IN 20 MARCH 1980 at 1548 hours a needle jumped on a seismograph at the University of Washington in Seattle. The machine had detected the first in a series of moderate-sized earthquakes that heralded the awakening of Mount St Helens after nearly a century of slumber. More quakes followed in quick succession. Three days later, instruments at dozens of monitoring stations were recording as many as 40 quakes an hour.
The mountain rumbled, growled, and shivered. On 27 March a plume of ash and steam spewed 6 kilometres into the air. This tentative eruption turned the mountain’s snow-capped peak a sooty black. Now, dirty and cracked, the mountain continued to flex its muscles. Soon a huge, prophetic bulge began to grow on its north slope.
Finally, on 18 May, Mount St Helens obliged the hundreds of reporters, tourists, and scientists who had gathered for the ‘volcano watch’. At 0832 hours, an earthquake measuring 5.1 on the Richter scale shook the mountain, dislodged its misshapen north face, and triggered an explosive blast 500 times as great as that which destroyed Hiroshima.
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Within seconds, the blast blew down, shredded, or scorched 500 square kilometres of forest. A plume of ash, cinder and pulverised rock rose some 25 kilometres into the sky, turning day into night. When the blowing material fell to Earth, it left deposits of coarse ash, or tephra, more than a metre thick near the volcano. Hundreds of kilometres to the northeast, traces of tephra smudged the ground.
Minutes after the eruption, fragments of pumice and gas, heated to 600 Degree C, poured from the ruptured dome and scoured the flanks of the mountain. Choking mud flows of boulders, sediment and melted glacial water swept down the Toutle River to the north and the Muddy River and Pine Creek to the south and east. The collapse of the north face of the mountain produced an avalanche of debris that filled 60 square kilometres of the North Fork of the Toutle River valley to a depth of 45 metres within 10 minutes.
When the temperatures fell and the dust settled, scientists boarded US Forest Service helicopters to inspect the damage. ‘It looked like a moonscape,’ says Lawrence Bliss, a botanist and leader of a multidisciplinary team of researchers from the University of Washington sent to study the biological effects of the eruption. ‘I really wondered several times if I had lost my colour vision,’ recalls John Edwards, a zoologist at the university, of his first view of the stark black-and-white landscape.
Because the mountain appeared so utterly devoid of life, the scientists had the rare opportunity to watch the reorganisation from scratch of a vast ecological system. It was a massive, ecological experiment provided free of charge. The rapidly assembled team began by pulling together what was known about the ecology of Mount St Helens before it erupted. The mountain itself is a relatively young volcano, about 40 000 years old. It is part of the active Cascade Range which is linked to other mountain ranges that, in turn, stretch south through Oregon and California.
Before it blew 400 metres off its top, Mount St Helens had an elevation of 2950 metres. Vegetation was sparse above the timberline, but it was ‘struggling its way back up the mountain’, recovering from previous eruptions, says Jerry Franklin, a forest ecologist in the team. The rest of the mountain was covered by evergreen forest. At lower altitudes, Douglas fir and western hemlock dominated; at higher elevations, true firs (species of Abies) and mountain hemlock. ‘These were magnificent forests composed of mature trees 75 to 150 centimetres in diameter and 40 to 70 metres tall,’ said Franklin.
Substantial areas of the virgin forest along the north slope had been logged during the previous 30 years, however, and there were several plantations in the western half of what was to become the area hardest hit by the blast. Yet in the undisturbed forest, subalpine meadows and clear, cold streams formed the only breaks in the old-growth forest.
The animals living in the volcano’s forests, meadows and waters were typical of the Cascadian fauna. There were, for example, Roosevelt elk and Columbia black-tailed deer in the woods, silver salmon and rainbow trout in the streams, and many species of small mammals and birds.
The first, and most surprising, discovery was that there were survivors even in the most devastated areas: the eruption had not extinguished the diversity of life. Along ridges facing away from the direction of the blast, for instance, small saplings lay bent to the ground, covered by snow 1 to 2 metres deep. This blanket of ice and snow protected many of the young trees from instant destruction by the heat, ash and ferocious winds of the volcanic blast.
If the snow had been heavier that year, more small trees would probably have survived, says Thomas Hinckley, a tree physiologist in the team. ‘But had the eruption occurred a month later, a greater number of trees closer to the mountain that are green today would have been dead.’
Many seedlings planted around the edges of the blast zone also survived the explosion. The survivors were well rooted and had enough foliage left unscathed peeking from the ash layer to continue photosynthesis. Stands of untoppled mountain hemlock and Pacific silver fir formed a ring around the flattened forests. These survived because they stood in the lee of ridges and gullies which deflected the blast.
The dense snowpack provided refuge for many smaller plants, notably lupins and weedy species such as common firewood and pearly everlasting. Below ground, burrowing animals such as the pocket gopher and many subterranean insects escaped death.
The unexpected behaviour of the insect survivors first alerted the investigators to the fact that something peculiar was happening on Mount St Helens: species were not recolonising the bleak mountain slopes as they should. ‘If you read any textbook on ecology, you read that, first, you get plants growing and then herbivore insects growing on the plants. That provides the basis for the predators, the scavengers, and the parasites,’ explains Edwards. ‘But in fact St Helens turned that upside down.’
According to Rick Sugg, an entomologist at the University of Washington, as soon as the dust settled, invertebrate pioneers began to arrive on the barren surface of the central blasted zone. They emerged from logs that had lain under the snow, climbed up out of the tephra or blew in on the wind. More than a thousand species arrived, but those able to live more than a few days were mainly small ground beetles belonging to the genus Bembidion and wolf spiders.
The beetles and spiders thrived on a diet of small arthropods falling from the sky. ‘Insect fallout’ is a normal part of Cascadian ecology; it consists of millions of dead and living flies, bugs, spiders and beetles that the prevailing wind picks up in the lowlands and deposits at higher elevations. Studies on nearby Mount Rainier show that on a typical summer day, between 8 and 10 tonnes of insects lie on its high snowy peak. With such a plentiful food supply, the scavengers did not need to wait for plants to revegetate the slopes.
The traditional first colonisers were not long in arriving, however. A few weeks after the blast, plants began to reclaim the barren slopes. But they, too, did not behave as predicted. According to Bliss, the plants did not appear in the same order as they would after a major disturbance such as a fire or logging. Then, the first plants to appear are annuals, followed by short-lived perennials that last one or two years, and, finally, the long-lived perennials.
On Mount St Helens, annuals able to adapt to the changed conditions were rare and so they played little part in recolonisation. Instead, one of the most important early colonists was the lupin, a perennial that lives for about five years. Lupins that endured the eruption acted as nurse plants for pearly everlasting and fireweed. These species, which had previously colonised logged parts of the mountain, were already able to cope with harsh conditions. The lupins helped them to establish by capturing their small, windblown seeds, trapping fine silt and, because they are legumes, by adding nitrogen to the leached soil. The lupin patches created their own small ecosystem, with aphids, ants and other insects. Even rodents travelled long distances across the volcanic landscape to reach the patches. The slow expansion of these oases suggests that one day many of them will grow into fully recovered meadows.
Meanwhile, underground, other changes benefited the lupin patches. According to Fiorenzo Ugolini, a soil scientist at the University of Washington, a primitive soil began to form beneath the armour-like surface of the pumice plains soon after the eruption. The hard outer crust, known as desert pavement, formed quickly as the volcanic materials were weathered by wind, rain and the acidic gases emitted by the still sputtering volcano. The crust consists of closely packed pumice stones, layered 3 to 40 centimetres deep.
Desert pavement does not hinder the formation of new soil. ‘It’s an example of nature protecting itself, in this case from further erosion,’ Ugolini says. Similar desert pavements, in Antarctica for example, have protected fragile soils for millions of years. Further, as the pavement weathers, it provides nutrients to the soil below. According to Ugolini, 13 of the 16 elements plants need to grow are found in the desert pavement of Mount St Helens. Finally, the porous pumice holds rainwater and offers shelter for seeds.
As terrestrial plants and animals struggled to live on the moonscape of the mountain, aquatic creatures faced equally dismal prospects in the lakes and ponds. Few vertebrates living around Spirit Lake survived the tremendous heat of the eruption and the crushing mud flows that followed. ‘Bodies of water were basically soup – cooked vegetation,’ said Edwards. This biological ‘sewage’ was a terrible habitat for anything but bacteria and mosquito larvae, which flourished in the broth of rapidly multiplying microorganisms. ‘The ponds and the lakes produced a prodigious pulse of mosquitoes the first year,’ Edwards reports. ‘They cleaned up the water and then disappeared.’
Return of the exiles
Gradually, over the past 10 years, reptiles and amphibians from outside the devastated area have journeyed to the lakes and ponds renewed by the mosquitoes. The return of fish has been more difficult. Clearly, there could be no natural recolonisation in closed bodies of water where there had been no survivors. Not everyone was keen on the idea of helping the process of recovery by reintroducing the fish. According to Franklin, the fish question created a controversy between scientists and state wildlife officials that continues to this day: ‘There’s been a conflict between getting a sport fishery right back on all the lakes and wanting to reserve those lakes to see how they develop under natural conditions – the scientific objective.’ So far, the compromise has been to restock some of the lakes outside the boundaries of the Mount St Helens National Volcanic Monument, a protected area established in 1982 as a resource for scientific study.
No similar tension arose over the return of big game to Mount St Helens. Large mammals came back voluntarily within weeks of the eruption to browse on the emerging vegetation. To ensure that populations would return to their original size, hunting was restricted within the reserve. Biologists predict that the number of game animals will continue to increase until evergreen trees begin to outgrow and replace the forage plants.
Small mammals did not recover so well. Of some 32 species living on the mountain at the time of the blast, only 14 have survived in the area. Many animals that were not killed or injured outright, died off slowly from a lack of food, water or shelter. Peter Frenzen, the Monument ¿ìè¶ÌÊÓƵ funded by the government, points out that those species that reappeared, such as the deer mouse and the golden-mantled ground squirrel, are adaptable generalists, with no very specific requirements for food or habitat.
Today, ten years after the quake and explosion that woke sleepers as far away as Vancouver in Canada, visitors to the Mount St Helens Monument can view the full spectrum of damage and recovery. The pumice plains still look bleak and forbidding, and the barren ground is relieved only by isolated islands of lupins. On the edge of the pumice plains, however, dogwood and beadlily, elder and huckleberry grow alongside the lupins, pearly everlasting and fireweed. Frenzen believes that willows, hardwoods and cottonwood trees will eventually appear among the scattered seedlings of coniferous trees now emerging from the rubble of boulders and stumps.
The forests bordering the flattened zone have shrugged off the persistent layer of ash that slowed their growth for the first two years. They have endured droughts, chemical changes in the soil, diseases and acid rain, all linked to the eruption. ‘Within the next 50 years, as the young trees mature, the weed and shrub dominated landscape will begin to take on the appearance of a young coniferous forest,’ predicts Frenzen. ‘Within a hundred years, a mature forest will cover most of the area. Within two hundred years, the area will begin to resemble the old-growth forest found in undisturbed portions of the Monument.’ Just in time, perhaps, for Mount St Helens to wake again and begin another explosive episode of fire, ash and devastation.
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NATURE VERSUS NURTURE: THE UNHELPING HAND OF SCIENCE
WHEN it comes to repairing a massively disturbed ecosystem, nature knows best. Many scientists monitoring the recovery of Mount St Helens believe that most of the human efforts to speed up the process have failed. For example, artificially seeding the area with grasses to control erosion did little good. Scattered from the air, many of the seeds washed off the steep ash-covered slopes. In retrospect, many scientists feel that the efforts may even have done more harm than good.
Likewise, removing woody debris from stream channels, banks and floodplains did not always assist recovery of the aquatic systems. Along the banks of some rivers and streams, restoration was held back because the debris could have provided much needed protection and ‘seed traps’ for species that would naturally encourage revival of the stream.
Early on, scientists assumed that salvaging the logs blown down by the eruption would be beneficial, uncovering surviving plants and allowing old soil and ash to mix. Subsequent analysis suggests that where trees were not manually replanted, the density and growth of surviving plants has been higher on sites that were not salvaged.
The message from Mount St Helens, the scientists say, is that human intervention is warranted only in commercially logged areas, or where communities and settlements are clearly at risk from floods or other incidents that follow in the wake of eruptions.