Why some leaves change color more than others in the fall

Warm light burns across a mountain slope as the sun sinks beneath the horizon line, leaving the high peaks glowing pink in alpenglow above the dazzling display of yellow aspen leaves on the south facing slopes below. A slow inhale brings the tickling smell of decay to my nostrils, the fragrance of leaves incorporated into soil. The colorful displays and rich odors are indicative that it is unmistakably autumn in the San Juan Mountains.

416 Fire fall colors colorado
Gambel oak glows red in the understory of the 416 fire while the distant hillside of Missionary Ridge glows yellow with aspen, but landscapes shaped by fire and showing evidence of the mix of deciduous and coniferous trees that make up the landscapes of the San Juan Mountains.Mike Remke

The celebration of autumn is rooted in senescence, when deciduous plants halt the production of chlorophyll and ultimately drop their leaves until the next growing season. This process begins to reveal the hidden complexity and variation of plants in our region. Chlorophyll is the rich green pigment that is crucial for plants to convert sunlight and carbon dioxide to sugar, the process known as photosynthesis. Chlorophyll is green because it absorbs the highest energy lights of violet, blue and orange but reflects shades of green — thus giving plant leaves a dense green appearance. 

As the days shorten and plants stop growing, chlorophyll begins to degrade, and other pigments become obvious. Most famous in the greater San Juan Mountain region is the fiery yellow-orange color of aspens. Aspens contain abundant xanthophyll and carotene pigments that give aspen its shades of yellows and orange. At lower elevations, the San Juans come to life with rich reds in oaks, maples and other shrubs. These reds are the results of anthocyanins that these species possess. Beyond these obvious patterns of trees and shrubs across large areas of the landscape, the great variability of color palettes that result from plants’ secondary pigments all come out on full display during the fall. 

With careful attention, one can recognize the magentas of fire weed in the subalpine and alpine, as every deciduous plant in the understory of forests and beyond shows its hidden colors. Amid the evergreen conifers, the contrast of the yellow aspens and reds of the understory feel particularly vibrant — revealing the complex ecological tradeoffs of forests. 

fall colors
Senescence in the alpine and other ecosystems besides forests is also rich with color pallets of pigments besides chlorophyll. Not only is autumn an exciting opportunity to see color, it’s an exciting time to get ready to ski.Mike Remke

Being deciduous means that each spring new leaves must be grown. Leaves, made of chlorophyll and other pigments, generally cost trees a lot of energy and nutrients. Thus, being deciduous has a huge cost. The constant replacement of leaves year after year requires these deciduous trees to come up with alternate strategies for energy and growth — aspen for example, tend to have large vast underground root systems to store energy from previous years. The decomposition of their leaves each year replenishes soil nutrients, and their broad leaves allow them to grow quickly — assuming they have light. Their significant annual investment in leaves prevents them from investing in other compounds that protect them from diseases, making them vulnerable to attack; thus, aspen tend to grow quickly, put on a colorful display and then slowly die off and thin out. 

In contrast, conifers remain evergreen through the year. With short growing seasons at higher elevations and soils with often limiting nutrients, being evergreen allows trees to reduce the cost of growth by maintaining needles — modified leaves — year after year. In a disturbance-free world, this conservative and carefully thought-out strategy wins in the plant kingdom of short growing seasons and poor soils; however, when disturbance such as fire is introduced, the benefits of growing fast and taking advantage of the recently recycled nutrients help favor the fast growth of other species. The result is a mosaic of color across the landscape — from deep greens of evergreens to the oranges, reds and yellows of deciduous trees. The blaze yellow of aspen often stands in the footprint of what was once a fire that raced across mountain sides — the dense red of oak, too, is a pattern shaped by flames. The colors of fall are more than just a display of pigment; they share stories of the ecology of a landscape. 

Fall is a season of change; the chaotic speed of summer growth slows to senescence, a colorful and decorative pause. Yet, even in this pause, the slow quiet persistence of growth remains evident. Autumn is a time to reflect on the fact that life is death recycled, and that there is beauty in the differences of the complex patterns of the world.

MIKE REMKE is a professor of biology at Fort Lewis College and a research associate with Mountain Studies Institute where his studies focus on the intersection between forest ecology and human dimensions of ecosystems. When Remke is not busy being a nerd, he is often out and about with his camera, bike or splitboard enjoying the rich scenery of the San Juans.