In situ growth stimulation of a temperate zone liana (Hedera helix) in elevated CO₂

Summary Lianas, which arguably benefit more than other growth forms from elevated CO 2 , have been associated with increasing turn‐over rates in tropical forests as observed in recent decades. Although rarely as prominent outside the tropics, an increase in abundance of climbing plants is likely to affect forest dynamics in the temperate zone as well. Hedera helix is the most abundant climber in western Europe and previous work in controlled conditions suggested CO 2 effects similar to those observed in tropical lianas. Here we present an in situ test of the hypothesis that this abundant climber will benefit from increasing CO 2 concentrations primarily under light limitation in the forest understorey, but much less in the forest canopy. To this end, we studied growth responses to elevated CO 2 for an entire growing season at the Swiss Canopy Crane site on the forest floor and 20–25 m above the ground in the forest canopy. The relative stimulation of length and biomass increment of shoots by elevated CO 2 (about 600 µl l −1 ) was indeed very pronounced in deep shade ( c . +60%), about twice as much as in the subcanopy ( c . +30%). Given the rapid depletion of non‐structural carbohydrate (NSC) pools at canopy closure in spring, carbon limitation under current ambient CO 2 concentrations must be substantial in the understorey. In contrast to the understorey, where CO 2 enrichment had no effect on NSC pools, there was a sharp increase in NSC concentration in subcanopy leaves exposed to elevated CO 2 . We conclude that rising CO 2 concentrations will allow Hedera to explore light‐limited understorey microhabitats more vigorously, which should increase the likelihood of reaching the forest canopy. There, however, Hedera benefits less from elevated CO 2 . Our results add one more explanation to Hedera's current success in European forests.