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

Question: How do temporal changes in plant communities occur after volcanic eruptions? What characteristics determine successional divergence or convergence? Location: The summit area Of Mount Usu, northern Japan, completely destroyed by 1-3 m of thick ash and Pumice during the 1977-1978 eruptions. Habitats were classified into three types: gullies where the pre-eruption topsoil was exposed due to the erosion of tephra (EG), gullies covered with tephra (CG), and Outside of gullies covered with thick tephra (OG). Methods: Plant community structure was monitored for 15 years from 1983 to 1997 in 14 2m x 5m permanent plots. The data were Summarized by species diversity, life form, and the detrended correspondence analysis. Results: The common species were perennial herbaceous plants, but habitat preferences differed between species. Seed bank species, including a nitrogen fixer Trifolium repens, were dominant in EG, and excluded the establishment of the later colonists. Pioneer trees slowly increased in cover. The detrended correspondence analysis indicated that species composition in the earlier stages did not differ greatly between plots. Thereafter, three patterns of temporal community changes Were observed: seed bank species persisted in EG, and in OG and CG forest development proceeded or community structure did not change greatly. Conclusion: Pre-eruption topsoil contributed to revegetation by the supply of seed bank and nutrients in the earliest stages, but resulted in the delay of forest development due to the persistence of seed bank species. Plant community divergence was driven by the persistence of earlier colonists.