Abrupt Drops in Water Table Level Influence the Development of Populus nigra and Salix alba Seedlings of Different Ages

摘要

Early seedling establishment is a demographic bottleneck for the sexual regeneration of many riparian tree species such as Populus nigra and Salix alba, two dominant species of European floodplains. Immediately after spring germination, seedling roots must track receding water tables to avoid desiccation. This may be especially difficult if the groundwater recession is too abrupt. In this study, seedling survival and growth under different abrupt drops in water table levels were examined using an experimental facility that allowed continuous and non-destructive measurements of above- and below-ground seedling development. A Cox proportional hazards regression model showed that the risk of mortality for a seedling was significantly lower in P. nigra than in S. alba, was lower for older than for younger seedlings, and increased with greater drops in water table level. In fact, almost no seedlings survived abrupt water level drops >40 cm. However, no substantial differences in any of the growth parameters were observed between species, age, and water table treatments, as shown by R-ANOVA and paired tests. This suggests that abrupt drops in water table level did not stimulate plant growth but only favoured the survival of the most resistant individuals. These findings should be considered when designing improved water flow prescriptions along managed river courses in Europe.