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Central and peripheral Hornungia petraea populations: patterns and dynamics

1 More or less continuous distributions tend to become fragmented towards species’ distribution limits. Peripheral or isolated populations of a species are predicted to have lower population sizes and densities than central populations, as a result of environmental and/or genetic stress. Population densities at the periphery may be reduced by decreased reproduction or higher interannual variation in reproduction. In particular, fecundity and survival are likely to be reduced by less favourable growing conditions. 2 We compared populations of the annual species Hornungia petraea (Brassicaceae) in two contrasting regions of the species’ range and at the population scale within regions. Ten populations in Italy (central to the species range) and 10 German populations (peripheral) were monitored for three growing periods from spring 1999 to spring 2001. 3 All life‐cycle stages, adult plant density, seed production and pre‐ and post‐dispersal seed bank density were studied in a nested sampling design and variation in various demographic factors was attributed to the effects of the two countries, of populations nested within country and to temporal effects. 4 Peripheral populations had higher densities than populations in the centre of the species distribution, both as adult plants and in the seed bank. 5 Fecundity was strongly influenced by temporal effects, but only affected to a minor degree by the study region. High interannual variation in fecundity was not reflected in high interannual variation of either adult plant density in spring or of the seed bank. Significant regional differences were, however, found in seasonal seed bank dynamics, which were more pronounced in peripheral German populations than in central Italian populations. 6 We conclude that seasonal seed bank dynamics are a key factor in explaining differences in H. petraea density patterns, particularly in central populations, where fewer seeds are incorporated into the seed bank.