Plant distribution and abundance in relation to physical conditions and location within Danish stream systems

The distribution of obligate submerged plants, amphibious plants and terres trial plants in streams was examined in relation to water depth, substrate and distance to the banks using univariate and mulitivariate analyses. The analyses were based on recordings in more than 40 000 quadrats (25x25 cm) i n 208 unshaded sites in the predominantly small and shallow (<1.6 m) Danish streams. Also, the distribution of plant abundance and richness from the s ource to the outlet of the stream system was determined. The submerged plants in Danish streams include 87 terrestrial, 22 amphibiou s and 30 obligate submerged taxa. The distribution of plant types was mainl y related to water depth and distance to the bank among the physical condit ions, while the type of bottom substrate had no significant influence. Terr estrial plants and amphibious plants (excluding Sparganium emersum) dominat ed in shallow water near the bank, but declined rapidly with increasing dep th and distance to the bank, reflecting the importance of dispersal by ingr owth from populations on the banks to the water among these plants. Accordi ngly, these two plant groups constituted a higher proportion of total plant abundance in small streams than large streams. S. emersum dominated on gre at depth and distance to the banks, probably reflecting the lengthwise disp ersal of this species from upstream to downstream parts of the stream syste m, the tolerance of the species to weed cutting and the adaptation to grow at low light intensities. Obligate submerged plants dominated at intermedia te depths and at all distances from the bank except from 0 to 50 cm. This d istribution reflects the ability of these species to disperse lengthwise in streams and live permanently submerged. The species number of all species and obligate submerged plants was lower i n the smallest stream sites compared to larger downstream sites, while ther e was no difference for terrestrial and amphibious plants. The downstream i ncrease of submerged species can be explained by the increase of habitat ar ea and the dispersal of plants with the current, implying that the species pool accumulates with distance from the source. This result is at variance with a maximum richness of submerged plant species predicted for intermedia te-sized streams according to the River Continuum Concept developed for lar ge North American streams having forested shallow upstream sections and uns haded, deep downstream sections both unsuitable to submerged plant growth. The results for Danish streams imply that both the longitudinal connection through the flowing water and the transversal connection to the local speci es on the banks are important for plant distribution in the streams.