Abstract
Plant zonation is conspicuous in wetlands. The cause is frequently assumed to be the direct physiological effects of physical factors (termed ‘stress’), however many experiments show that competition and facilitation also cause zonation patterns. We conducted a field experiment with freshwater marsh emergent plants to test the causes of zonation along a single stress gradient: flooding duration. We constructed an experimental wetland with ten flooding levels to ensure that the environmental conditions represented the full range of potential flooding levels, from never flooded to continually flooded. We planted ten common marsh plants with varied ecology along the flooding duration gradient. We grew them alone and in mixture for three years and measured changes in the minimum and maximum limits, the mode and the range of distribution, and interaction importance. The mode of distribution did not shift, whether species were grown alone or with neighbours. We found strong effects of competition under low flooding stress. We found no effects from facilitation under high flooding stress. Flooding duration alone controlled the lower limits of plants. The effects of competition were intense enough to eliminate half of the species within three growing seasons. Our experiment showed that competition and physical stresses, but not facilitation, controls the zonation of emergent macrophytes along a flooding duration gradient, at least in freshwater wetlands. Models guiding wetland restoration need to include competition as well as flood duration as causal factors, but not facilitation.
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Data Availability
Data used in the production of this manuscript are available at DOI: https://doi.org/10.6084/m9.figshare.12937031.v1.
Code Availability
R code for statistical analyses is available in Supplemental Materials.
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Acknowledgements
We thank the many hard-working field assistants, including Tiffany McFalls, Michaelyn Broussard, Jack Siegrist, Meghann Clark, Jennifer Tynes and Glen Hurst.
Funding
Initial funding for the experiment was provided by National Science Foundation grant DEB-0129024. Thereafter, costs of analysis and manuscript preparation were borne by the co-authors, who, like Thoreau, had neither salary nor grant support.
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PK conceived and designed the experiment. DC helped to set up the experiment and collect the data. DC analyzed the data and produced the figures. DC and PK co-wrote the manuscript. Both authors contributed critically to this draft and gave final approval for publication.
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Campbell, D., Keddy, P. The Roles of Competition and Facilitation in Producing Zonation Along an Experimental Flooding Gradient: a Tale of Two Tails with Ten Freshwater Marsh Plants. Wetlands 42, 5 (2022). https://doi.org/10.1007/s13157-021-01524-4
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DOI: https://doi.org/10.1007/s13157-021-01524-4