Frankenia salina and Jaumea carnosa performance in a restored salt marsh

The Stress Gradient Hypothesis (SGH) predicts facilitation will become
more important than competition where abiotic stress is high, and the
framework successfully predicts positive interactions between species in
many systems. Fewer studies have focused on intraspecific facilitation,
and to our knowledge none examine intraspecific interactions in Pacific
Coast salt marshes of North America. We used two species that tend to
occur in large, monospecific patches to test for intraspecific
facilitation in a restored California marsh, where tides and evaporation
during summer create moisture and salinity gradients across elevation. We
tested performance of Frankenia salina or Jaumea carnosa in restoration
plots using two treatments: clustered plantings to promote facilitation,
and widely spaced plantings to limit interaction between individuals. We
characterized the soil water potential gradient and measured plant
survival, cover, physiology, and susceptibility to herbivory across
elevation and planting treatments. Soil water potential declined sharply
with elevation, suggesting plant stress should be higher upslope. However,
tissue water potential was unaffected by elevation, and survival was high
– suggesting growing conditions remained benign. A seawater addition
treatment did not alter the response of plants or plant interactions to
the abiotic gradient. At nineteen months, intraspecific competition
prevailed across the entire elevation gradient for both species, with less
transplant cover in clustered plantings. However, clustered Frankenia
fared better during a transient period of heavy rabbit herbivory. Aside
from this temporary benefit, clustering did not reduce stress and strongly
suppressed growth – suggesting that restoration designs for the high marsh
should minimize competition.