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.