Ecosystem functions of plant diversity: Comparisons from a large-scale marsh restoration experiment in California, USA

Although the promotion of biodiversity has been recognized as an important
conservation goal, salt marsh restoration typically focuses on
re-establishing dominant foundation species. Salt marsh restoration
projects that add or remove sediment to optimize marsh elevation often
result in a bare landscape following construction. Restoration managers
must decide whether to plant and, if so, which species. This decision can
be difficult because few studies have examined the ecological functions of
individual species, especially those that are less abundant. Within a
major salt marsh restoration project in Elkhorn Slough, California, where
17,000 plants of five high marsh species were planted, we investigated how
rarer species and the naturally recruiting dominant (Salicornia pacifica)
differ in ecosystem function. We evaluated 31 different metrics related to
blue carbon, plant productivity, environmental effects, and community
interactions. No single species had the greatest ecological function
across this suite of metrics, and measured effects were mainly
independent, with only 16 of 435 pairwise comparisons revealing a strong
correlation. We found significant differences among species for 18
metrics, revealing key contrasts in ecosystem function, with significant
effects of marsh elevation and interaction between effects of species and
elevation on some of these functions. Salicornia pacifica scored highest
for metrics such as recruitment and canopy height but other species
outperformed Salicornia in other metrics. Frankenia salina had the
greatest ecological function in the highest number of metrics, including
cover and belowground biomass carbon content, but other species had higher
rates of photosynthesis and harbored fewer individuals of invasive
arthropods. We recommend planting a suite of less common species at
restoration sites to provide more diverse functions across the landscape.
Frankenia salina in particular is recommended for its tolerance of
hypersalinity and low moisture conditions. Our demonstration of the value
of complementing restoration of the dominant foundation species with
restoration of less common species is applicable to restoration of other
ecosystems beyond salt marshes. The approach we implemented, evaluating a
large suite of functions for multiple species across a restored landscape,
can serve as a model for investigations of the importance of biodiversity
for enhancing multifunctionality in other restored
ecosystems.