May 2, 2000
Biological legacies a key of ecological rebirth after Mount St. Helens eruption
Jerry Franklin was one of the first ecosystem scientists to visit Mount St. Helens after the eruption 20 years ago this May 18. Appearing particularly stark to the University of Washington researcher was the 125,000-acre blast zone, which looked like a moonscape, uniformly gray and, from initial appearances, sterile.
Investigations would, however, reveal that even in desolate-looking areas there were often what Franklin began to call “biological legacies” – whole plants protected by snow, seeds, spores, root balls from which new plants could sprout, even downed trees as well as debris that offered footholds for other plants to take hold.
The importance of these remnants, the varied ways survivors and invading plants have both competed and depended on each other and the role happenstance can play revealed that classical studies of plant establishment were often too simple for what happens after major disturbances. Work at Mount St. Helens came at an important turning point in thinking about vegetation both for scientists interested in natural processes and for those interested in new ways of managing landscapes disturbed by human activities such as logging.
On and around the mountain, layers of snow and soil helped preserve plants and animals. At the higher elevations the relic sites were the size of a living room or smaller, according to UW botanist Roger del Moral, whose teams have worked at sites above 3,500 feet on the north side and above 4,500 feet on the south side of St. Helens.
One school of thought said the plants in those biological hotspots would colonize outward causing a re-greening of the area. Another said the red alderberry, slide alder, goatsbeard and other relic plants, many of which prefer shady conditions, would wither because the trees were flattened or obliterated in the blast.
But what del Moral and his fellow researchers found was that the patches tended to die back mainly at the edges. The exposed soil became aircraft carriers offering a safe landing for seeds carried in the air or tumbled along the ground by the wind, he says.
Wind-borne seeds landed everywhere, of course, but those in the die-back areas had decided advantages. And, it turns out, the next-generation of these long-distance invaders – and not the struggling understory bushes and shrubs, which made the survival of many of the invaders possible in the first place – were then able to spread into adjacent areas.
This is what del Moral and his co-authors on more than a dozen scholarly papers about the recovery mean when they say that “stochastic processes” are most important during early succession and landscape effects. The first invaders have not succumbed to competition even 20 years later and they have greatly influenced the patterns of vegetation found today. This “luck of the draw,” more than any other factor appears to dictate the nature of which plants inhabit areas after a major disturbance.
Dead snags, downed trees part of legacy
Besides surviving plants, seeds, spores and such, scientists found that another key biological legacy at Mount St. Helens was the standing dead trees, snags, downed trees and their root balls and other woody debris, according to Franklin, a professor of forest resources. He says these structures act as “lifeboats” for plants, insects and animals by providing shelter and long-term sources of nutrients and energy.
In addition, such structures add richness to a recovering site. Without them, all the plants, brush and trees would be uniformly young. With them, the habitat has more complex ecosystem functions and can be used by more kinds of animals.
“Animal species dependent upon large decadent trees or snags are able to re-establish themselves in relatively young stands that have legacies of this type,” Franklin says. “Otherwise they would wait for decades or even millennia for development of such structures.”
Franklin was among the first ecosystem scientists to take what was learned about legacies at Mount St. Helens and other sites disturbed by natural forces and apply it to human-caused disturbances. Human-imposed disturbances – for example clearcutting a site – typically removes much more of the ecosystem, in a more uniform manner, and is repeated more frequently than natural disturbances.
Retaining some living trees, dead snags, downed logs and other woody debris when timber is harvested is one way Franklin and other foresters and landowners are trying to make human activities more similar to natural disturbances.
Learning from natural disturbances
This is a major shift from 20 years ago, when crews harvesting trees were often required to pile up and burn the woody debris left after logging, according to Tom Hinckley, professor of forest resources.
Today’s logging sites are “messy” and reflect a change in management that Hinckley believes was triggered by what scientists learned following the eruption of Mount St. Helens in 1980 and the Yellowstone wildfires of 1988.
Another example of how disturbances aren’t uniform comes from the live trees outside the blast zone, visited by Hinckley and groups of students and foresters every year since 1981. While young trees recovered in as little as two seasons, the old-growth silver firs in the area are still in decline. Silver firs, with their stiff needles and rigid branches, kept their coatings of volcanic ash, which reduces the amount of sunlight that reaches their needles and affects tree vigor.
In other cases roots and water have provided more than one surprise. In the Clearwater Drainage near Bear Meadow, in areas where trees were destroyed during the blast, some of the slopes suffered landslides but not until 10 years later. These mass slope failures – slides that started at the top of slopes and carried away everything down to bedrock – were probably the result of tree rootballs finally rotting away. It can take 10 years, Hinckley says, and that should be kept in mind for sites that aren’t replanted with trees, including urban slopes cleared to make way for buildings.
Studying other major disturbances
Work at Mount St. Helens, which was funded by the National Science Foundation, the U.S. Forest Service’s Gifford Pinchot National Forest and Pacific Northwest Research Station, has broadened into international research for UW scientists. For example, del Moral has studied plant succession after eruptions in Japan and the Russian Far East. This summer he’ll work at Kamchatka in Russia and next spring plans work at Mount Etna in Sicily. Franklin is collaborating with researchers as far away as Australia who are trying to help managers use ideas about biological legacies for managing natural resources including fish, rangelands and forests.
“Disturbances are like editors – they selectively remove or modify elements of an ecosystem while leaving others intact,” Franklin says. “Major disturbances such as Mount St. Helens have taught us how complex these events really are and the importance of the incredible biological legacies that they inevitably leave behind.”