Graduate Thesis Or Dissertation
 

Responses of forest understory plants to burial by volcanic tephra from Mount St. Helens, Washington

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  • The effects of burial by tephra (volcanic aerial ejecta) on forest understory plants were examined northeast of Mount St. Helens, Washington, in the area where the 18 May 1980 eruption deposited tephra but did not destroy canopy trees. At six sites along a tephra depth gradient from 2-15 cm, understory plant cover and density were recorded during 1980, 1981, and 1982, in plots on undisturbed tephra and in others cleared of tephra during summer, 1980. Bryophytes failed to grow through 4 cm of tephra; both cover and density of herbaceous vascular plants were reduced over 70% by 7.5 cm of tephra, and mostly eliminated with 15 cm of tephra. Microsites with thin tephra, especially erosion channels and to a lesser extent large logs, were vital to the survival of many individual plants. At sites with 15 cm of tephra, most herbaceous cover in 1982 was in erosion channels. Seedlings were common but most forest herbaceous species were not represented among them; total seedling cover was very low. Individuals of many herbaceous species occasionally grew through tephra deposits exceeding 10 cm in depth. Examination of such plants of 109 species indicated that, in some, pronounced modifications in growth form occurred, including the development of long internodes on normally short rhizomes and an upward reorientation of the direction of growth. Most herbaceous species examined had produced perennating buds and roots in the tephra. All of the 28 shrub species examined produced adventitious roots in the tephra. Subsurface parts on eight common species of forest herbs, Achlys triphylla, Arnica latifolia, Clintonia uniflora, Erythronium montanum, Rubus lasiococcus, Smilacina stellata, Tiarella trifoliata, and Valeriana sitchensis, were examined in detail during 1981 and 1982 on plants in tephra and in a tephra free area in the Oregon Cascade Range. Species varied greatly in the rate of movement of roots and stems into the tephra; except for Erythronium, which moved neither its corm nor roots upward, species with large rhizome or stolon systems had the lowest proportion of material in the tephra. For most species, plants in tephra had greater underground biomass per unit leaf area than plants in tephra free areas; this condition became less pronounced by 1982.
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