Individual study: Accelerating the restoration of a salt marsh through soil amendments, planting and creek excavation at Tijuana River National Estuarine Research Reserve, California, USA
O’Brien E.L., & Zedler J.B. (2006) Accelerating the restoration of vegetation in a southern California salt marsh. Wetlands Ecology and Management, 14, 269-286
Southern California salt marshes have been reduced to less than 10% of their historical area due to urban development and agriculture. Area continues to decline at Tijuana Estuary as sediments elevate intertidal areas more rapidly than compensated for by sea level rises. Restoration thus necessitates excavation, which creates large, bare surfaces that have proven difficult to vegetate (Zedler et al. 2003). Our objectives were to determine if ecosystem functions could be restored effectively by adding tidal creeks, amending soils, and varying halophyte transplant spacing. In addition, we sought to demonstrate the value of large-scale experimentation in a restoration site. Effects of three planned treatments were determined for halophyte plantings (herein), plus additional responses of sedimentation patterns and use by invertebrates and fish (Larkin et al. in press).
Study site: Tijuana River National Estuarine Research Reserve (Tijuana Estuary) is located in southwestern San Diego County (32º34'N, 117º7'W), California, southwest USA. Due to the dry climate, the estuary receives freshwater only during large winter rainfall events. Sedimentation events occur in years with heavy streamflow.
Experimental design: Between October 1999 and February 2000, the top 2 m of soil were removed from an 8-ha kidney-shaped restoration site to expose the historical marsh plain. A channel dredged along the north edge provided tidal input to six conceptually divided, appriximately1-ha cells. Every other cell was further excavated to include a tidal creek network, patterned after natural network. Tidal creeks were connected to tidal action in February 2000 after soil amendments were rototilled into experimental plots and native halophytes were planted.
Each cell (with or without tidal creeks) had 18 experimental plots on the marsh plain, each of which measured 1.6 x 1.4 m. Plots were located both near and far from the cell centre and randomly assigned each a soil treatment and a cluster spacing. The total was 108 plots (3 soil treatments x 2 distances from cell center x 3 cluster spacings = 18 per cell x 6 cells).
Treatments: The three field treatments in a site that was excavated to reinstate tidal flows and restore salt marsh were: i) adding tidal creeks; ii) planting seedlings in tight clusters; and iii) rototilling kelp compost into the soil.
Five species of native halophytes (three perennial forbs: marsh jaumea Jaumea carnosa, California sea lavender Limonium californicum and saltwort Batis maritima; a perennial sub-shrub: alkali heath Frankenia salina; and the short-lived seablite Suaeda esteroa) were planted near (5 m) and far (12 m) from tidal creeks, in clusters at distances of 10, 30 and 90 cm, and in plots with and without kelp compost, as well as a rototilled control.
The magnitude of responses was the reverse of expectations, with tidal creeks having the least effect and kelp compost the most. In cordgrass Spartina foliosa plots, kelp compost did not affect soil organic matter, but plants were taller (by about 11 cm) and denser (47% more stems). On the marsh plain, kelp compost significantly increased soil organic matter (by 17% at 0-5 cm; and 11.5% at 5-20 cm), total Kjeldahl nitrogen (45% at 5-8 cm; p < 0.001) and inorganic nitrogen (35% at 5-8 cm), and decreased bulk density (16% at 0-5 cm and 21% at 5-8 cm depth) compared to controls. Survivorship of kelp compost-treated plantings increased, along with growth (>50% increase in a growth index at 20 months after planting).
Planting seedlings in tight clusters (10 cm apart) on the marsh plain increased survivorship by 18% (compared to in 90 cm loose clusters), but did not increase growth rates.
Tidal creek networks increased Batis maritima and Jaumea carnosa survivorship by > 20%. Kelp compost had a strong, positive influence on vegetation establishment by ameliorating some of the abiotic stress.
Simultaneous studies documented positive effects of tidal creeks on sediment removal and fish access to the marsh plain, as well as important functions of tidal pools, which formed with wave-generated resuspension of sediments (Wallace et al. 2005, Larkin et al. in press).
Restoration implications: Results suggest that in this region, species-rich salt marsh vegetation can be enhanced by: i) introducing native species that recruit slowly (all except Salicornia virginica); ii) amending soils that are low in organic matter or exposed to intermittent drying; iii) planting seedlings in tight clusters, e.g. 10-cm spacing near tidal creeks, and iv) based on long-term evaluations of the site , planting seedlings in large bare areas in order to reduce chances of the planting site becoming dominated by Salicornia virginica.
Follow-up research indicated that shallow depressions (~5 cm) on the marsh plain reduced dominance by Salicornia virginica and allowed a rare annual, S.bigelovii, to persist at the site (A.Varty & J.Zedler, in review).
Larkin D.J., Madon S.P., West J.M. & Zedler J.B. In press. Topographic heterogeneity influences fish use of an experimentally-restored tidal marsh. Ecological Applications
Wallace K.J., Callaway J.C. & Zedler J.B. (2005). Evolution of tidal creek networks in a high sedimentation environment: A 5-year experiment at Tijuana Estuary, California. Estuaries, 28, 795-811.
Zedler J.B., Morzaria-Luna H.N. & Ward K. (2003). The challenge of restoring vegetation on tidal, hypersaline substrates. Plant and Soil, 253, 259-273.
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