Abstract
Plant species composition in wetlands and on lakeshores often shows dramatic zonation, which is frequently ascribed to differences in flooding tolerance. This study compared the growth responses to water depth of three species (Phormium tenax, Carex secta and Typha orientalis) differing in depth preferences in wetlands, using non-linear and quantile regression analyses to establish how flooding tolerance can explain field zonation. Plants were established for 8 months in outdoor cultures in waterlogged soil without standing water, and then randomly allocated to water depths from 0 to 0.5 m. Morphological and growth responses to depth were followed for 54 days before harvest, and then analysed by repeated-measures analysis of covariance, and non-linear and quantile regression analysis (QRA), to compare flooding tolerances. Growth responses to depth differed between the three species, and were non-linear. Phormium tenax growth decreased rapidly in standing water >0.25 m depth, C. secta growth increased initially with depth but then decreased at depths >0.30 m, accompanied by increased shoot height and decreased shoot density, and T. orientalis was unaffected by the 0- to 0.50-m depth range. In P. tenax the decrease in growth was associated with a decrease in the number of leaves produced per ramet and in C. secta the effect of water depth was greatest for the tallest shoots. Allocation patterns were unaffected by depth. The responses are consistent with the principle that zonation in the field is primarily structured by competition in shallow water and by physiological flooding tolerance in deep water. Regression analyses, especially QRA, proved to be powerful tools in distinguishing genuine phenotypic responses to water depth from non-phenotypic variation due to size and developmental differences.
Highlights
Plant species zonation is a characteristic feature of water depth gradients in wetland environments and lake shorelines (Seabloom et al 2001; Strayer and Findlay 2010)
Morphological and growth responses to depth were followed for 54 days before harvest, and analysed by repeated-measures analysis of covariance, and non-linear and quantile regression analysis (QRA), to compare flooding tolerances
Phormium tenax growth decreased rapidly in standing water .0.25 m depth, C. secta growth increased initially with depth but decreased at depths .0.30 m, accompanied by increased shoot height and decreased shoot density, and T. orientalis was unaffected by the 0- to 0.50-m depth range
Summary
Plant species zonation is a characteristic feature of water depth gradients in wetland environments and lake shorelines (Seabloom et al 2001; Strayer and Findlay 2010). The factors associated with flooding that stress plants in deep water include the limited ability of most wetland species to assimilate inorganic carbon from water (Colmer and Pedersen 2008), light attenuation and its effects on photosynthesis and development (Mommer and Visser 2005), and oxygen deprivation in below-ground rhizomes and roots growing in anaerobic sediments (Bailey-Serres and Voesenek 2008) These abiotic stresses, and the extent to which they can be avoided or tolerated due to ecophysiological adaptations that confer flooding tolerance, become increasingly growth limiting as water depth increases (Sorrell and Hawes 2010).
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