Seasonal pattern of germination and seed longevity in the invasive species Heracleum mantegazzianum

We investigated the effects of different temperature regimes and dry storage on germination of H. mantegazzianum (Apiaceae, native to Caucasus) seeds in the laboratory and linked the results with studies of seasonal seed bank depletion in a common garden experiment and under field conditions. Seeds were collected at seven sites in the Slavkovský les region, Czech Republic, cold-stratified for 2 months and germinated at seven temperature regimes. Under all temperature regimes, fresh seeds germinated to significantly higher percentages than older (1, 2, 3 years) seeds. For all storage lengths, seeds germinated best at alternating day/night temperatures of 20/5 °C. The length of the germination period had a significant effect only at low constant temperatures of 2 and 6 °C, where germination percentage increased between 2 and 6 months. Seasonal germination exhibited a distinct pattern, with rapid depletion of seed bank by the first spring after seed burial. Non-dormant seeds were present in the soil early in spring and late in autumn. The higher summer temperatures prevented dormancy breaking and another cold period of at least two months below 10 °C was needed to bring non-germinated seeds out of dormancy. The results suggest that (1) seed dormancy of H. mantegazzianum was not completely broken until the first spring, but that some seeds re-enter or retain dormancy during high summer temperatures and that (2) the threshold needed for breaking the dormancy was achieved gradually during the cold autumn and winter months. However, in a small fraction of seeds the dormancy breaking process took several years. Of seeds buried in 10 different regions of the Czech Republic, on average 8.8% survived 1 year, 2.7% 2 years and 1.2% remained viable and dormant after 3 years of burial. The ability of even small fraction of H. mantegazzianum seeds to survive for at least 3 years can result in re-invasion of this species into controlled sites.

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