Individual study: An assessment of transplant success of three salt marsh plants (Juncus maritimus, Leptocarpus similis and Schoenoplectus pungens) into tidal wetlands near Christchurch, New Zealand
Thomsen D., Marsden I.D. & Sparrow A.D. (2005) A field experiment to assess the transplant success of salt marsh plants into tidal wetlands. Wetlands Ecology and Management, 13, 489-497
A proposed new saltmarsh creation site was located at the Linwood paddock area near Christchurch (New Zealand), formerly sand dunes but reclaimed for pasture by the addition of treated biosolids from sewage and industrial waste. To assess if the soil created from these wastes was suitable for the restoration scheme, a field experiment was undertaken to compare the transplant success of three species of saltmarsh plants in plots containing natural estuarine mud with those planted in soil from this proposed new wetland site. Three common, native perennial middle marsh species, Juncus maritimus, Leptocarpus similes and Schoenoplectus pungens occuring in nearby marshes were selected as the test plants. Natural stock and nursery-reared stock derivedfrom local seeds, were used. A split-block design was established in unvegetated areas within a nearby existing J.maritimus dominated salt marsh to compare the effects of soil type (natural estuarine mud or reclamation soil) and plant source (field collected stock or nursery stock) on plant survival and growth.
Study area: The study area is within the Avon-Heathcote Estuary (43º31'S, 172º43'E) close to Christchurch on the east coast of South Island, New Zealand. Field plots were established in March 1998 within an area of mature salt marsh 1.5 km south of the proposed saltmarsh creation area. The marsh has a low species diversity comprising mostly J.maritimus, with Australian samphire Sarcocornia quinqueflora at higher elevations. There were suitable gaps amongst the vegetation to allow for experimental transplanting without disturbing existing plants.
Plant stocks: Natural field stock was obtained from a natural salt marsh 3 km north of the experimental area (also a natural saltmarsh) with similar physical conditions and tidal inundation patterns. Nursery-reared stock (grown from seed collected from the former site) was obtained from the Christchurch City Council nursery.
Experimental design: A split-block design was used to investigate the effects of soil type (natural estuarine mud or restoration area soil), plant species (J.maritimus, Leptocarpus similes and Schoenoplectus pungens) and plant stock (field collected or nursery stock).
The design consisted of 36 plots (0.5 m x 0.5 m) spanning a topographical gradient from above the existing river channel to the high tide mark, and contained 144 individual plants.
At the beginning of March 1998, the plots were excavated to 15 cm. One half of the plots were filled with sieved reclamation topsoil and the other with estuarine mud. Both substrates had similar concentrations of phosphates, nitrates and trace metals. All plots were compacted by foot to obtain a surface elevation similar to the surrounding marsh and to reduce sediment loss through tidal wash.
One week later, each plot was planted with four individuals of the same species, two from natural stock and two from nursery stock. Plants were trimmed to 20 cm height after planting. A small sample of naturally growing plants was also trimmed to this height to test for any effects of trimming. Every 2 weeks from March to December 1988, the plots were checked and any tidal debris or seaweed removed to minimize this potential cause of mortality.
Environmental conditions: At time of planting and at the end of each month until December 1998, pH and salinity were measured in surface soil samples (0–10 cm depth) from each plot. The particle size distribution and organic content of the two soil types were also determined at the time of planting.
Plant survival, condition and growth: Survivorship was measured in April one month after planting, and in May, August and December. At the end of August 1998 (4 months after planting) just prior to spring growth, the height of all transplants was measured, and again at the end of December 1988. To avoid destructive sampling (i.e. pulling up plants) indirect estimates of standing biomass were calculated.
All individuals of all three species appeared healthy one month after planting. However by the end of May, S.pungens appeared to have died back in the usual seasonal manner, but it failed to regenerate in the following spring. All J.maritimus and L.similis individuals were alive in May, 2 months after planting. In August, all L.similis and 87.5% of J.maritimus plants were alive. From September onwards most plants of both species appeared stressed (chlorotic wilting shoots). Despite this, in December (nine months after transplanting), L.similes survival was still 100%, whilst survival of J.maritimus had dropped to 73%. It was also apparent that nursery-sourced J.maritimus had a higher mortality than plants obtained from natural stocks, and that natural stocks had significantly higher biomass than nursery stocks.
Conclusions: Results from this study suggest that L.similis and to a lesser extent J. maritimus would probably establish and grow in soil formed from the treated sewage biosolids at the proposed saltmarsh creation site. Transplant success might be enhanced by sourcing natural stock as opposed to using nursery-reared stock, and management regimes may be necessary to reduce salinity extremes.
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