"The recovery process on Mount St. Helens results from a tapestry of events woven over a patchy environment."
--Roger del Moral and David M. Wood
The eruption of Mount St. Helens in Washington State in 1980 provided the scientific community with a rare opportunity to watch how the flora and fauna in an area recover after a major disturbance. Since the eruption, an interdisciplinary team of UW researchers has been studying these recovery processes. Their findings are challenging previously-held theories about how ecological communities become established and change over time.
The landscape of pre-eruption Mount St. Helens that many Pacific Northwesterners had come to know and love was one of dense evergreen forests and crystal clear lakes and streams. The "moonscape" in some areas that resulted after the blast stands in stark contrast to the image of that verdant past.
On May 18, 1980, at 8:32 am, a large earthquake triggered an avalanche of debris, unroofing the mountain. Steam and rock debris blasted out of the mountain as mudflows cut a swath through the valley forests below. Volcanic ash rained from the sky, while flows of hot gases and pumice spilled out of the crater. Great areas of the mountain's flanks were devastated by the impacts, searing heat, and deposits of volcanic material.
UW professors Roger del Moral, Larry Bliss, Jerry Franklin, John Edwards, and their colleagues have found that vegetation has been slowly recovering within the blast zone through two main mechanisms: expansion of plants that survived the eruption, and dispersal of plant seeds into the zone from surrounding unaffected areas. The fact that surviving plants and animals have played a central role in the early stages of ecological recovery in the blast zone came as a surprise to the researchers; they did not anticipate that survivors would be so diverse or widespread.
Del Moral notes that factors such as a slope's location and the direction it was facing, as well as the amount of snow cover at the time of the eruption, were very important factors in the survival of plants and the subsequent recovery of plant communities. "For example, that the eruption was in May when there was still a lot of snow on the ground made a difference to the degree of recovery in those areas where there were survivors," explains del Moral. "In the blast zone, the trees were killed, but many of the understory species survived simply because they were covered with snow. Had the eruption happened in August, the devastation would have been far greater to the plants and animals."
Acknowledging that these unpredictable or "stochastic" factors play an important role is a significant departure from traditional successional theories of ecology, which hold that recovery begins when "pioneer" plants are dispersed into the area. Those early-colonizing plants supposedly are followed by a deterministic succession of species, from pioneers through what's known as climax species, which constitute a single, inevitable endpoint of the development of an ecological community. "To some degree that's true," says del Moral of the St. Helens site, "but to a large degree the outcome is a matter of chance." That finding not only has important theoretical implications, but practical ones as well for environmental restoration efforts.
"What is growing on a particular piece of land is not preordained, and it's not inevitable," asserts del Moral. "There are many alternative stable endpoints. That's perhaps a little unnerving to some people. But it's also an advantage. If your job is to restore, say, a coal mine tail or an open pit mine, you don't need to recapitulate natural successional processes—you don't have to start with pioneers and build it up and wait 30-40 years. You can actually jump-start the process by planting species that are perfectly happy growing there."
These findings are the result of painstaking measurements made on hundreds of 10 x 10-meter square plots systematically laid out in barren areas of the blast zone. From year to year, the researchers inventoried what plant species were present, and measured microtopographic features, soil water content, texture, pH, organic content (usually nil), and nutrients, such as nitrogen and phosphorous (often only 2-5% or less of normal forest soils).
With what seems like excruciating patience and persistence, nature transforms a desolate landscape into one capable of supporting life. "At first, a new site is uninhabitable by any species," says del Moral. "Then, within a few years or so depending on the site, abiotic [physical] processes lead to some small fraction of the area becoming inhabitable. Erosion, for example, might remove deposits that were smothering vegetation; frost cycles might break down rocks; soil weathering occurs."
Meanwhile, there is a rain of nutrients out of the sky--either wet, in rainfall, or dry in the form of insects or pollen. Edwards has studied the process in detail. An insect is blown out of its forest and is dumped in the middle of nowhere and dies, giving organic matter to the soil. Spiders are very important because the natural dispersal of many spider species occurs via ballooning: Millions of baby spiders will be sent out, and some will float over the area, and they'll land and die, donating their precious organic material to the soil. Seeds blow in on the wind and eventually begin to sprout in the nooks and crannies next to rocks or on the edges of small erosion features.
The researchers also have found that the successful dispersal of seeds to the blast area seems to be a recovery-limiting factor, more so than competition between plants. Plants that are found to be growing in barren sites in the blast zone are simply the ones that are physically able to get there; they're not necessarily the ones that grow better, or faster, or are better for reinitiating plant communities. Sites that lie near unaffected areas of vegetation recover faster because seeds can reach them. As in the popular saying about real estate, a key to recovery is: "Location, location, location."
From studying Mount St. Helens and similar kinds of volcanoes, for example Ksudach Volcano on the Kamchatka Penninsula in Russia, which erupted earlier in the twentieth century, researchers have tried to piece together how long it will take for recovery processes to converge to a stable endpoint. Many believe it may take anywhere from 200 to 1,000 years for that to occur.
Nonetheless, many areas around St. Helens have already lost their jagged, barren appearance as fire-scarred tree snags begin to topple. Conifer seedlings that survived, buried in snow, have been growing for more than 15 years now. "Some areas are getting pretty lush," says del Moral. "But it's still very sparse in most places. You can look out on the landscape for 100 meters and you can't see any plants. There are exceptions around water, springs, and creeks coming out of the crater but on the whole, it's still a very barren landscape--not the moonscape that everybody talked about in 1981, but more like, say, the Mojave desert in August, without the shrubs."