Effects of wrack burial in salt-stressed habitats:: Batis maritima in a southwest Atlantic salt marsh

In coastal salt marshes, mars of wrack (dead plant stems) that are deposited on the marsh by high tides can kill underlying vegetation and initiate secondary succession. The importance of wrack disturbance in northwest Atlantic salt marshes has been a topic of recent debate. The importance of wrack disturbance in southwest Atlantic salt marshes, which experience a very different climate regime than do northern marshes, has rarely been examined. Working in a Georgia salt marsh, we documented a pattern of positive association between wrack and Batis,maritima biomass: and conducted experiments that indicated that wrack was beneficial to Batis maritima. Sampling indicated that natural wrack deposition was correlated with areas of vigorous Batis growth and mild soil conditions. Natural wrack deposition occurred primarily at the highest elevations occupied by Batis maritima (the high Batis zone). Batis plants in the high zone were taller: with more and larger leaves. and contained several times the biomass/unit area than Batis plants at lower elevations. High zone soils had lower salinities, better percolation rates, and a lower organic content than did soils from lower elevations. Experimental manipulations demonstrated that deposition of wrack was partially responsible for these patterns. In each of two experiments: soil salinities were lower and plants taller and larger in the presence of wrack compared to in its absence. Although wrack lowered salinities and enhanced plant growth, the effects were not large enough to completely explain the differences between Batis zones. Instead, wrack probably reinforced pre-existing gradients in flooding and salinity caused by differences in elevation and terrestrial runoff. Our results contrast with previous studies from northern marshes. Because of geographic differences in climate and plant phenology, northern marshes are more likely than southern marshes to receive patches of wrack thick enough to kill underlying vegetation. Plants covered by thin layers of wrack, as commonly occurs in southern marshes, may often grow through the wrack rather than suffer mortality. Also because of climatic differences, wrack is more likely to benefit plants by ameliorating salinity stress in southern marshes, where soil salinities are often hypersaline, than in northern marshes where soils are rarely hypersaline. Although ecological processes may differ between northern and southern salt marshes, these differences may be predictable based on an understanding of geographic variation in climate.