Mount St. Helens Anniversary

After 30 Years, Still Lessons from Natural Disturbances

May 18th marked the 30th anniversary of the eruption of Mount St. Helens and SFR researchers to this day use what's being learned there to challenge established thinking about how landscapes evolve and rebound — even from something so bleak that Professor Jerry Franklin, who visited St. Helens within weeks of the eruption, said, "The blast zone looked like a moonscape, uniformly gray and from initial appearances, sterile."

However, research soon revealed that even in desolate-looking areas there were often what Franklin began to call "biological legacies" — plants protected by snow, along with seeds, spores, root balls from which new plants could sprout, and even downed trees and 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.   Recent work by SFR researchers augments initial investigations at St. Helens, which 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.  Important theoretical and management implications continue to emerge.

An early epiphany had to do with the important roles played by biological legacies that survived and contributed to post-disturbance ecosystems.  For example, 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.  This is a major shift from 30 years ago, when crews harvesting trees were often required to pile up and burn the woody debris left after logging.  Management implications have been huge and have included reconsideration of salvage logging on federal forest lands after wildfires or other disturbances.

A March 2, 2010 paper in Frontiers in Ecology and Environment reviews the underappreciated value of ecosystems that develop between the time trees are removed — by natural processes such as eruptions or fires, or by harvesting — and when they again dominate. These so-called "early-successional" forest ecosystems attract and sustain numerous species and are characterized by highly productive plant species that thrive without a tree canopy above, according to authors who include Franklin and first-author Mark Swanson, an SFR alumnus, ’99, ’07, now on the Washington State University faculty.  Another key finding is that, in a very real way, we are not starting "from scratch." Even after an eruption, an ecosystem inherits many values and functions from pre-disturbance elements. Also, a large disturbance like the St. Helen’s 1980 eruptions creates diversity at scales from the very small (snowbank communities and hummock communities) to the very large, the blast zone.

Other findings, by Professor Tom Hinckley and his students, have shown how disturbances aren’t uniform —  different species, stand compositions, and ages are impacted differently and recovery is different.  His work has focused on forests outside the blast zone that were covered with ash. While young trees recovered in as little as two seasons, old-growth silver firs underwent extensive decline, dieback, and mortality; their stiff needles and rigid branches that held onto the ash reduced the amount of sunlight that reached their needles and made them less vigorous. This was especially true in mixed stands where other old-growth species, such as western hemlock and Douglas-fir shed ash faster, recovered sooner, and took competitive advantage over the weakened silver firs, perhaps even further slowing their recovery.  Hinckley has also looked at how snow and erosion have shaped recovery in the blast zone. For example, many very small, but surprisingly old, “advanced regeneration” mountain hemlock and silver fir trees were covered in snow when the eruption occurred. Those sapling-sized trees survived and today they form stands over 30 feet (10 meters) tall with many of the trees producing seed for the last several years.

Other UW researchers also continue work at Mount St. Helens, including studying the relationships between vegetation and environment, the effects of the eruption on insect species, the dispersal of the ash plume caused by an eruption, and the nature of small volcanic earthquakes occurring before and during an eruption. This 30-year-long natural experiment will continue in the coming decades to be an unprecedented learning experience in how ecosystems respond to such a major disturbance.

The green trees in this picture were buried underneath snow at the time of the eruption of Mount St. Helens, allowing them to survive. Photo credit: Tom Hinckley.