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Functional Redundancy among Tidal Marsh Halophytes: A Test

Gary Sullivan and Joy B. Zedler
Oikos
Vol. 84, No. 2 (Feb., 1999), pp. 246-260
Published by: Wiley on behalf of Nordic Society Oikos
DOI: 10.2307/3546719
Stable URL: http://www.jstor.org/stable/3546719
Page Count: 15
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Functional Redundancy among Tidal Marsh Halophytes: A Test
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Abstract

A priori, we identified species groups among eight halophytes of the southern California tidal marsh plain, using cluster analysis on growth form and life history traits which are commonly employed to produce 'functional groups'. We then tested the efficacy of the groupings by assessing biomass and nitrogen accumulation functions of each species in two greenhouse experiments that differed in soil moisture and nutrient availability. Plants were grown for 6 mo in both monotypic and multispecies microcosms. Functional attributes differed among species, including members of the a priori groups. We found a 2-fold range in tissue nitrogen concentrations and a 2.5-fold range in biomass accumulation. Rhizosphere root distributions differed, and root-to-shoot ratios ranged 3-fold. The presence of neighboring species altered the biomass or nitrogen concentrations of four species. A posteriori cluster analysis based on microcosm performance produced different species groups than found in the a priori analysis. Two species pairs appeared functionally redundant, although the annual and perennial pair would likely differentiate in time, and species in the second pair tend to be found at different elevations across the marsh plain. We argue that the designation of functional groups should be based on measured performance and objective classification procedures. Rather than assuming functional redundancy, we recommend testing for similarity of group members under varied conditions, e.g., alone and with their common neighbors and under benign and stressful conditions.

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