Factors restricting recruitment of Ascophyllum nodosum L (Le Jolis)

Ascophyllum nodosum dominates a wide zone on the shore. Juveniles occur high in the zone where canopy is continuous and grazing animals few. Settlement could account for the restricted range if it were enhanced under canopy or on the upper shore. Settlement was monitored under canopy and in the open at four shore levels during spring 1983. Propagules settled everywhere. Settlement does not restrict plant establishment. Germlings caged and not caged were placed under canopy and in the open through the zone to evaluate the effects of animals and the physical environment (shore level x frond cover) on survival and growth. Animal impact overwhelmed environmental effects both summers (1982, 1983). Impact increased down the shore. For caged germlings, survival was dependent upon canopy high in the zone. Survival was greatest ( +2.0 m in the understory) where growth was slowest. Growth increased 5 to 10 times down the shore, doubling in the open. A size-based exclusion experiment indicated that larger animals were most effective. Individual snails were caged with germlings and only Littorina littorea had significant effects. Smaller snails (\u3c0.6 cm) removed fewer germlings. When impact was related to size, Littorina obtusata and Acmaea testudinalis were effective but of secondary importance. Winter may afford a seasonal lapse in impact on the upper shore. Animals were less abundant in winter (1983--4), though larger L. littorea did not decline significantly. Caging experiments demonstrated that animal impact was important during the winter. Ascophyllum shares key features with climax tree species. Ascophyllum germlings are at a selective advantage in the understory. Suppression of growth in Fucus vesiculosus and Ascophyllum was significantly different. Ascophyllum suppressed in the understory high on the shore were transplanted low on the shore. Growth increased significantly. Suppressed juveniles are critical in regenerating lost cover, a \u27juvenile plant bank.\u27 Ascophyllum juveniles of intermediate sizes grow under Ascophyllum canopy but were more numerous under Fucus vesiculosus, poised for replacement