Improving our knowledge of regeneration processes in riparian woodlands could help guide restoration projects by enhancing the natural regeneration potential of riparian tree species. In western Europe, Populus nigra, is a major component of riparian woodlands, but the spatial extent of this species has decreased due to river management activities. In this study, the timing of Populus nigra seed dispersal and variations in seed germinability through time were monitored. An experiment was also devised to measure both seed germination and seedling survival in changing hydrologic conditions. The seed dispersal period lasted 12 weeks along the Garonne River. Individual poplar trees displayed variations in seed release patterns. On average, seed germinability remained rather low and close to 28%. However, large differences in germinability were observed between trees and over time. A continuously wet substrate was the most favorable condition for germination and seedling survival. A change from wet to submerged conditions increased germination and reduced seedling survival. In the case of a change from either wet or submerged to dry conditions, there was no survival possible if the dry conditions persisted more than one week. Substrate type had a significant effect on germinability only in moderate moisture conditions. Populus nigra tolerates decreases in the optimum minimum flow if coupled with at least a brief rise in river level on a weekly basis.
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Vol. 22 • No. 3