Long-term studies of succession : Colonisation and seed banks

Abstract: Using data from long-term studies of the vegetation in two areas under primary succession, patterns of colonisation and the accumulated seed bank during plant succession were studied.At century-old small islands in Lake Hjälmaren, Sweden, the genetic structure of differently old populations of Moehringia trinervia and Lythrum salicaria was studied, using allozyme variation for M. trinervia and style-morph frequencies for the tristylous L. salicaria. Both species showed a low colonisation rate but differed in their genetic structure. For the highly selfing M. trinervia, the genetic structure of the colonisers was preserved due to a low immigration rate, and founder populations of different ages did not differ in their genetic structure but differed from mainland source populations. Neither the perennial self-incompatible L. salicaria showed a temporal trend, probably due to low seed set and high longevity of the colonisers, which retards frequency-dependent selection towards an even style-morph distribution.Small-scale seaward migration of species on a land uplift shore along the Baltic Sea was largely related to allogenic factors. Due to stochastic variation in the sea level and to wave action, the continuous seaward migration of species may be temporarily retarded. During the 15 years studied, Ellenberg indicator values related to allogenic vegetation change showed a greater temporal change than values related to autogenic vegetation change. At both study areas, the similarity between the seed bank and the extant vegetation decreased with increasing successional age. At Lake Hjälmaren, the deepest soil contained species only found in earlier successional stages. At the sea shore, the seed bank and the vegetation were completely dissimilar at sites older than 200 years.Species attributes and Ellenberg indicator values differed in most cases between seed bank and vegetation. Attributes connected with early successional species were associated with the deepest sampled seed bank while attributes of late successional species were associated with the surface soil, indicating that early successional species still after a century of primary succession were present in the deepest soil horizons.The Ellenberg indicator values indicated that the seed bank reflects several successional stages now extinct from the vegetation.