Control of Plant Species Richness and Zonation of Functional Groups along a Freshwater Flooding Gradient

Abstract

We investigated the role of plant interactions in producing the zonation of strong competitors, i.e. clonal dominants, and weak competitors, i.e. interstitials. In addition, it was tested whether the effect of plant interactions on species richness depends on the position on the flooding gradient. During one growing season vegetation canopy was removed at a low elevation, dominated by Phragmites australis and at a high one, dominated by tall forbs, mainly Urtica dioica. The seed bank was examined and in half of the plots seeds of clonal dominants from high elevations and interstitials from low elevations were sown to ensure that seeds of both groups were present at both zones. At both elevations, removal of vegetation canopy resulted in a strong increase of interstitial species, but interstitials from low elevations failed to establish in cleared plots at the tall forb zone. This could not be attributed to the absence of seeds and we concluded that conditions unfavourable for germination, rather than plant interactions determine the zonation of interstitials from the P. australis zone. Many seedlings of tall forb dominants emerged in cleared plots at the low elevation. However, number of seedlings rapidly declined during the first year. Hence, abiotic conditions, most probably flooding, prevented seedling establishment of tall forb dominants at zones dominated by P. australis. Canopy removal increased species richness at the low elevation in the first year only, whereas at the higher elevation species richness in cleared plots remained higher throughout the second year when the canopy was no longer removed. We therefore concluded that species richness of freshwater shorelines is controlled by abiotic factors in the frequently flooded zone and by plant interactions at higher elevations. [KEYWORDS: Water tidal wetland; seed-bank; emergent macrophytes; fen vegetation; salt-marsh; communities; england; growth; level; competition]