Initiation and maintenance of vegetation mosaics in an Arctic salt marsh

1 Vegetation mosaics of grassland and exposed soil are often the outcome of intense herbivory and represent alternate stable states, although the vegetational and soil processes that both initiate and maintain the states in response to herbivory are poorly understood. 2 The vegetation mosaic resulting from grubbing by geese was examined on the intertidal flats at La Pérouse Bay, Manitoba. Soil properties of the alternate states were measured in conjunction with the ability of transplanted tillers of Puccinellia phryganodes to grow in exposed soil and intact swards. Simulated grubbed patches (2.5–40 cm diameter) were prepared at three sites and the soil properties and inward clonal growth of the grass in each patch were examined during two growing seasons. 3 Where an organic layer was still present on exposed areas, soil properties were similar to those in the rooting zone beneath intact swards, except that soils were hyper-rather than hyposaline. Older exposed mineral soils, where the organic layer had been eroded, were similarly hypersaline, but were also low in soil moisture and total nitrogen, and compacted with a low infiltration rate. Transplanted tillers survived in intact swards, but few survived in any hypersaline soils. 4 Changes in soil properties over two seasons depended on size of experimentally created patches. Again, high salinities developed in soils of the larger patches, and there was a loss of nitrogen and soil moisture and an increase in bulk density with increasing patch size. Although inward clonal growth of P. phryganodes took place from plot perimeters, this is likely to be curtailed by high salinities in plots over 20 cm diameter. 5 As in tropical Africa, intense herbivory can lead to an alternate stable state in this Arctic salt marsh. Small patches of exposed soil are characterized by deteriorating soil conditions and the inability of vegetation to re-establish. As patches coalesce with continued herbivory, an ecological sere (the intertidal salt marsh) is lost.