Ivy (Hedera helix L.) dynamics in riverine forests: Effects of river regulation and forest disturbance

Ivy (Hedera helix L.) favours moist nutrient-rich substrates of the floodplains of Western Europe. In this study we investigated ivy dynamics in two forest reserves of the upper Rhine. The aim was to determine the influence of flooding on ivy development following the cessation of cuttings at two forest sites of contrasting flooding regimes: the Rhinau Reserve is currently flooded while the Erstein site has not been flooded for 30 years. We also examined the impacts of severe storms in 1993 and 1999 and a long-lasting flood that occurred in 1999.Our results show that the population of Rhinau was smaller and younger because ivy is severely limited by long periods of anoxia, even though it is favoured by regular short-period floods. Indeed, juvenile growth was more rapid at Rhinau where nutrient and moist conditions are more favourable than in Erstein, but mortality was higher because of the long-lasting flood of 1999. At Erstein, the ivy showed a tendency to clump around several big trees, especially oaks and ashes, which may make the host tree vulnerable to windfall. Uprooted or broken ivies were found to survive better than the fallen host tree but could not climb to another trunk highlighting a strong dependence of ivy on its hosting tree.