November
Glacier Gorge: Rhythms of Air
I’ve always enjoyed the line from Louisa May Alcott’s novel Little Women about November: “‘November is the most disagreeable month in the whole year,’ said Margaret, standing at the window one dull afternoon … ‘That’s the reason I was born in it,’ observed Jo pensively.” I, too, was born in November and I identified with Jo. Now, far away from the Ohio childhood in which I read Little Women, I am leaning into a disagreeable November headwind as I climb the Glacier Gorge trail. The weather has been mixed over the past weeks: days of warm sunshine alternating with early winter snowstorms. Now the trail is clear but muddy in places and already packed into mounded ice where the path remains shaded at this time of year. A strong gust momentarily staggers me and I hear the weird creaking of the surrounding trees, a sound like a wounded animal moaning. This is the season for blowdowns and I glance warily upward at the trees swaying along the trail.
Winds of Change
I first heard stories of a great blowdown in Glacier Gorge in early 2012. An online search revealed photographs and posts by hikers, pinpointing the event to November 21, 2011. That day fell within a windy week during a month in which sixteen days had wind gusts exceeding 50 miles per hour in the subalpine forests at Loch Vale.
Windy winter days in the park can be both exhilarating and frightening. The wind creates a sense of urgency, of things on the move. If I’m lucky enough to have it at my back, I move along quickly on snowshoes or Nordic skis. If the wind is strong enough to start the trees swaying and creaking, the exhilaration takes on an undertone of menace, particularly where beetle infestations have left numerous standing dead trees.
The park is always windy, but from October to March it is more so. Average daily maximum wind speed in the subalpine forest can be nearly 50 miles per hour, even close to the ground, with gusts of up to 90 miles an hour. The jet stream of consistently eastward-flowing air over North America is referred to as a stream because it represents a concentrated flow, like a stream channel. And, like a water stream, the jet stream alternates through time between straight, meandering, and even braided. Sometimes the full force of the jet stream blasts over Colorado. At other times, the jet breaks up into branches and one of these passes across the state. Although extending high up into the atmosphere, the topography down where we live affects the surface expression of the jet stream. The Rockies are so windy because rapidly descending jet stream air cascades over the crests of the individual ranges in patterns that reflect details of the local topography.
Trees are remarkably strong. An enormous weight of snow can settle on them in a few hours during an intense storm and they can bend without breaking in a strong wind. In the spirit of scientific inquiry, while seeking to understand how debris carried on floodwaters can scar tree trunks, I have taken the largest branch that I can lift and swung it against a tree trunk as hard as I could. My exertions leave not a mark on the trunk of a mature tree. But even the strongest trees can snap or topple in exceptional winds. Trees can be severely damaged when wind speeds exceed about 60 miles an hour, but it is the gusts—which can be up to two times the hourly average wind speed—that do the most damage. Steep terrain of the kind present in Glacier Gorge can funnel and accelerate the wind, creating speed and the turbulence expressed in gusts.
The sound and the fury of trees being broken and toppled must be amazing. Visiting sites where trees more than a foot in diameter have been sheared off by wind, I think of the battles fought with cannons on wooden sailing ships. One of the greatest dangers during these battles was flying splinters, although splinter seems too dainty a word for shredded wood pieces several inches across and more than a foot long being hurled through the air by the force of a cannonball smashing into a ship’s wooden hull. I can readily understand how such a piece of wood could wound or kill sailors, having come upon jagged splinters up to four feet long and a foot across that were shot dozens of feet through the forest when a tree snapped during a wind storm.
Mature tree snapped by high winds during a blowdown near Loch Vale.
More commonly, the entire tree is uprooted and toppled, ripping up a wide slab of soil as it goes over. The size and shape of the tree and its roots, the strength of the tree trunk, and the depth and moisture of the soil all influence whether a tree breaks or topples. The shallow-rooted spruce, fir, and pines common in the subalpine forests of Rocky Mountain National Park are particularly prone to uprooting, and the resulting blowdowns are relatively common across the Southern Rockies. I realized how common once I started noticing areas in the park with large numbers of downed trees. Valley forests along Glacier Creek, Icy Brook, the North Fork Big Thompson River, North St. Vrain Creek, and Hunters Creek all had blowdowns during the winter of 2011–2012. Hidden Valley, Mill Creek, Bighorn Creek, the Big Thompson River, and the aptly named Wind River are among the areas in the national park with large blowdowns during the past few decades. Most portions of the subalpine forest in the park experience some level of blowdown every ten to twenty years.
This might seem as if blowdowns would prevent forests from ever reaching the august status of old growth, but blowdowns are typically small in size. Like a wildfire that burns one patch of forest and then leaps across a valley, leaving unburned forest between charred stands of trees, wind gusts can topple one tree or a few trees in a grove, while leaving neighboring trees untouched.
I started searching out blowdowns to investigate how they influence the logjams I had been studying for a few years before the 2011 blowdown along Glacier Creek. Trees that pull up a root mass and topple across a creek form an effective barrier to small pieces of wood floating down the creek, not least because most of the branches of the fallen tree remain intact as it falls. If the first year’s snowmelt after a blowdown is large enough to carry much wood down the stream channels, at least some of the smaller wood pieces will lodge against the downed trees that bridge or partially span the channel. The accumulation of wood helps to trap other wood moving down the channel and soon the smaller pieces of wood are tightly packed into a jam that ponds water upstream and creates a small waterfall on the downstream side of the tree. If the jam ponds enough water during snowmelt peak flow to send the water spilling over the stream banks, a smaller side channel can form. Flow in the new channel can erode the banks and topple more trees, creating additional logjams. Given enough time, the valley bottom can become a maze of smaller channels that branch and rejoin around logjams. These portions of the valley are hot spots that trap bits of leaves and twigs in pools and eddies. Microbes, bacteria, and stream insects feed on the plant detritus. Trout, ouzels, riparian spiders, and songbirds all feed on the stream insects. Wetland plants not found elsewhere along the stream flourish in the wet soils. All this starts from a violent wind that blows down the big trees.
A tree toppled during the blowdown along Glacier Creek pulled up a wide, flat slab of soil with its roots. The straight line at the base of the root slab is a metal tape 3 feet long.
By the time I first visited the blowdown along Glacier Creek, in July 2012, the park service had cut dozens of trees that had fallen across the trail, creating a clear path through the chaos. As part of my research, however, I left the trail to crawl and climb over, under, around, and through the tangled mess of downed trees. Well before the end of the day, I developed my own Dr. Seuss–like chant: One bruise, two bruise, red bruise, blue bruise. The exercise left me deeply appreciative of trails.
At first, I thought the area looked like someone had lobbed hand grenades into the forest, leaving toppled and broken trees strewn randomly. Then I gradually perceived a pattern as I hiked the perimeter of the blowdown area and climbed the valley wall to the uppermost extent of damaged trees. The swath cleared through the forest by the wind came down the east side of the Glacier Creek valley, leaving trees snapped 5 to 20 feet above the ground. Then the wind turned down the valley floor, uprooting many of the trees but toppling them in different directions. The pattern likely reflected a microburst, a type of downburst in which cold air accelerates downward, forming an extremely powerful downdraft that spreads out when it hits the ground surface.
As with blowdowns documented in other regions by forest ecologists, the intense winds that sheared off mature trees did not flatten everything in their path. Undamaged trees remained standing amidst the piled trunks of their former neighbors. One standing tree had even caught the upper portion of the adjacent tree when the latter sheared off. The number of standing, unaffected trees increased gradually toward the perimeter of the central area of greatest damage, but I also found little pockets farther away where six or seven trees had been toppled while the surrounding forest remained undamaged.
The size and intensity of blowdowns vary widely. In October 1997, winds estimated at 120 to 150 miles an hour in the Routt National Forest just northwest of Rocky Mountain National Park destroyed trees over nearly 25,000 acres. The affected area consisted of hundreds of smaller blowdown patches averaging about 60 acres in size, separated by equal-sized patches of undisturbed trees. The blowdowns occurring in Rocky Mountain National Park during the winter of 2011–2012 also mostly consisted of relatively small patches of uprooted trees among undisturbed forest.
Blowdowns leave a messy-looking forest, but they likely contribute to forest health by helping to maintain diversity of tree age and vegetation species present within the forest, just as fires do. Subalpine forests are particularly susceptible to blowdowns because of more shallowly rooted trees, shallow or poorly drained soils, and more frequent extreme winds than lower elevation montane forests. Stand-killing forest fires occur only infrequently in the subalpine zone. This makes blowdowns even more important as a local disturbance that kills some trees, but also opens new sites for herbaceous plants and other tree species, such as aspen or lodgepole pine, to germinate.
The Beetles Are Coming!
Blowdowns are not the only natural disturbance shaping the subalpine forests. Part of the challenge in understanding how these forests change through time is the synergy among blowdowns, beetle kill, and wildfire, particularly as climate warms in the next few decades. The integrated picture that emerges from individual scientific studies is a sort of perfect storm descending on subalpine forests in the national park under a warming climate: Warmer winter temperatures help mountain pine beetles and promote a longer wildfire season. Fire-injured trees can be more susceptible to beetles and to blowdown. Blowdowns can trigger beetle outbreaks. Blowdown-affected stands can burn more severely. All signs point to likely increases in the frequency, extent, and severity of disturbances to subalpine forests. This in turn points to smaller areas of old-growth forests in future.
The most visually apparent sign of forest die-off in the national park and surrounding national forests at present is the trees killed during the past decade by mountain pine beetles. When I started my logjam surveys on the eastern side of Rocky Mountain National Park, I felt a sense of urgency. I had watched broad extents of forest on the western side of the continental divide change from green to orange within a few years and I knew that I had a relatively brief window of time to document patterns of instream wood before the pine beetles reached the east-side forests and started to change patterns of tree mortality and wood recruitment to rivers. So I worked day after strenuous day one summer, egging myself on during periods of fatigue with a chant running through my head: The beetles are coming! The beetles are coming!
The mountain pine beetle is a native species that creates at least localized forest die-off every few decades. The current beetle infestation in western North America may be the largest and most intense that has occurred within the past few hundred years, although some ecologists dispute this. Whether unprecedented or just unusually extensive, the current beetle epidemic has killed millions of trees and altered forested landscapes in ways that many people find unsettling.
Considering just the numbers, it’s impressive that any stand of forest can reach old-growth status. Bark beetles as a group range from northern Canada to northern Mexico and from sea level to 11,000 feet in elevation. Seventeen different species of bark beetles live within Rocky Mountain National Park. Different species of beetles feast on lodgepole pine, ponderosa pine, limber pine, Engelmann spruce, subalpine fir, and Colorado blue spruce, which together constitute a large proportion of the conifer species in the region.
There may not be a single “smoking gun” explanation for the current extensive outbreak around the national park, but the relatively even-aged forest that has regrown largely in the absence of fires after widespread deforestation in the late nineteenth century is commonly assumed to be part of the explanation.
Progressively warming climate likely also plays a role in the current beetle success (to consider the infestation from a beetle-centric viewpoint). Longer summers equate to more beetle reproduction—perhaps twice as much. Biologists Jeff Mitton and Scott Ferrenberg discovered that mountain pine beetles are now reproducing twice each year instead of once, resulting in exponential increases in beetle numbers. Working at a field site at 10,000 feet elevation with a long-term temperature record, Mitton and Ferrenberg noted that air temperatures have increased over the last forty years. The pine beetles have responded by metaphorically getting a jump on spring. The beetles start their flight season more than a month earlier than indicated by historical records and they just keep going, flying twice as long as in the past.
Adult beetles emerging from trees in late spring search for a living tree that they attack en masse. The beetles start to excavate egg galleries within a day and lay eggs within a few days. Speeded along by warm temperatures, the eggs develop more quickly during the summer, resulting in adults that emerge in August. It is this summer generation that has not previously been present in beetle populations. All those beetles need room to grow, so the species has also expanded geographically, killing trees 450 miles farther north in Canada and 2,000 feet higher in the Rocky Mountains than reported from previous beetle outbreaks.
Low winter temperatures (around –40°F for more than a week) can kill beetle eggs and larvae wintering under a tree’s outer bark and thus limit beetle survival, but such prolonged cold snaps are becoming uncommon. The warmer winters and drought-stressed trees present in the Front Range during the past two decades probably favor widespread beetle infestation.
Confronted by a massive die-off of trees, park service officials face some unenviable decisions. On the one hand, they are supposed to minimize interference with natural processes. On the other hand, no one wants to have visitors killed by falling trees. Trees in high-use areas including road corridors, campgrounds, parking lots, and visitor centers can be selectively or completely removed, as in the case of the Timber Creek campground on the western side of the park, which I now call Timberless Creek. Trees that the park service describes as high-value trees important for shade, visual screening, and esthetics are being sprayed with the insecticide carbaryl, which must be applied directly to the trunk of every single tree each year until the beetle outbreak eventually subsides. As park service literature demurely puts it, “there are adverse impacts with chemical spraying,” so carbaryl is not a viable option for preserving wide swaths of forest.
If trees could shiver with fear, they should be doing so now. One study published in 2009 has already documented a rapid increase in tree mortality rates within unmanaged old-growth forests in the western United States during the past fifty years. The rate at which individual trees die (rather than mass die-offs in fires or blowdowns) has been doubling at intervals of seventeen to twenty-nine years across varying elevations, tree sizes, tree ages, and tree species. Even the young trees are dying faster than they used to. The scientists who conducted the study attributed these changes to climate warming and increased water stress in trees.
Other plants will grow where these trees die. Spruce and fir species of subalpine forests might be replaced by pine species now characteristic of drier and more open montane forests. At lower elevations, montane ponderosa pines might be replaced by the pinyon pines and junipers now growing in the chaparral zone, or by dry grasslands. These are the types of changes that have occurred during the past 10,000 years after the Pleistocene glaciers retreated.
Individual plant and animal species altered their geographic distribution as the warm, drier period of 7,000 to 5,000 years ago, for example, gave way to much colder temperatures about 4,000 years ago. In each period of change, individual plants and animals do not just pack up and move. A great many individuals die and some species go extinct. With luck and room to move, some of their offspring colonize newly suitable habitat. The difference in this particular period of climate change is that human alterations have severely restricted the ability of many species to disperse to new habitats. Cities, roads, dams and reservoirs, or farm fields: for some species of plants and animals, these constitute impassable barriers. The other difference in this period of climate change is that humans are present to witness and understand the causes and consequences of the change.
Living Downwind
Hiking up to the old-growth forest along Glacier Creek on this windy November day, I reach the outlet of Mills Lake only by leaning so far forward into the wind that I am mostly looking at my feet. Even though the morning is well advanced, much of the lake remains shadowed by the low-angle sunlight and the surrounding bedrock walls. A snowshoe hare and I regard each other for a long moment. The hare, which in summer turns brown but for its hind feet, has now transitioned back to a coat of winter white. Another blast of cold wind makes my eyes water. The hare turns back into the protection of the forest.
I lean into the wind, thinking about the rigors and dramas of winter. Metaphors of sleep and death aside, a lot happens in the national park during the winter. Avalanches roar down steep slopes. Glaciers grow—or at least, they used to. Patches of forest topple and snap in blowdowns. And the wind brings in not only snow, but also dust. More and more dust, recently.
Aeolus was the god of the winds in classical mythology, and scientists refer to wind-blown dust as aeolian inputs. Mostly this dust is silt and clay particles less than a tiny fraction of an inch in diameter, but the composition of these particles varies widely. Some of the dust particles are the nitrates and mercury that I described in connection with Loch Vale and the park’s high center. Other particles are calcium picked up by the wind from eroding soils or phosphorus and carbon derived from bits of dead plants blown in on the wind. Dust has been blowing into Rocky Mountain National Park for millennia, adding important minerals to the soils. We know this because the dust shows up in sediments accumulating in lake beds, meadows, and streams across the park. The dust also collects in glacier ice and in each year’s snowpack. Vertical cores through the sediment and ice reveal that dust inputs vary through time. Dust deposition increased between 1850 and 1900, for example, and then declined for a while. The decades of increased dust correspond to the period when people aggressively disrupted native vegetation across the region, harvesting timber in the mountains and plowing the shortgrass prairie into croplands.
Dust deposited across broad sections of the American West has increased substantially during the past twenty years. This is clearly visible in Arizona, where the frequent occurrence of giant dust storms known as haboobs makes the news. The increased dustiness is also visible in the dirty snowpack that now characterizes the Rocky Mountains each year. Unless fresh snow has just fallen, the snowpack can be so dirty that it appears distinctly tan or gray even from the elevation of a commercial flight over the mountains.
A more precise indicator of the increased dust inputs comes from the National Atmospheric Deposition Program (NADP), which has monitored calcium and other dust inputs at numerous locations across the United States since the 1970s. At monitoring sites on the western slope of the Rocky Mountains, calcium deposition has increased by 400 to 500 percent during the past twenty years as dust storms from Arizona, New Mexico, and Utah increasingly blow across the region. Some of this dust from the southwestern United States does not make it over the continental divide, but enough does to show up at NADP sites on the eastern side of Rocky Mountain National Park.
I huddle into a sheltered spot among the trees at the edge of Mills Lake, enjoying the wintry look of my favorite lake in the park. High bedrock ridges south of the lake block much of the low-angle sunlight of November and the lake remains shaded for much of the day, creating a scene composed in shades of gray and white. No one else is here today and the lake feels isolated and pristine. I get the same feeling at Loch Vale, which lies just across the ridge crest to the northwest. The pristine look is deceptive, however.
Loch Vale is one of the NADP sites. My visits there make me appreciate the heroism and dedication of the scientists who actually conduct the NADP sampling. The phrase above that describes the NADP sounds innocuous: “monitored calcium and other dust inputs … since the 1970s.” What this means in practice is a trip into what is commonly a remote site, strenuously difficult to access, every Tuesday. No matter what Tuesday happens to coincide with—Christmas, a major blizzard, torrential rainfall, or a heat wave—those in charge of each NADP station will do their utmost to collect the samples. C. L. Rawlins’s book Sky’s Witness provides an eloquent account of backcountry trips to collect NADP samples in the Wind River Range of Wyoming. My colleague Jill Baron of the US Geological Survey oversees the NADP sampling at Loch Vale, collecting the samples herself when her technicians are unable to make the trip for some reason (such as Christmas on a Tuesday). I have accompanied them on one of these autumn trips, driving up to the national park in the dark of very early morning, hiking along the frosty trail as the rising sun warmed us, then leaning into the winds at the Loch to collect the samples of stream and lake water.
Apparently, the only thing that can shut down the weekly sampling is the government. The September 2013 floods damaged every road leading into the eastern side of Rocky Mountain
Loch Vale in winter.
National Park, causing long detours for the weekly sampling trip. But the coup d’état to the sampling came during the shutdown of the federal government that followed the flood. All national parks around the country were closed, even to researchers. I was not along on the first trip during the shutdown, but Jill and her coworkers had climbed the long, steep trail up to Loch Vale before they were intercepted by law enforcement officers. Jill is a petite woman, but determined. During the ensuing discussion, handcuffs were mentioned. Things never reached that point, but the sampling was not completed. While politicians argued and postured in Washington, decades of continuous sampling were interrupted.
It is precisely because of such determination and dedication—passion—that we know that dust deposition has increased enormously in the Rocky Mountains. Why the increase? Probably multiple reasons. The southwestern United States has had long and severe droughts during the past twenty years. Drought decreases vegetation cover and soil moisture, allowing wind to more readily erode the soil. That appears to explain about half of the increase. The rest may reflect more disturbance of the soil from off-road vehicles, oil and gas development, and sprawl of urban and suburban areas as population increases across the Sun Belt.
The consequences of the increased dust deposition are equally diverse. The most solidly documented consequence is changes in snowmelt. As melting snow concentrates dust at the surface, the darker surface absorbs more solar energy and melts faster. The snowpack now melts up to a month earlier in the headwaters of the Colorado River basin on the western side of Rocky Mountain National Park, reducing runoff and water supplies by creating a shorter duration of snowmelt-fed stream flow. Some of the earlier melting reflects warming temperatures, but some reflects dust. One study of a relatively small mountain basin in the San Juan Mountains of southern Colorado found that snow cover disappeared from twenty-one to fifty-one days earlier in direct proportion to dust concentration on the snow surface. The effects across many small basins quickly add up. Cores of lake sediments from the eastern portion of the Upper Colorado River basin indicate that dust accumulation increased six-fold by the early twentieth century as a result of grazing, crops, and other land uses. The dust shortened the duration of snow cover by several weeks, causing peak river flow at Lees Ferry, Arizona—a flow gage on the Colorado River just upstream from the Grand Canyon—to occur on average three weeks earlier and decreasing annual runoff by more than 1 billion cubic yards, or about 5 percent of the annual average.
Thinking about these numbers, I remember the animated version of the Dr. Seuss story “Horton Hears a Who!” Horton is an elephant aware that a tiny dust speck contains a teeming world of miniature creatures. When Horton’s fellow large creatures think he is crazy and taunt him with boiling the dust speck, the “Whos” all chant together, “We are here, we are here” until the bigger creatures hear them and stop short of boiling the Who-world. The billions of dust particles making vital water supplies vanish back into the atmosphere across the western United States might as well be shouting “We are here, we are here,” but most of us have not yet begun to hear them even if we notice that late-season snowpacks seem to be dirty.
The less obvious consequences of increasing dust in the air involve human health. The occurrence in central Arizona of valley fever, a sometimes-fatal respiratory infection caused by wind-borne fungal spores, increased by almost ten times between 1998 and 2011. Asthma has also become increasingly common in parts of the rural western United States once known for good air quality.
The more subtle effects on park ecosystems of increased dust remain largely unknown. Aeolian dust can be an important source of carbon and other nutrients for alpine soils. As the snowpack melts in spring, carbon dust in the snowpack enters the soil, creating a boom in soil microbial populations. As microbial numbers decline later in the spring, nutrients become available for plants at the onset of the growing season. How these interactions among soils, microbes, and alpine vegetation will respond to more nutrients is an open question. Also unknown are the potential effects of the other materials blown in on the wind—pesticides, artificial fertilizers, heavy metals such as copper or cadmium, and industrial compounds.
The Glacier Creek watershed is one of the most scenic areas of the national park, from the view at the Storm Pass trailhead west to the peaks of the continental divide, to the backdrops of Bear Lake, Mills Lake, and Loch Vale. I can easily lose myself in the immediate details of my work here, but as I hike through and contemplate this superb scenery, I try to remember the invisible processes that underlie the geology, climate, and plant and animal communities creating the scenery. And with every staggering gust on this November day, I particularly remember that we all live downwind.