Large‐scale field studies inform adaptive management of California wetland restoration

Abstract

Salt marsh vegetation provides the structure that supports key wetland functions and the success of wetland restoration efforts depend on the establishment of vegetation, which can take decades. Field experiments aimed at identifying the factors that limit the colonization rate of marsh vegetation can be useful for guiding restoration efforts. We used field experiments and additional measurements to determine the causes for persistently low vegetation cover in a restored salt marsh in southern California. First, we characterized the soil properties of the marsh and found that high salinity and low soil moisture increased with elevation and improved with vegetation. Using this information, we conducted two field experiments-one in the high marsh and the other in the mid marsh to determine the effectiveness of different remedial actions (e.g. irrigation, soil decompaction, soil amendments, and planting and seeding different species) on increasing vegetation cover; the mid marsh experiment evaluated planting and seeding only. In the high marsh, plantings increased cover, and irrigation in combination with soil decompaction facilitated natural plant establishment. In the mid marsh, increases in vegetation cover varied by species planted. In both experiments, the marsh dominant (Salicornia pacifica) naturally recruited, increasing cover whereas seeding did not increase cover. In general, the magnitude and pace of vegetation growth was greater in the mid marsh relative to the high marsh that received less tidal inundation. Collectively, our results show how information gained from experiments conducted during wetland restoration can inform the processes underlying the establishment of vegetation.

Researcher effects on the biological structure and edaphic conditions of field sites and implications for management

“…Ecological memory, like other ecosystem attributes, develops through time; thus, older constructed and restored ecosystems should have greater resistance and resilience to disturbance than younger constructed and restored ecosystems (Johnstone et al, 2016). Our results support this assumption; researcher effects were less severe in the Alabama constructed marsh, which is twice as old as the California constructed marsh (Figure 1; CA‐CON = 17 years, AL‐CON = 34; Beheshti et al, 2022, 2023; Vittor et al, 1987). However, even after three decades, AL‐CON has not developed the resiliency necessary to recover from researcher direct and indirect effects, which are still evident >2 years after the initial disturbance (Rinehart and Dybiec, personal observation).…”

“…Our natural site was Kendall-Frost Marsh (CA-NAT; 32 47041.0 00 N, 117 13046.4 00 W) and our constructed site was San Dieguito Lagoon (CA-CON; 32 58047.0 00 N, 117 14043.6 00 W). CA-CON was initially constructed in 2006 and planted with Pacific cordgrass (Spartina foliosa) and pickleweed (Sarcocornia pacifica) in 2008, with subsequent grading and planting activities occurring from 2009 to 2022 (Beheshti et al, 2022(Beheshti et al, , 2023. At both sites, Pacific cordgrass dominates low-elevation habitat, pickleweed dominates high-elevation habitat, and mixtures of these species occur at intermediate elevations (Walker et al, 2022;Walker, Rinehart, et al, 2021).…”

Testing strategies to enhance transplant success under stressful conditions at a tidal marsh restoration project

“…Thus, it is plausible that the dredged sediment resulted in a more stressful environment. Although the bulk density was not significantly different across our sites, the average bulk density of our site (1.77 g/cm 3 ) was significantly higher than many other published bulk density values published for similar systems (O'Brien & Zedler 2006;Beheshti et al 2023). This suggests that a common stressor in these bare areas may be soil compaction and warrants future investigation of decompacting measures, either through soil amendments or mechanical disturbance (Beheshti et al 2023).…”

“…While we did not test non-salt hardened plants in our irrigation study, the lack of irrigation proved to be impossible for plants to overcome, with 100% mortality in just under 3 months. Targeted irrigation has historically aided restoration efforts in high tidal marshes (Beheshti et al 2023) and other stressful habitats: deserts (Abella et al 2015), mine exploration areas (Elliott & Turner 2021), seasonally dry oak forests (Badano et al 2009), etc. Based on the significant differences in performance with watering and the higher moisture at the west site, our plant performance may be limited by available moisture, or a stressor ameliorated with increased moisture, such as salinity.…”

“…Neighborhood effects and clustering designs have been examined for both interspecific and intraspecific facilitation between plants (Silliman et al 2015;Duggan-Edwards et al 2020;Shaw et al 2020), showing clustering can improve performance in stressful areas. Actively ameliorating localized stress with other actions like short-term irrigation has shown mixed success in facilitating the growth of transplanted plants (Porensky et al 2014;Xie et al 2019;Beheshti et al 2023). As an alternative to ameliorating stress, practitioners can also utilize plants that have a higher tolerance for stress, targeting specific species (Armitage et al 2006) or larger planting container sizes (Herriman et al 2016).…”