Causes of plant community divergence in the early stages of volcanic succession

Abstract Question: How do temporal changes in plant communitiesoccur after volcanic eruptions? What characteristics de-termine successional divergence or convergence?Location: The summit area of Mount Usu, northernJapan, completely destroyed by 1-3m of thick ash andpumice during the 1977-1978 eruptions. Habitats wereclassified into three types: gullies where the pre-eruptiontopsoil was exposed due to the erosion of tephra (EG),gullies covered with tephra (CG), and outside of gulliescovered with thick tephra (OG).Methods: Plant community structure was monitored for15 years from 1983 to 1997 in 14 2m 5m permanentplots. The data were summarized by species diversity, lifeform, and the detrended correspondence analysis.Results: The common species were perennial herbaceousplants, but habitat preferences differed between species.Seed bank species, including a nitrogen fixer Trifoliumrepens, were dominant in EG, and excluded the establish-ment of the later colonists. Pioneer trees slowly increasedincover.Thedetrendedcorrespondenceanalysisindicatedthat species composition in the earlier stages did not differgreatly between plots. Thereafter, three patterns of tem-poral community changes were observed: seed bankspecies persisted in EG, and in OG and CG forest devel-opmentproceededorcommunitystructuredidnotchangegreatly.Conclusion: Pre-eruption topsoil contributed to revegeta-tion by the supply of seed bank and nutrients in theearliest stages, but resulted in the delay of forest develop-ment due to the persistence of seed bank species. Plantcommunity divergence was driven by the persistence ofearlier colonists.Keywords: Early colonists; Forest development; Gully;Mount Usu; Permanent plots; Seed bank.Nomenclature: Ohwi & Kitagawa (1983)

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