liability. Strong winds and heavy snow can put tremendous pressure on limbs that have leaves. Some of the worst tree damage occurs during late-spring snowstorms when wet snow collects on trees that have already leafed out. So for trees that produce broad leaves, it’s better to spend the winter bare.

As autumn progresses, trees prepare for winter by stopping the production of chlorophyll, the green pigment that captures light. The leaves gradually change color as nutrients are withdrawn and transferred to roots and stems. At the same time, enzymes digest the cells at the base of the leaf stalk forming an abscission layer or scar. When digestion is complete, the leaf falls off.

Dormancy is brought on by a change in the levels of plant hormones. Short day length is the most important environmental cue that stimulates the onset of dormancy and leaf fall. For that reason, in cities, trees closest to street lamps are often the last to lose their leaves. Drought will also hasten the onset of leaf fall, causing trees to shed their leaves earlier following a dry spring and summer. On the other hand, superabundant watering or hard pruning stimulates vigorous growth which delays the onset of dormancy.

Some years, you may notice that many trees continue to hold their leaves much longer than usual. A wet spring and early summer can stimulate growth and delay the onset of leaf fall. If an abnormal period of prolonged cold weather hits before most trees drop their leaves,  leaf tissues can freeze and be killed before sufficient enzymes are produced to cause abscission and leaf drop. As a result, many trees may enter winter still carrying some of their leaves. Trees that have been overly pruned and watered can hold onto almost all their leaves, and are most at risk of breakage from severe winter storms. Leaves without abscission layers will eventually fall, and unless next year’s are damaged by an unseasonable cold snap, we can be sure to see new leaves appear again in the spring.

"Field Notes" is produced by the Montana Natural History Center.

(Broadcast: "Field Notes," 10/09/18. Listen weekly on the radio, Sundays at 12:55 p.m., Tuesdays at 4:54 p.m. or Fridays at 4:54 p.m.,  or via podcast.)