Performance of seven species in clustered versus uniform plantings in a large-scale high marsh restoration project

Large-scale restoration projects are an exciting and largely untapped opportunity to use an experimental approach to inform ecosystem management and test ecological theory. This paper takes advantage of a $10M marsh restoration project with long-term monitoring to test the relative importance of facilitation and competition across stress gradients. 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 performance of clustered and uniform plantings across the high marsh elevation gradient for three years. There was a strong effect of elevation on plant performance and clear signs of plant stress related to soil conditions. Clustering slightly improved 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 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.

Methods
This dataset contains:

Early survival data for species planted in clustered versus uniform treatments across an elevation transect, including:

Distichlis spicata
Extriplex californica
Frankenia salina
Jaumea carnosa
Spergularia macrotheca

Growth and later survival data collected from marked plants at designated "High" and "Low" elevation locations

All species above, plus Limonium californicum and Triglochin concinna at High elevation only

Transplant and exotic cover data collected on point intercept transects across elevation in 2019, 2020, and 2021

See Methods and Appendix S1 for further details on data collection methods and statistical analyses.

The attached archive includes a read-me file that details the content of each .csv in the data set.