Does elevated CO2 facilitate naturalization of the non-indigenous Prunus laurocerasus in Swiss temperate forests?

Summary

• 1
  An increasing abundance of the non-indigenous evergreen woody plant species Prunus laurocerasus has been observed in the understorey of Swiss temperate forests. We addressed the question whether rising atmospheric CO₂ concentration contributes to the success of this species in a comparative test with four co-occurring native species (Ilex aquifolium, Hedera helix, Fraxinus excelsior, Carpinus betulus).
• 2
  We grew plants from germination to the end of the third growing season in open-top chambers exposed to either ambient or two elevated CO₂ concentrations (500 and 660 µmol mol⁻¹) in a deeply shaded forest understorey (1·2–3·2% of full sun).
• 3
  Species differed greatly in their response to CO₂. Biomass growth in Prunus increased by an average of 56% at the two elevated CO₂ concentrations compared to ambient CO₂; there was no significant difference between 500 and 660 µmol mol⁻¹. In contrast the native Ilex, with the same functional traits, a similar life history and occurring in the same habitat, showed no significant CO₂ response.
• 4
  A particularly large and nearly linear CO₂ effect on seedling growth was observed in the liana Hedera with 100% more biomass and 137% longer stems at 660 µmol CO₂ mol⁻¹ compared to ambient CO₂. Seedlings of the deciduous tree species Fraxinus produced 43% more biomass at elevated CO₂ (no significant difference between 500 and 660 µmol mol⁻¹), but there was no significant CO₂ effect on Carpinus seedlings.
• 5
  Our results indicate that elevated CO₂ might contribute to the current spread of Prunus in natural forests. The strong CO₂ response in Hedera suggests an increasing rate of tree colonization with rising CO₂. Increasing dominance of non-indigenous understorey species and accelerated liana colonization of canopy trees could both have far-ranging consequences for forest community dynamics and composition.