Disturbances are ubiquitous among salt marshes, and disturbances such as litter removal may form stressful environmental conditions. The stress-gradient hypothesis (SGH) predicts that the relative importance of facilitation and competition between species will vary inversely to each other across abiotic stress gradients. However, how intraspecific interactions vary across disturbance levels, and whether they follow the SGH has not been investigated. To test the SGH with intraspecific interactions within Suaeda salsa individuals, we assessed individual survival in established fully mapped salt marsh plots with two treatments, disturbed, litter removal and control, undisturbed plots. Recently developed spatial statistics were applied to distinguish between random processes, intraspecific facilitation, scramble competition, and contest competition underlying the spatial patterns at different spatial scales, growth stages, and disturbance levels. We found evidence that intraspecific interactions among S. salsa individuals across disturbance levels tended to support the SGH, but this support depended on time and space. Intraspecific interactions were more likely to show positive density dependence in disturbed plots, particularly at the seedling period and when individuals were separated by <9 cm. However, positive density dependence was not detected in undisturbed plots. The protective effects of litter on seedlings, which were lost in undisturbed sites, may have been at least partly simulated at high seedling densities. Mortality in disturbed sites was not random, and facilitation had a more important influence than competition on population dynamics in high disturbance sites. However, the influence of competition on population dynamics, specifically scramble competition was also clearly important.
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This research is funded by National Natural Science Foundation of China (No. 31100313), the Foundation for Outstanding Young Scientist in Shandong Province (No. BS2013HZ012), and the Program of Science and Technology Service Network Initiative, Chinese Academy of Sciences (No. KFJ-EW-STS-127). We thank the reviewers and editor for their comments and suggestions, which have been very helpful for improving the manuscript. We would also like to thank Christine Verhille at the University of British Columbia for her assistance with English language and grammatical editing of the manuscript.
Communicated by Stephen Brewer.
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Zhang, L., Wang, B. Intraspecific interactions shift from competitive to facilitative across a low to high disturbance gradient in a salt marsh. Plant Ecol 217, 959–967 (2016). https://doi.org/10.1007/s11258-016-0621-x
- Intraspecific facilitation
- Intraspecific competition
- Salt marsh
- Temporal scale
- Spatial point pattern analysis