Properties of the New Volcanic Substrate of Mt. Usu with Regards to Natural Revegetation

概要

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The 1977-78 eruptions of Mt. Usu deposited a thick ash-pumice mantle on the summit caldera. This deposit was immediately re-worked by erosion. Air-fall tephra and re-worked materials provided new substrates for plants. Natural revegetation occurred but not in a uniform manner. To check if different revegetation patterns reflected noticeable variations in the substrate, investigations were made on the substrate in 1983-84. Tephra depth generally controlled plant survival. Pebble-granule sized pumices overlaid on the alluvial fans were more quickly invaded than silty-sand ash, probably because they offered better protection for seedling growth. Potential moisture was lower in pumice than in ash but actual moisture was probably higher on pumiceous fans due to better irrigation and less evaporation of the white surface. Actual moisture was also high in gullies and this favored plant establishment. Content in nutrients was generally low but this did not impede seed-plant invasion. Moisture and 20-45℃ surface temperatures around vapour fumaroles could explain the extensive invasion of algae and mosses. Substrate instability after the eruption had damaging aspects but also transformed an initially poor habitat: a thick tephra mantle overlaid with ash, into better habitats for plants such as thinner new tephra, gullies and pumice-overlaid alluvial fans.
北海道大学の論文
1986-08-20