Temporal trends in tidal marsh restoration at Hester Marsh, Elkhorn Slough, California

Understanding restoration trajectories and their sensitivity to climate is
critical for designing effective adaptation strategies for restoration
projects. Tidal marsh restoration often involves initial bare earth
conditions that may be stressful to colonizing plants, especially on high
elevation marsh platforms built to be sea-level resilient. Under these
circumstances, stressors such as soil salinity may increase over time, but
can be mitigated by strong rainfall. At Hester Marsh, a large tidal marsh
restoration site in Elkhorn Slough, California, we evaluated passive
restoration success, tracking colonization by plants whose seeds arrived
naturally on tides, and active restoration success, monitoring
greenhouse-grown transplants. Our investigation revealed non-linear
restoration trajectories with high climate sensitivity, at the scale of
the entire landscape and of individual plants. We found strong effects of
drought on marsh restoration success indicators. Restoration success
decreased dramatically in the first area to be completed, which
experienced drought conditions following construction. In contrast,
restoration success declined more slowly in the second area, which had
consecutive rainy years following construction. Both passive and active
restoration showed strong differences across these areas and across dry
and rainy years. Facilitation can sometimes improve conditions for later
arriving plants, but we found higher mortality of seedlings under existing
vegetation than in bare areas. Thus, plant colonization may slow over time
both due to increasing abiotic stress and through competition by early
colonizers. Our findings lead to concrete recommendations for climate
adaptation strategies for tidal marsh restoration. Since we found that the
first year following construction appeared to have the least stressful
conditions, we recommend managers invest especially heavily in supporting
plant colonization through passive and active restoration during this
early window of opportunity. We also found plant size and species affected
drought tolerance and recommend larger plant sizes and hardy species be
incorporated into active tidal marsh restoration. Furthermore, we
recommend planning for phased completion of restoration projects to
generate a mosaic of areas with different trajectories and increase the
probability that some areas will be completed during optimal climate
conditions. We thus illustrate how an understanding of climate sensitivity
of restoration trajectories can enhance restoration success.