Primary Succession & Species Diversity of Flora on Mount St. Helens, 32 Years Post-Eruption

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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...
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