Mechanisms producing plant zonation along a water depth gradient: a comparison with the exposure gradient

Abstract

A 15-month field experiment was performed in the emergent zone of a freshwater riverine marsh along the Ottawa River, Canada, to determine whether the mechanisms producing plant zonation along the exposure gradient of freshwater shorelines also accounted for the zonal pattern along the water depth gradient. Three species (Carex crinita, Acorus calamus, and Typha angustifolia) were chosen, having contiguous distributions along a gradient of water depth. Ramets of each were planted within and beyond the field distributions of each, both in the presence and in the absence of the natural vegetation of each site. Although there was strong evidence of growth depression due to the presence of neighbouring plants (interference), there was no evidence of a differential response between species, between sites on the water depth gradient, or a combination of the two. As well, thee was no evidence that the water depth gradient represents a general gradient of decreasing productivity; rather, there was a qualitative change below the low-water level. These results are contrary to previously published results obtained along the exposure gradient of freshwater shorelines, where the effects of plant interference do vary both along the exposure gradient and among species. Key words: plant zonation, interference, water depth, exposure gradient.

Historical changes in herbaceous wetland distribution induced by hydrological conditions in Lake Saint-Pierre (St. Lawrence River, Quebec, Canada)

“…Hydrology is also linked to other factors (exposure, litter accumulation, fertility gradients, exotic species introduction) previously shown to influence the complex mosaic of wetland plant assemblages (Day et al, 1988;Keddy, 1989;Shipley et al, 1991;Lavoie et al, 2003). Comparison of wetland assemblages over low-level episodes in 1931 and 1999 revealed that recent wetland plant assemblages reflect nutrient enrichment, drier wetland conditions and proliferation of aggressive taxa (Hudon, 2004).…”

Ecological Dynamics I: Vegetation as Bioindicator and Dynamic Community

“…However, the hierarchy of causes has been controversial, because the effects of environmental factors on species and habitats differ, and many factors are correlated (van Diggelen et al 1996). Experiments by Shipley et al (1991) suggest that the mechanisms contributing to plant zonation along a freshwater depth gradient are not the same as the mechanisms producing zonal patterns along the exposure gradient as described by Wilson and Keddy (1986). Species order along a water depth gradient may be due instead to the different physiological response of each species modifi ed by interspecifi c competition, that the stresses at each depth are species-specifi c (Shipley et al 1991).…”

“…Experiments by Shipley et al (1991) suggest that the mechanisms contributing to plant zonation along a freshwater depth gradient are not the same as the mechanisms producing zonal patterns along the exposure gradient as described by Wilson and Keddy (1986). Species order along a water depth gradient may be due instead to the different physiological response of each species modifi ed by interspecifi c competition, that the stresses at each depth are species-specifi c (Shipley et al 1991). Menges and Waller (1983) examined the distribution of herbaceous species in southern Wisconsin fl oodplain forest in relation to elevation and concluded that most species had defi nite elevation optima.…”

“…In Grime's (1979) model, competitive interactions between plants are weak and relatively unimportant in determining local distributions in environments having either high levels of density-independent mortality (disturbance) or low level of productivity (stress). As either the level of disturbance or stress decreases, the degree of interspecifi c competition increases in intensity and importance (Shipley et al 1991). Grime (1979) hypothesised that the relationship between plant species richness and biomass is unimodal, with richness being highest at intermediate levels of biomass.…”

Ecophysiological responses of nine floodplain meadow species to changing hydrological conditions

“…The lower distribution limits are abiotically constrained by flooding stress (Vervuren et al 2003;van Eck et al 2004), which strongly affects plant performances by reducing gas exchanges between plant and atmosphere (Crawford and Brändle 1996;Dat et al 2004). In contrast, competition is recognised as the most important factor at the upper distribution limits (Grace 1990;Keddy 1990, Shipley et al 1991Budelsky and Galatowitsch 2000), although studies suggested that drought might contribute to restrict flood-tolerant species to low elevations (Silvertown et al 1999;Lenssen and de Kroon 2005). Consequently, as flooding gradients would correspond to a shift from abiotic to biotic constraints, the intensity of stress induced by a change in hydrology should differ according to the direction of change.…”