DISTURBANCE OF SALT-MARSH VEGETATION BY WRACK MATS IN GREAT-SIPPEWISSETT-MARSH

To quantify disturbance to salt marsh vegetation, and to test the noti on that disturbance and species richness are related, we studied distu rbance of vegetation by 195 wrack mats that had become stranded over G reat Sippewissett Marsh. The mats varied in area, thickness, residence time, and elevation of stranding. Mats that were large, that had resi dence times of 3-4 months, and were stranded at higher elevations prod uced the most damage, ranging from total eradication to a decrease in shoot density of underlying vegatation. Thickness had no effect. About 70% of mats caused no damage, and of those that did, damage was usual ly 50% or less of the area beneath the mat. Plant species were differe ntially distributed along the tidal excursion within the marsh, and th ere were more species (8-9) high in the intertidal range than lower (4 -6 species). Vertical distribution of species, species richness, and r ate of annual disturbance were not well correlated, although highest a nd lowest disturbance rates corresponded to highest and lowest species richness. Most of the increases in species richness, however, occurre d over a very small range of disturbance, so narrow that it seemed unl ikely that disturbance affected richness. It seems more likely that fa ctors other than disturbance rate control vertical position of species in salt marshes. It also appears that the effect of disturbance by wr ack is less important than has been claimed.