When leaves morph from green to gold, orange and red, is it cooler autumn temperatures that trigger the change? To some degree, yes, but the most influential factor is steadily declining sun exposure as the amount of daylight dwindles with each passing fall day.
This and other leaf-changing facts of autumn life are described in Why Leaves Change Colors, a science-based summary published by the U.S. Forest Service (Northeast Area).
Some other items of interest:
• Chlorophyll is just one of 3 pigments that play a role in leaf coloration. The others: carotenoids and anthocyanins.
• A string of cool nights, but no freezing temperatures, tend to promote brighter colors. The reason: Veins in the leaves contract in cool weather, retaining more of the color-yielding sugars produced during daylight hours.
• Typically, the showiest leaf displays follow a warm, wet spring, normal summer conditions and sunny fall days with cool nights.
Meanwhile, here's a brief sampler of fall colors created by black oak trees in Yosemite National Park last autumn, as documented by Yosemite videographer Steven Bumgardner (aka Yosemite Steve), curator of the splendid short-film series Yosemite Nature Notes:
There you have it. Feel free to impress your friends with your newfound knowledge. Where have you spotted the most impressive display of fall foliage this season?