Does elevated CO 2 facilitate naturalization of the non‐indigenous Prunus laurocerasus in Swiss temperate forests?

Summary1 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 CO 2 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 CO 2 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 CO 2 . Biomass growth in Prunus increased by an average of 56% at the two elevated CO 2 concentrations compared to ambient CO 2 ; 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 CO 2 response. 4 A particularly large and nearly linear CO 2 effect on seedling growth was observed in the liana Hedera with 100% more biomass and 137% longer stems at 660 µmol CO 2  mol −1 compared to ambient CO 2 . Seedlings of the deciduous tree species Fraxinus produced 43% more biomass at elevated CO 2 (no significant difference between 500 and 660 µmol mol −1 ), but there was no significant CO 2 effect on Carpinus seedlings. 5 Our results indicate that elevated CO 2 might contribute to the current spread of Prunus in natural forests. The strong CO 2 response in Hedera suggests an increasing rate of tree colonization with rising CO 2 . 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.