Growth and morphology of Scirpus lacustris and S-maritimus seedlings as affected by water level and light availability

1. Adults of the emergent macrophytes Scirpus lacustris ssp. lacustris (S.l. lacustris), S. lacustris ssp. tabernaemontani (S.l. tabernaemontani) and S. maritimus occur along a gradient in water depth from deep to shallow water. This study examined whether seedlings of these taxa respond differently to changing hydrological conditions. 2. Seedlings of both S. lacustris subspecies showed the highest relative growth rate (RGR) under terrestrial growth conditions, whereas S. maritimus did so under submerged growth conditions. In all three taxa, shading reduced the mean RGR of terrestrial seedlings more strongly than that of submerged ones. 3. Scirpus maritimus and S.l. tabernaemontani maintained an erect growth form under water, whereas S.l. lacustris produced numerous long, floating leaves. 4. Under terrestrial growth conditions the specific leaf area (SLA) did not differ between taxa. Under submerged growth conditions the SLA differed as follows: S.l. lacustris > S.l. tabernae montani > S. maritimus. Irrespective of taxon and water level, the SLA was increased by shading. 5. Growth of all three taxa was reduced considerably after seedlings were transferred from terrestrial to submerged growth conditions. This effect was stronger with increasing age of seedlings. When transferred the other way round, seedlings of S.l. tabernaemontani and S. maritimus adapted quickly to the terrestrial growth conditions, whereas the thin leaves of S.l. lacustris partly dried out. 6. It was concluded that although seedling establishment of all three Scirpus taxa will be most successful under terrestrial conditions, subsequent fluctuating water levels may act as a strong selective force. This may determine the distribution of Scirpus taxa along a gradient in water depth during seedling establishment.