Unfriendly neighbors: When facilitation does not contribute to restoration success in tidal marsh

Large-scale restoration projects are an exciting and often untapped opportunity to use an experimental approach to inform ecosystem management and test ecological theory. In our $10M tidal marsh restoration project, we installed over 17,000 high marsh plants to increase cover and diversity, using these plantings in a large-scale experiment to test the benefits of clustering and soil amendments across a stress gradient. Clustered plantings have the potential to outperform widely spaced ones if plants alter conditions in ways that decrease stress for close neighbors. Here, we test whether intraspecific facilitation improves restoration outcomes using a suite of seven high marsh species native to central California salt marshes. We also applied a biochar treatment to test whether soil amendment boosts restoration success. We compared the performance of clustered and uniform plantings across the high marsh elevation gradient for 3 years. There was a strong effect of elevation on plant performance and clear signs of plant stress related to soil conditions. Clustering slightly improved the survival of one species out of seven, although clustering did not benefit that species in a follow-up experiment under more stressful conditions. By contrast, clustering had strong negative effects on the growth and/or cover of all species tested. The stressors in this system-likely related to compaction and soil salinity-were not mitigated by neighbors or biochar. The prevailing negative effect on seven species from distinct evolutionary lineages lends strong generality to our findings. We therefore conclude that for this and similar high marsh systems, intraspecific facilitation confers no benefits and practitioners should space plants widely to minimize competition. To take full advantage of the learning opportunities provided by large-scale restoration projects, we recommend including experimental treatments and monitoring the response of multiple species across years to refine best practices and inform adaptive management.