Raised atmospheric CO2 levels affect soil seed bank composition of temperate grasslands

Abstract

Co-ordinating Editor: Martin Zobel Abstract Questions: Soil seed banks buffer plant populations against environmental variability. But environmental changes can have profound impact on them. Several studies addressed the effect of climate change on aboveground vegetation, but studies on changes in the seed bank are rare. Thus, we studied the seed bank of a temperate grassland at a longterm FACE (FreeAir Carbon dioxide Enrichment) site, with the following questions: (a) Are there general differences in the species composition between aboveground vegetation and seed bank; (b) what are the impacts of elevated CO2 (eCO2) on seed density and species composition of the seed bank; and (c) are there differences in the functional traits of the seed bank species under eCO2 versus ambient CO2 (aCO2)? Location: Temperate grassland, Gießen, Germany. Method: Thirty soil cores were taken in six FACE rings. Emerging seedlings were identified to species level after germination and aboveground vegetation was sampled. From the seed bank data, we derived diversity measures and weighted means of species traits, e.g., seed longevity and regeneration type, and compared eCO2 (+20% CO2 above ambient conditions) with aCO2 treatment. Results: Nonmetric multidimensional scaling ordination revealed a clear separation between seed bank and aboveground vegetation but no clear CO2 effect. Analyses revealed higher seed densities under eCO2. Species diversity and Shannon diversity were not significantly affected. Evenness decreased significantly under eCO2. There are shifts in functional traits of seed bank species. Seed density of longtermpersistent species increased, while shorttermpersistent species decreased. Seed densities of species with generative reproduction increased under eCO2, while numbers of species with vegetative reproduction decreased. Conclusion: The observed trait compositions of the seed bank under eCO2 indicate that species relying on generative reproduction and production of longtermpersistent seeds have a competitive advantage under eCO2. The changes in the plant communities described above may lead to profound changes in the supply of grassland ecosystem services.