I've noticed that for the autumn leaves in my area (Great Lakes region of United States), if a tree's leaves are changing or falling asymmetrically, they almost always change first on the East-facing side of the tree.

I can't find any explanation for this phenomenon online; but in the spirit of "showing my work", I'll give some hypotheses from this non-biologist. :)

  • If there is a temperature at which direct sunlight destroys chlorophyll or reduces its effectiveness, there might be pronounced coloring effect on eastern leaves. The air temperature at dawn is usually 5-10 degrees Celsius less than at sunset.

  • If direct sunlight instead prolongs color change or leaf detachment, it could be that this asymmetry is caused by an increase in cloud cover in the mornings relative to the afternoons.

Any thoughts? I've noticed this in past years, but this year seems really extreme: many trees in the region have lost all of their eastern leaves but have a western side covered in bright orange leaves; they also had orange eastern leaves while the western sides were green, so wind gusts aren't the culprit.

  • $\begingroup$ Nice question. Do you have a photo to illustrate this? $\endgroup$ – RHA Oct 31 '18 at 6:43

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