Ellenberg’s water table experiment put to the test: species optima along a hydrological gradient
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An important aspect of niche theory is the position of species’ optima along ecological gradients. It is widely believed that a species’ ecological optimum takes its shape only under competitive pressure. The ecological optimum, therefore, is thought to differ from the physiological optimum in the absence of interspecific competition. Ellenberg’s Hohenheim water table experiment has been very influential in this context. However, the water table gradient in Ellenberg’s experiment was produced by varying the soil thickness above the water table, which confounded the potentially disparate impacts of water table depth (WTD) and soil depth on species growth. Accordingly, here we have re-evaluated Ellenberg’s work. Specifically, we tested the hypothesis that physiological and ecological optima are identical and unaffected by interspecific interaction. We used the same six grasses as in Ellenberg’s experiments, but in our mesocosms, WTD was varied but soil depth kept constant. The design included both an additive component (with/without plant interaction) and a substitutive component (monocultures vs. species mixtures). The results show that the physiological optima along the hydrological gradient varied greatly between species, even in the absence of interspecific interaction. Within species, however, physiological and ecological optima appeared identical in most cases, irrespective of the competition treatment. We conclude that the ‘physiological capacity’ of species largely determines where they are able to persist and that any impact of interspecific interaction is only marginal. These findings are at variance with Ellenberg’s rule, where competition is considered to shift the distribution of a species away from its physiological optimum.
KeywordsHohenheim groundwater table experiment Hydrological niche Ecological optimum Physiological optimum Water table depth
The authors would like to thank Sibylle Bauer, Philipp Glaab, Stefanie Meier, Benedikt Müller, Daniel Peterlik, and Christoph Schmid for help during setup und harvest of the experiment; Ingeborg Lauer for general maintenance; and the gardeners of the Botanical Garden of Regensburg for technical support. We greatly appreciated the valuable input from discussions with Jonathan Silvertown (Open University of Milton Keynes, UK) and Richard Michalet (Université Bordeaux, France). We thank two anonymous reviewers and John Hodgson for their constructive comments that helped to improve the manuscript.
Author contribution statement
MB and PP conceived the study, MB designed and executed the experiments and analyses, and MB and PP wrote the manuscript.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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