One of the joys of living in Colorado is the gorgeous gold of the aspen in fall. Other regions may boast more colorful foliage—the reds and purples of the hardwood forests to the east, for example—but nowhere else do we get the combination of cobalt blue skies, spectacular mountain scenery, and shimmering golden leaves. Such a treat is not to be missed, so we recently joined some friends and went leaf “peeping.”

It’s a bit tricky to time things just right. This year’s peak color was a bit ahead of average, due to a late summer drought, but a number of websites keep everyone aware of what’s happening where and when. (I particularly like the map on the FOX 31 News site.) Even easier is to keep an eye on my friends’ Facebook pages. When the aspen photos start appearing, it’s time to roll.

Some of the best aspen stands are along the western slope, but that was a bit far for us, living in Colorado Springs. Rather, we chose a route that would get us home in plenty of time for dinner, and headed up Ute Pass to Woodland Park. From there, we turned south on Hwy. 67, planning to eat lunch in the gold mining town of Victor. Finally, we’d head back down the mountain on unpaved Gold Camp Road.

I was glad to discover that I wasn’t the only photographer—the driver was too. He made frequent stops, where we all piled out to ooh and ahh and snap picture after picture. While I have plenty of aspen photos, I can always try for a better one.

One thing I immediately noticed was that the gold aspen came in clumps. In some places, the trees were still bright green, while others had already dropped their leaves. Aspen largely reproduce by sending out runners, to the extent that entire hillsides can be covered by one tree with multiple trunks, all connected by their shared roots. These cloned trees are genetically identical, and therefore respond to fall’s changing temperatures in the same way. Some groups of clones are more sensitive to the cold than others. I realized that the different colors were an indication of how the trees were connected underground.

Then I started thinking. I know why trees drop their leaves. But I don’t know how they do it. It’s not like they have muscles and can simply let go. Later that evening, I looked it up. I learned a lot.

The sciency name for leaf drop is abscission. It turns out that there is a special layer of cells, called the abscission zone, right where the leaf (or flower, or whatever) separates from the rest of the plant. As fall progresses, these cells begin to separate from one another, creating a clean gap between the leaf and the stem. The whole process is regulated by proteins produced by the plant, which turn genes on and off according to the temperature.

Being a photographer, I wasn’t just looking for gold leaves—I wanted something special, memorable, what would make a unique photo. While most of the foliage was “aspen gold,” some of the trees had turned partially or totally red. Why would they do that?

As I’m sure you know, leaves turn color in the fall because the green chlorophyll is broken down and reabsorbed by the plant, leaving the red and yellow pigments behind, no longer masked by green. Carotenoids are responsible for yellow leaves, and the presence of anthocyanins creates red. What I didn’t know is that high sugar levels encourage the leaves to produce anthocyanins, and that’s the key.

Even though the leaves are no longer green, they’re still photosynthesizing (though not as efficiently) due to the other pigments still present. The end result of photosynthesis is sugar. Typically, the sugars produced in the leaves are then moved throughout the plant, and especially to the roots, where they’re stored.

Now, remember the abscission zone? That cell layer not only allows the leaf to eventually drop, it also impedes the transfer of molecules—such as sugars. You see where this is going?

Yes, the abscission zone traps the sugars in the leaf. If enough sugar builds up, the leaf makes anthocyanins, and turns red instead of gold.

It’s all a balancing act. Coo, cloudy weather reduces the sugar levels in the leaves, while warm sunshine increases them. Not all parts of a tree have the same exposure—outer leaves shield the inner ones, for example. Additionally, aspen clones differ in their sensitivity to both sunlight and sugar levels. Leaf color depends on the weather that year, the exact location of the tree in its microclimate, and the genetics of that clone.

But enough of botany. Sometimes, I like to put all that aside and simply enjoy the trees. They’re pretty!