The calendar has turned to October, and here in the Rocky Mountain west, the splendor of autumn is fully upon us. The climate is ideal, and with each passing weather front, a fresh layer of snow is left gleaming on the highest alpine summits. Meanwhile, the deciduous forests of the lower mountain valleys are ablaze with color, and much of the region’s fauna is preparing for the inevitable onset of shorter days, long cold nights, and the heavy snows of winter.
While high-country summers are relatively brief, daytime temperatures under a searing sun can be uncomfortably warm, and frequent monsoon storms out of the Gulf of Mexico are often violent and unpredictable. But later in August, as the sun tracks more to the south and monsoonal flows are gradually cut off, temperatures moderate and dryer air comes predominantly from the west. Skies remain mostly clear, with cool bluebird mornings, pleasant sunny afternoons, and chilly starry nights.
Naturally, fall weather patterns also produce occasional storms, but they are typically the result of colliding warm and cold air masses, or a passing low-pressure system. Such disturbances can generate significant rain, shifting winds, and some lightening, but they are seldom as strong or potentially perilous as the thunderstorms of summer. Nevertheless, as significant precipitation falls over the highest terrain, it more frequently does so in the form of snow.
By mid-September, the sun has dropped further in the southern sky, and days are cooler still. Afternoon breezes dry out the forest vegetation, and nighttime readings regularly drop into the 30s. This lack of moisture and the occasional frost choke off the production of chlorophyll in most leafy plants, reducing the green pigment essential to the process of photosynthesis, and allowing the yellow, orange, and red pigments (which are always present) to show through in brilliant display.
The gradual progression first occurs in the smaller broadleaf trees and shrubs of the forest understory, particularly in low-lying riparian areas where the coldest air settles. Various shades of glowing scarlet, golden yellow, russet, and deep purple appear among the sumacs, serviceberry, chokecherry, and dogwoods. Soon thereafter, the oranges, reds, and russets of Rocky Mountain Maple, Ashleaf Maple (Boxelder), and Gambel’s Oak show, but are eventually outdone by the magnificent yellows, orange, and gold, of the vast aspen forests and towering streamside cottonwoods.
Of course, elevation largely determines climate, and the highest reaches suffer long brutal winters and short growing seasons. On the alpine tundra above treeline, spring doesn’t arrive until June or early July, and autumn is only about eight weeks away. The tallest plants grow to about six inches in an environment where winds can often exceed 150 miles per hour. A few scattered evergreen shrubs grow in dense ground-hugging domes, usually in low spots or on the leeward side of large boulders. Fall colors on the tundra are largely limited to lichen-covered boulder fields and talus slopes.
The Subalpine zone stretches from around 9,000 feet to treeline at 11,500 feet. Comprised of mainly coniferous forest, including Subalpine Fir and Engelmann Spruce, most of the visible trees are evergreens with leaves (needles) that remain green throughout the year. In terms of fall color, only variable shades of green stand out in sharp contrast with the broad expanses of yellow and orange aspen that dominate the lower Montane forest.
Above all else, the splendid transformation of the aspen forests define autumn in the Rocky Mountains. Because aspen trees grow as clones through a network of shallow roots, vast stands are genetically identical, and all turn the same shade of color at the same time. Depending on elevation, peak fall colors usually occur between mid-September and mid-October, and by late October the leaves have fallen and the aspens, with white bark and blackened scars resembling peering eyes, stand as bare traceries against the sky.
Even as the mountain flora braces for winter, reptiles, amphibians, birds, and mammals are busy preparing in earnest.
Reptiles, including turtles, snakes, and lizards, are cold-blooded and their body temperature varies with that of their surroundings. Once colder weather arrives, all activity halts and they hibernate alone or in communal dens.
Amphibians are also cold-blooded and hibernate at the onset of cooler weather. Salamanders, toads, and frogs begin life in fresh water, but later, most live on dry land. Because their skin is thin and moist, they are more susceptible to dehydration than reptiles, and must live near water at least part of the year. By September, they burrow deep into leaf litter, soft earth, or the mud of bogs and remain inactive until spring.
Innumerable birds and waterfowl spend at least a portion of the summer months feeding, nesting, or rearing young in the high-country. Some are year-round residents, while others are only passing through on their way to northern nesting areas (spring), or southern wintering grounds (fall). With the ability of flight, several species migrate great distances, and from July to November, many songbirds, ducks, geese, and raptors either fly far to the south, or move to lower elevations to wait out the cold of winter.
While illustrious V-formations of countless ducks and geese exemplify the great fall migration over the central, mountain, and Pacific flyways, the less conspicuous movements of thousands of rutting deer and elk mark their passage from the high mountain pastures of summer, to lower-elevation forests and grasslands of their winter range. Meanwhile, solitary moose stick to higher-elevation riparian areas, while mountain goats and Bighorn Sheep move to south and west-facing slopes below treeline.
Autumn is a time of plenty for many creatures, and some utilize every waking moment preparing for winter. Picas, tree squirrels, beaver, and muskrats are among those who work tirelessly in the gathering and storing of provisions enough to see them through until spring. In the meantime, bears, marmots, skunks, and raccoons are fattening up for winter hibernation.
Once the heavy snows come, those remaining active will stay close to home and live off accumulated supplies, or rely on unique hunting or feeding abilities to find sufficient nourishment at a time when food is generally scarce. Others will simply den up and go to sleep, while still others will depart for areas of more favorable weather conditions. Either way, autumn is a colorful time, and an intense time of preparation.