relating to the effects of one laminarian 

 species (or groups of them) on another. 

 His experiments indicated that the species 

 forming a surface canopy, Alaria 

 f istulosa , was not a competitive dominant 

 because other laminarian species did not 

 invade clearances when this canopy was 

 removed. Canopies of Laminaria spp. , 

 however, appeared to preclude the 

 recruitment of Alaria . The rhizoidal 

 growth pattern of I. longipes ensured that 

 relatively rapid vegetative growth would 

 allow this species to reform a canopy 

 quickly, and so exclude other species from 

 invasion. Two other canopy removals 

 indicated that Agarum cribosum had little 

 effect on other species, but that its 

 cover increased when Laminaria spp. were 

 removed. 



These experiments clearly show that 

 canopies of individual species affect 

 other species. The nature of these 

 effects, however, is not clear. 

 Presumably, canopy effects were the result 

 of light inhibition to understory species 

 (see Section 2.4). Canopies may also 

 impede the arrival of propagules to the 

 substratum. One of the main problems, 

 however, was the lack of information about 

 reproduction for individual species. The 

 apparent failure of some species to 

 recruit successfully and the success of 

 other species could be a reflection of the 

 seasonal occurrence of propagules, which 

 may not have been available for some 

 species at the time of the clearances. In 

 addition, percentage cover of canopies was 

 used as a measure of abundance, and it was 

 not clear whether plants that remained 

 after clearances of one species increased 

 in size, or whether significant 

 recruitment occurred. 



Many intertidal studies have shown 

 that free space is a major requirement for 

 successful recruitment of sessile species 

 (e.g., Dayton 1971, Connell 1975). This 

 is probably the case for subtidal regions 

 as well, although there is little 

 experimental evidence for this in 

 Macrocystis forests. Sessile organisms, 

 particularly algae such as articulated 

 corallines, fleshy red algae and stipitate 

 laminarians, may impede spore fall from 

 larger plants, may already occupy 

 available substrata, may have toxic 



effects on settling spores, and may reduce 

 light levels below threshold values for 

 spore development. 



Field experiments by Reed and Foster 

 (1984) have shown that a combination of 

 these effects can be important to kelp 

 recruitment in central California. They 

 experimentally removed canopies of over- 

 and understory plants: Macrocysti s , 

 Pterygophora , articulated corallines, and 

 encrusting corallines. The most abundant 

 understory kelp in the forest, 

 Pterygophora , reduced light levels to the 

 substratum by up to 90%. Removal of this 

 species resulted in a higher recruitment 

 of kelp relative to control areas. 

 Clearance of articulated and encrusting 

 corallines, leaving patches of bare 

 substratum, did not yield more recruits. 

 Lower recruitment occurred in areas where 

 the branches of articulated corallines 

 were present, suggesting that the branches 

 themselves impede spore fall and 

 subsequent development, or that they may 

 severely reduce irradiance to the 

 substratum. Reed and Foster (1984) 

 concluded that the major factor affecting 

 recruitment was the reduction of 

 irradiance caused by the Pterygophora 

 canopy. 



This is the most thorough set of 

 experiments yet published involving 

 manipulations of algal abundances in 

 Macrocystis forests. Most combinations of 

 factors (i.e., the presence and absence of 

 each canopy) were used. A balanced 

 experimental design, however, would have 

 allowed statistical tests of all factors 

 and their interactions, thus providing 

 stronger evidence for their conclusions. 



Pearse and Hines (1979) cleared a 

 Macrocystis canopy from a 20 x 10 m plot 

 and left one area uncleared as a control 

 in a kelp forest off Santa Cruz, 

 California. After 3 months, significantly 

 more laminarians, of several species, 

 recruited into the cleared plot. This 

 difference was attributed to light 

 inhibition caused by the intact 

 Macrocysti s canopy. Light measurements 

 indicated that only about 0.2% of surface 

 light reached the bottom under the canopy, 

 while 3.8% reached the bottom in the 

 cleared plot. The long-term effects of 

 this differential recruitment were not 

 recorded. 



98 



