Helophyte

Abstract: 1. Colonization in a reintroduced population of European beavers in the Loire Valley was studied between 1974 and 1999. It followed a discontinuous remoteness model and a scattered distribution, beavers occupying only 25% of the river system over the 2800 km explored. 2. After 5 years, the colonization rate reached 104.2% year–1 of new sites occupied (SD 75% year–1), before dropping over the next 20 years. Nevertheless, the number of new colonies per km (0.125) remained stable throughout the years. 3. Populus nigra, Salix alba and Fraxinus angustifolia were the dominant woody species in beaver sites, often associated with some herbaceous species. 4. The length of willow grove dominated by S. alba and P. nigra (x) was the best predictor of beaver home range (y), fitting the equation y=−0.742x + 5.9. Long-term maintenance of the population requires a minimum of 1.79-km of willows per colony. 5. In cutting tree trunks, beavers stimulate shoot development from the remaining stumps. They rejuvenate riparian forests, increase the number of tree stems and help stabilize the banks. Their effect on woody plant morphogenesis may have consequences for the helophyte communities used as food or habitat by other aquatic species.

Abstract: . Drift of aquatic macrophyte propagules was investigated in a wetland along the River Rhone, during the first flood after the growing season (i.e. in the winter of 1995–1996). Input and output drift were studied at the beginning, around the top, and at the end of the river overflow in the upper reach of a cut-off channel. The soil propagule bank was sampled along the study area before and after the flood. The amount and composition of viable propagule drift and bank were determined, analysed and compared. Drift densities and richness were on average higher at the outlet of the channel than at the inlet (respectively: 23.2 vs 13.1 viable propagules/100 m3 of water and 8.7 vs 2.6 taxa per sample). Immigrating taxa were mostly in the form of helophyte seeds, whereas numerous resident hydrophyte species left the disturbed area rather as vegetative propagules. Temporal variability in propagule bank structure was weak, and mean bank densities did not change before and after the flood (respectively: 33 047 ± 10 510 vs 35 653 ± 15 070 viable propagules/m2 of ground, including Chara). However, the density of Elodea canadensis significantly increased after the flood while that of Eleocharis acicularis decreased. This contrast suggests that flood responses vary among species. Despite a broad overlap in the taxa (18 out of 25 were common both to drift and bank collections), no significant relationship occurred in composition or structural changes between flood drift and propagule bank. Flood acted as a means of distribution of existing propagules and also as a provider of new vegetative dispersal units.