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Science Of The Season: Why Autumn Brings Changes For Leaves

Sep 26, 2019

Some trees have already turned a brilliant gold, like these ones near Maroon Creek Club in Aspen.
Credit Alex Hager / Aspen Public Radio

Autumn is officially here. The nights are getting colder, the days are getting shorter, and aspen trees on hillsides around the valley are starting to turn from green to gold. 

It's exactly those changes — less sunlight and chillier temperatures that are causing the leaves to change. 


Trees use leaves as a means of absorbing sunlight. Using a substance called chlorophyll, the sun’s energy is captured and converted to energy that the tree can use to grow. When the days grow shorter, trees can no longer efficiently capture sufficient sunlight, so they shed those leaves.

“The tree has a budget it needs to meet every year,” said Jim Kravitz, Naturalist Programs Director at the Aspen Center for Environmental Studies. “And it meets its budget through its leaves. It is producing more leaves, it is producing roots, it is getting thicker, it is getting taller. The tree has to do all of this in a very short period of time.”

Before the leaves fall off, the green chlorophyll recedes, causing the brilliant colors that draw leaf peepers from far and wide.

“The yellow is always there,” Kravitz said. “The green is more prominent. That's the chlorophyll. And the tree draws back the chlorophyll every year when the growing season ends, exposing the yellow.”

Kravitz said the center receives calls from across the state with people asking for a prediction of when the leaves will reach their golden peak. This year, some have expressed concern that a hot, dry summer has caused the aspens to turn later than usual.

“This is the same question we get every single year,” Kravitz said. “If you said the average date was, let's say the 21st of September, I think most years are within a week of that or 10 days of that. So the 14th or the 28th.”