Effects of water depth and substrate on growth and morphology of Eleocharis sphacelata:: implications for culm support and internal gas transport

The response of growth and morphology to water depth (6 cm below to 32 cm above substrate) and substrate type (sandy versus organic substrates, with NPK added in excess of plant requirements) was studied for Eleocharis sphacelata. Depth and substrate interacted strongly in their effects on structure and growth. In sandy substrates, new culm biomass remained constant with depth, but the morphology changed from a high density of short (<0.75 m) culms in the shallowest treatment to a low density of tall (>0.9 m) culms in the deepest treatment. Culm height also increased with depth in the predominantly organic substrates, but the number of culms did not differ significantly and hence culm biomass increased with depth. Allocation to culms increased with depth, and tended also to increase with increasing organic matter content of the substrate. Root growth was strongly correlated with growth of the rhizome material, and rooting depth and root cortical porosity remained high in all treatments. The diameter of the tallest culms was affected by depth, but not substrate, increasing from <5 mm in the shallowest treatment to >7 mm in the deepest treatment. This decreased resistance to internal gas flow from 1.57 +/- 0.30 to 0.97 +/- 0.03 Pa s mm(-2) with increasing depth. In contrast to the three deeper treatments, the short, narrow culms in the shallowest treatment did not produce inflorescences. These responses suggest conflicting demands for minimising resistance to gas transport in deep water, but maximising mechanical strength when the water is shallow or below the substrate surface. Short, narrow culms may limit this species' competitiveness in shallow water and thus explain its frequent displacement from shallow environments. (C) 2002 Elsevier Science B.V. All rights reserved.