Individual study: Revegetation trials in salt-marsh degraded by lesser snow goose Anser caerulescens caerulescens grazing at La Pérouse Bay, Manitoba, Canada
Handa I.T. & Jeffries R.L. (2000) Assisted revegetation trials in degraded salt-marshes. Journal of Applied Ecology, 37, 944-958
Extensive coastal flats at La Pérouse Bay in Manitoba, northern Canada, have been subject to intensive foraging by increasing numbers of lesser snow goose Anser caerulescens caerulescens, with former saltmarsh sward degraded to mudflats largely devoid of vegetation. Revegetation trials were undertaken to determine the ability of two native plant species to revegetate the degraded areas.
Transplanting: Plugs of two dominant graminoids, Puccinellia phryganodes (a sterile grass reproducing by clonal propagation) and the sedge Carex subspathacea (rarely producing seed due to heavy goose grazing) were used. Both species can establish from single leaves and other plant fragments generated by goose grazing.
Areas were selected where intact and degraded patches of vegetation were present. For each species three sites were chosen where patches of remnant sward (mostly 20 m² or less in area) existed. The species transplanted was that dominant in the sward. Four replicated pairs of 1 x 1 m chicken wire exclosures were erected at each site, one on the intact sward and one on a degraded patch (< 5 m distant).
Transplants were cored from a patch of the appropriate species. Each cored plug was 2 cm in diameter x 4 cm in depth, typically consisting of three to five tillers. Each exclosure was planted with 42 plugs (7 columns x 6 rows), 7 cm apart. The 20 plants in the inner matrix were designated experimental plants and scored for growth and senescence, the external ones acted as a buffer zone. Plugs of P.phryganodes were planted on 17-21 June and C.subspathacea on 25-28 June, 1996. Each enclosure in degraded areas was subdivided into four plots, each randomly assigned one of the following treatments:
1) bare sediment (no treatment control);
2) nitrogen and phosphorus fertilizer addition;
3) peat mulch addition;
4) fertilizer and mulch addition.
Fertilizer and mulch application: Fertilizer (inorganic salts) was sprinkled across the plots at the time of transplanting (10.5 g of nitrogen/m² as NH4Cl and 4.45 g phosphorus/m² as NaH2PO4.2H2O). Peat mulch was taken from beneath nearby willow Salix spp. shrubs and was applied in a 5 mm layer by hand.
Monitoring: Growth, senescence and death in the growing seasons of 1996 and 1997, except those in the intact sward (only examined in 1996) was recorded. The basal area of plants was measured. On three dates in 1997, 12 x 4 cm turves were collected from the buffer zone of treatment plots, including the intact sward in three of the four exclosures to measure above ground biomass. Turves were clipped live plant biomass dried and weighed.
A range of soil characteristics were analysed from samples taken in June 1996 and 1997.
Establishment of plants: All P.phryganodes plants transplanted into exclosures containing intact patches survived the first season, whilst some in the degraded soils had died by 29 July 1996. Some transplants did establish in the degraded sediments, and those treated with fertilizer and peat exhibited higher growth than those in bare untreated sediment after the two growing seasons. The ratio of the average basal area of plants in treated plots compared with bare plots was between 1.7 and 4.0:1 at different sites.
C.subspathacea did not establish well in degraded sediments with some dying by 28 July 1996 and subsequently. By the second growing season more than 75% of transplants in the degraded soils had died. No treatment enhanced growth. Establishment of P.phryganodes was greater than C.subspathacea in part because of the former species higher salt tolerance.
Growth rate and mortality of plants both varied between sites and years, reflecting variation in the frequency of hot, dry weather from late June to early August, and the salinity and water content of soils during summer.
Soil characteristics: Soils in degraded sites lacked an organic layer, and had significantly higher bulk densities, clay content, surface temperatures and evaporation rates compared with soils with inact swards.
Conclusion: The extent of soil degradation and vegetation loss makes it unlikely that unassisted revegetation will occur at some sites, even in the absence of goose grazing. However, the removal and transplanting of soil/vegetation plugs has limited application because of labour costs in large-scale habitat restoration projects. It could be used on a smaller scale however to create small diverse patches or islands of vegetation which might facilitate the establishment and spread of plants around them if goose-grazing intensity is reduced.
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