z 

 < 



O 



CO 



=> 

 z 



1000 



100 



10 - 



\ 



\ 



\ 



V. 



15 



25 



(1969) 



(1970) (1971) 



AGE IN MONTHS 



35 

 (I972) 



Figure 15. A survivorship curve for 

 individually-tagged Macrocystis pyrifera 

 plants that recruited to a kelp forest off 

 Del Mar, California in September 1969. 

 The original cohort was 387 plants (re- 

 drawn from Rosenthal et al. 1974). 



population at a depth of 15 m in the Point 

 Loma kelp forest. They found that only 

 19% of plants survived the first three 

 months after recruitment, and 2% after 

 nine months. Some plants survived to an 

 age of 7 years. The life span of fronds, 

 however, is only about 6 months (North 

 1971b). 



Kain (1982) compared the short-term 

 growth rates of fronds of Macrocystis 

 pyrifera from three sites in southern New 

 Zealand with those at a site in southern 

 California. The relative growth rates of 

 stipes and laminae from the different 

 populations were similar. By a series of 

 morphometric measurements, however, Kain 

 (1982) determined that plants from the 

 most exposed site in New Zealand more 

 closely resembled those in California than 

 those in the other populations in New 

 Zealand. 



As Macrocystis plants grow through 

 the water column, they have fewer shading 

 interactions with progressively fewer 

 species. Pearse and Hines (1979) found, 

 for example, that many species of large 

 brown algae recruited into an area near 

 Santa Cruz, California recently cleared of 

 sea urchins. Macrocystis recruited at 



4-5/m 2 , Laminaria setchel 1 i i at 4-12/m 2 , 

 Pterygophora californica at 4-6/m 2 , 

 Nereocystis at 1-2/m 2 , and Cystoseira at 

 1/m 2 . Macrocystis eventually grew to the 

 surface, forming a canopy, while the 

 species that were shaded below declined in 

 abundance. 



If effects among species decrease in 

 importance once plants reach the surface 

 canopy, intraspecific events must assume 

 more importance. Dense stands, about 4 

 plants/10 m 2 , provide a "forest" effect, 

 with deep shading beneath the canopy (see 

 cover photo). Darwin (1860) remarked on 

 the calming effect which dense stands of 

 Macrocystis have on turbulent inshore 

 waters. This reduced flow can also affect 

 carbon assimilation and nutrient uptake 

 (see Section 2.6). At the other extreme 

 of density, solitary plants do not usually 

 fare well. They may be ravaged by 

 herbivorous fishes, especially halfmoon 

 and opaleye, which appear to be attracted 

 in numbers to isolated plants (North and 

 Hubbs 1968, LOSL 1983). Because the frond 

 meristem is at the tip, it is easily 

 damaged by fish grazing, and once 

 destroyed, frond growth stops. These 

 plants may also be more susceptible to the 

 effects of severe water motion without the 

 dampening effect of nearby plants. 

 Between these extremes, the effects of 

 density on Macrocystis are equivocal. 

 North's (1971b) observation of plants at 

 three densities showed that those in the 

 densest stands (some 15 stipes/m 2 , and 7 

 stipes/m 2 ) could grow faster than those at 

 1/m 2 . He attributed this difference to 

 some unknown localized factor affecting 

 growth. Neushul and Harger (in press) 

 planted adult Macrocystis plants at 

 different densities and found that, over a 

 period of one year, the number of fronds 

 per plant increased for plants growing at 

 low density, stayed about the same for 

 those at the medium density, and decreased 

 for those at high density. These 

 differences were also reflected in the 

 weights of plants, with those at the 

 highest density weighing the least (see 

 also Section 5.5.2). 



The effects of density on algal 

 growth are far from resolved, however, and 

 may be of some importance to the dynamic 

 relationships of plants in kelp 

 communities (c.f. Schiel and Choat 1980). 



47 



