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

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 mumol 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 mumol 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 mumol 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 mumol 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.