Geoff Hibbs May 4, 2012 Primary Succession & Species Diversity of Flora on Mount St. Helens, 32 Years PostEruption On May 18 1980, Mount St. Helens, a stratovolcano in western Washington, erupted. The eruption scoured the slopes of the volcano removing vegetation and soil from the vicinity of the mountain. 32 years after the eruption, vegetation has returned to the mountain. The author’s main interest in this field is the methods by which vascular plants re-establish themselves after cataclysmic, soil-removing volcanic eruptions, and if some plants are better suited than others to re-establishing on volcanic tephra and other eruptive debris.
The author believes that it is important to understand how volcanism affects succession and diversity patterns, due to the ongoing activity of Mount St. Helens, and the potential for eruptive activity from the other mountains of the Cascade volcanic arc in the future. In order to understand the processes leading to the current species diversity of Mount St. Helens, one must understand the disturbance that prefaced vegetation establishment. According to Tilling, Topinka, and Swanson of the United States Geological Survey, the 1980 eruption of Mount St.
Helens was the most significant volcanic event in the United States since the eruption of Lassen Peak in California in 1915. The mountain’s eruptive activity began in early March of 1980, with a series of minor earthquakes and small eruptions. This period lasted until May 18, when the weakened northern slope of the volcano was shaken by a large earthquake, and collapsed in a massive landslide. The slide itself covered roughly 24 square miles of the terrain north of Mount St. Helens in material up to 150 feet thick (Tilling, Topinka, and Swanson).
Behind the slide, a lateral volcanic blast, released from the pressure of the overlying rock, traveled at speeds of up to 700 miles per hour in a fan-shaped flow northwards from the mountain. This blast, ejecting 208 million cubic meters of old rock, ash and new magma, destroyed 230 square miles of forested area (Harris pp. 205-6), and stripped soil down to bedrock in the immediate vicinity of the volcano. The eruption, although not of great duration, drastically altered vegetation patterns on and near Mount St. Helens.
However, the eruption may only have altered the immediate-term vegetation and succession patterns, as a link has been described between the older pumice deposits on Mount St. Helens, and how they may indicate how vegetation patterns and succession trajectories form. Prior to the 1980 eruption, vegetation patterns on Mount St. Helens were already atypical of the other Cascade volcanoes. The classic Cascade volcano subalpine environment is a well-defined mixed tree clump and open parkland environment consisting of Abies lasiocarpa, Tsuga mertensiana, Chamaecyparis nootkatensis and Pinus albicaulis.
In contrast, Mount St. Helens presented a strange combination of upper and lower elevation conifer species, including Pinus contorta, A. lasiocarpa, T. mertensiana, A. procera and A. amabilis, along with the hardwoods Alnus sinuata and Populus trichocarpa (Kruckeberg 1981). P. trichocarpa, being a low-lying species normally found in riparian areas, was found as high as tree line. Kruckeberg (1981), as well as del Moral and Bliss (1981) suggest that this strange combination of high and low elevation trees is closely linked to the underlying pumice deposits from prior eruptions of Mount St.
Helens. The vegetation patterns prior to the 1980 eruption are thought to have been immature, not having developed sufficiently to mirror the classic vegetation patterns found throughout the other Cascade volcanoes. This may indicate that the previous large eruptions of Mount St. Helens have been frequent enough to impede the growth of vegetation and the formation of distinct vegetation patterns (del Moral and Bliss). Since Mount St.
Helens is a volcano and therefore physically separate from other nearby mountains with alpine environments, this pattern of frequent eruptions and isolation from alpine flora appears to significantly decrease the richness of species composition on the volcano. The isolation of the volcano and its associated vegetation also affect how revegetation occurs following any large eruption. Research of vegetation plots in the years following the 1980 eruption showed several different primary and secondary succession patterns, influenced by complex interactions between flora, fauna and erosion.
Swanson (1981) states that initial revegetation of the volcanic blast area began at its edges, where soil strata and various vegetation still existed after the eruption. Existing plants produced seeds that germinated and began to colonize the blast zone from the outside edges inward. The interior of the blast zone, in areas where soil escaped removal by the blast but was sterilized by a thick covering of hot tephra, appears to have followed a different pattern. Volcanic tephra is very porous, and poor in nutrients required for plant growth, such as nitrogen and phosphorous (McKee et al).
However, certain plants are able to germinate and grow through the tephra deposits. Both Swanson and Topinka (2004) indicate that very hardy pioneer plant species, such as Epilobium angustifolium and Lupinus lepidus that are well adapted to colonizing recently disturbed areas, were the basis for other plants to follow. These plants, by rapidly producing deep taproots, are able to reach soil buried beneath the highly porous, nutrient-poor tephra deposits left by the eruption, and therefore flourished on the blast zone.
As these plants grew, they attracted burrowing animals that feed on subsurface plant matter. Topinka identifies species of pocket gophers that survived the eruption as a significant factor in soil mixing. These gophers, as they feed on roots, carve tunnels through the tephra and underlying soil strata . This mixture of soil and ash is then exposed by melt water from snow in the spring and summer. As the pioneer species grow, drop foliage and eventually die, they add an organic layer to the new soil layer (Swanson), providing a substrate for secondary plants to revegetate an area.
In areas where tephra deposits were shallow, top-killed flora were able to quickly send out new growth that penetrated the tephra. The majority of these plants were perennials that survived the blast by being buried beneath snow during the eruption (Stevens et al & McKee et al). These plants then produced seed crops, which germinated and began to colonize the area (del Moral and Bliss). Initially, the seedlings did not flourish due to germinating on the nutrient-poor tephra. However, he shallow tephra here was washed away by successive years of spring melt water as well as being blown away by winds on the southern aspects of the mountain (Pincha), eventually exposing the underlying soil and allowing for permanent plant establishment.
This complex interaction of pioneer species, burrowing fauna and erosion set the stage for further colonization by invading secondary flora. According to del Moral et al (1993), dispersion of new species is predominantly wind-driven. In a 2008 study on the eastern flank of Mount St. Helens, del Moral et al found that 10. 3 of the species within a vegetation plot were dispersed by wind, and a mere 0. 11 were dispersed by animals. The 1993 study found that, on the debris flow caused by the collapse of the landslide that triggered the eruption, 19 out of 24 species growing on the debris were most likely wind dispersed. Adams and Dale show that initial colonizers of a site may take up available resources at the beginning of reestablishment, slowing the establishment of following species. Species that have been shown to do this include Populus trichocarpa, Pseudotsuga menziesii, Salix species, and Alnus species.
These native plants produce large seedlings from small seeds that out-compete other vegetation for resources, and they are all light enough to be dispersed by wind. While these seedlings may not live for very long in the nutrient-poor volcanic debris, they have been shown to play a vital role in seed production and dispersion. Short-lived seedlings on the debris slide were found to be acting as seed sources by Adams and Dale. They found that P. trichocarpa that released seeds after the eruption produced nearly 400,000 seedlings per hectare just two months following he eruption. Several thousand Salix seedlings of various species were also recorded. While many of these seedlings die, native seedlings were also found that appeared to be flourishing on the debris flow without prior conditioning of the flow by early successional species. These established seedlings continue to grow and produce seeds, giving the species an edge over others. Some species of plants were able to remain sheltered under logs that were blown down in the initial blast, and have since been found to be growing well inside the blast zone.
These zones, known as refugia, shielded seedlings and low-lying plants from the blast. Adams and Dale found that the microclimates produced by refugia such as boulders and fallen trees are enough to permit the establishment and growth of seedlings. These plants did not play a role in colonization of the blast zone, however. Fuller et al found that when refugia were created in the eruption, the surviving plants merely helped facilitate the establishment of pioneer species within the refugia. The pioneer species would then use the refugia as a “stepping stone” to invade the blast zone and tephra deposits.
This research shows that at the edges of the blast zone and debris flow, flora are well established, and are producing seedlings that are slowly recolonizing into the blast zone. Aiding the recolonization is Lupinus lepidus. According to del Moral et al (2005), this legume is the most abundant herbaceous plant on Mount St. Helens. Its symbiotic relationship with a nitrogen-fixing organism allows it to grow especially well in the nutrient-poor tephra. This nitrogen fixing also carries over into the soil that it inhabits. When the lupine plant dies out, it leaves behind nutrient-enriched soils for other plants o colonize, usually various pioneer species. By enriching the soil in which it grows, L. lepidus also speeds up the trajectories of successional patterns. The plant improves soil fertility, results in more seed trapping and soil stabilization. By causing these positive factors to become prevalent in soil strata, the amount of soil organic matter can accumulate more rapidly and aid in the establishment of other species on Mount St. Helens. While Mount St. Helens is currently the most active volcano in the Cascade Range, it is only one of several large stratovolcanoes.
The potential for the surrounding volcanoes to once again become active is considered by scientists to be very high, and the United States Geological Survey expects several of these volcanoes to become active again at some point. It is important for us to understand the ecological succession processes that occur following volcanic eruption for this reason. When Mount St. Helens erupted in 1980, it left behind it what at first appeared to be a scarred, sterile landscape. In the 32 years that have passed, ecologists have been reassured to find that nature is even more resilient than previously thought.
Wind-distributed seed propogules and residual plants sprouting from vegetation under the tephra colonized and prepared sites for invasion by secondary plants. Subterranean animals that survived have helped to mix soil with ash and organic matter, furthering site preparation. Wind and snow melt water continue to remove old tephra deposits, exposing more soil strata for plants to recolonize. Notwithstanding further eruptive actions, a vast mosaic of vegetation should be expected to quickly form over the slopes of Mount St. Helens.
