Chasing Aspen Colors

Each September, aspen leaves start changing from green to shades of golds, oranges, and even reds. As summer finally surrenders to autumn, entire mountainsides across the Southern Rockies burst into a blaze of color. This spectacle causes leaf-peepers from far and wide to flock to the mountains, because what better way to celebrate the arrival of autumn than with an aspen colors drive? I'll be the first to admit there's something delightful and quaint about packing a picnic and driving the narrow, winding mountain roads in search of the most colorful aspen stands. It's a tradition I look forward to every autumn.

Photo: approaching Aspen, CO from Independence Pass

Leaf-peeping is a big fucking deal in Colorado, despite dictionary definitions indicating that this pastime occurs primarily in New England. Collins Dictionary, for instance, defines a leaf-peeper as "a tourist who visits New England in autumn to see the changing colours of the foliage." From what I have heard, a visit to New England during fall color is indeed a life-changing experience; but, the Southern Rockies boasts excellent fall color too. Beginning in September, countless news stations, local papers and websites post updates on timing of leaf color and publish articles with titles such as "The top 10 places to go to see peak color in Colorado" and "Fall Color: Where and When to Go." These trite articles are helpful in planning leaf-peeping excursions, because timing is everything: a few days too early and leaves are mostly chartreuse; a few days too late and only the last few leaves cling stubbornly to branches where they dance wildly in the wind. A gust finally sets them free and they become flecks of gold swirling down canyons and hillsides. They litter the forest floor, paint pathways gold, and begin the process of decay, which contributes to the tantalizing scent of autumn.

Lake City, CO

In September 2015, C and I timed our aspen colors trip to Southwestern Colorado just right and most of the photos on this post are from that glorious trip. First stop: the small mining town of Creede and the 17-mile Batchelor Historic Loop, a scenic drive which showcases mines perched on jagged mountainsides and next to avalanche chutes, an overgrown cemetery, an old townsite, and other vestiges of an area with a rich mining history. This loop begins at the north end of Main Street, where iconic volcanic rocks tower over the town and stand as sentinels to the mouth of an imposing canyon. Earlier that day, the man at the visitor center described the mining loop as a gravel road with a few steep inclines, but that it was easily passable by a 2WD. We entered the canyon, took a right and then another right (that was our mistake), and soon began a long ascent. We entered an aspen forest that was a kaleidoscope of golds, oranges, and reds, interspersed with forest green spruce trees.

As we climbed, the road became increasingly technical. C maneuvered the car around washouts, straddled potholes, and negotiated tight switchbacks. The man at the visitor center must have had great confidence in our driving, we commented, as we waved to the 7th high-clearance 4WD car we passed. We detected a concerned look from the driver and finally it dawned on us that somewhere we had gone wrong. Very wrong. We looked at the map and immediately saw our mistake: we had gotten off the loop before it had really even begun. If we continued a bit farther we could rejoin the 2WD loop again. Turning around didn't make sense at this point, so we continued to bump along. We cringed when we heard a loud scraping noise as the undercarriage of our car grazed a rock. Several minutes later we reached the Batchelor Mining Loop and sighed a sigh of relief.

During the trip, we also drove the scenic byway hwy 550, which connects Silverton and Ouray by way of Molas Pass. The weather turned cold and alternated between snow and rain. As we approached Ouray, I saw some of the best fall color I had ever seen.

The view of the valley. Ouray is around bend.

Another scene from the same pull off:

The science behind fall color

If you are wondering what causes the leaves of aspen trees to change from earthy greens to the fiery colors of autumn, here is some basic botany: All summer long trees produce chlorophyll, which is the green pigment that captures sunlight and converts it, along with carbon dioxide and water, into oxygen and sugars. This process allows trees to manufacture their own food, so they can grow and function. As fall approaches and the days become shorter, aspen and other deciduous trees lessen their production of chlorophyll. When days are so short that there isn't enough sunlight to sustain photosynthesis, the tree signals to the leaves that their photosynthetic service is no longer needed. It communicates this by producing an abscission layer or cork-like cells where the leaf meets the stem. The abscission layer slows the flow of sugars from the leaf to the tree and ultimately causes chlorophyll production to halt. When this happens, the green fades and yellow (xanthophyll) and orange (carotenoid) pigments that have been there all along become visible.

So the above explains why leaves turn yellow and orange, but what causes the ellusive red aspen that leaf peepers such as myself so delight in seeing? More botany: The pigment responsible for red (and purple in other plants) is anthocyanin. Interestingly, studies by CSU researchers in the 1970s (Chang, Fechner, and Shroeder, qtd. in Rozell) demonstrated that anthocyanin is only present in some aspen trees, suggesting that it is a genetic trait. Also interesting is that aspens with the ability to produce anthocyanin don't produce it every year. Curiously, anthocyanin is only produced in the fall, which prompts the question: Why would a tree expend energy to produce a pigment in a leaf that is about to fall off? One theory is that this color may offer protection from UV radiation while another is that red may ward off insects (Rozell 2014). Good stuff.

As mentioned above, 2015 was a particularly good year for fall color -- at least in southwestern Colorado. Why is that? Are there certain factors that contribute to good fall color? The short answer is yes. The best weather for good aspen colors is warm, sunny days and cool, but frost free, nights. Warm and sunny days cause the green pigment to break down quickly, while allowing for increased production of anthocyanin. Cool nights slow the rate at which anthocyanin leaves the leaf. On the other hand, causes for disappointing years include an early frost, which doesn't allow anthocyanin to be converted from sugars. Moisture can also impact color: a dry year such as 2018 can cause leaves to drop before much color change. Conversely, too much moisture prior to leaf change can reduce color intensity. Finally, the health of a stand affects color: a healthy stand typically has more intense colors. On the other hand, when tree health is compromised by pathogens such as Marrsonina and ink spot leaf blight, leaves can turn brown and fall to the ground before they have a chance to change color (Shonle).

Above photo: Rocky Mountain N.P., September 17, 2018

As I finish writing this post on September 21st, aspens across the Southern Rockies are changing colors. The window to see peak fall color is short, so pack a picnic, grab your camera, and drive to the mountains for some leaf-peeping!

Trivia: Have you ever wondered why aspens quake?

It is because their petioles (leaf stems) are flat and not round,

which causes their leaves to flutter and quake with the slightest breeze.

Sources:

"Aspen fall colors: planning your fall foliage experience." (2017). Colorado State Forest Service. https://csfs.colostate.edu/aspen-fall-colors/

Rozell, Ned. (2014). "Maverick red aspens in a world of gold." University of Alaska Fairbanks, Geophysical Institute. http://www.gi.alaska.edu/alaska-science-forum/maverick-red-aspens-world-gold

Shonle, Irene. (2015). "Why some aspens leaves turn red." The Weekly Register Call. http://www.weeklyregistercall.com/2015/10/15/why-some-aspen-leaves-turn-red/

#leafpeeping #aspen #fallcolor #southwesterncolorado