Long-term survival in soil of seed of the invasive herbaceous plant Heracleum mantegazzianum

Seed persistence in soil is an important ecological trait enabling a species to survive at a particular site. How long the seed persists depends on its physical and physiological characteristics as well as on biotic and abiotic environmental conditions. In alien species, the ability to develop a persistent seed bank is associated with their ability to naturalize and become invasive. We carried out an experiment to find out how long the seeds of Heracleum mantegazzianum, one of the most invasive monocarpic species in Europe, are able to persist in soil and explore the effect of environmental conditions on seed depletion over a long period of time. The seeds were buried at 10 localities in different regions of the Czech Republic to cover a range of climatic, edaphic and geographic conditions. After seven years of burial, 0.1% of the seed of H. mantegazzianum, averaged across the localities, were viable, and none survived at five localities, at one of which the seed bank was completely depleted after five years and at one already after two years. The highest recorded survival at the end of the burial period was 0.5% of seed. In general, the average seed-bank survival was very low after five years and never exceeded 1%. We found a significant correlation between the percentage seed bank depletion and the content of total carbon and organic carbon; the effects of other soil characteristics, climatic and geographical factors were not significant. High percentages of seed persisted after the first year at localities with a low carbon content but the percentage depletion after seven years was slower at localities with a high carbon content. In addition, low carbon content and high seed survival after the first year was recorded at localities with nutrient poor soils. The course of seed bank depletion in H. mantegazzianum corresponds to its classification as short-term persistent. Therefore, monitoring well beyond the reported period of seed bank persistence is recommended after full eradication of this species from a site.