(1) the "northern lethal boundary" 

 corresponding to the lowest winter temper- 

 ature at which a species can survive; (2) 

 the "northern growth boundary" correspond- 

 ing to the lowest summer temperature 

 which, over a period of several months, 

 permits sufficient growth for plant 

 maintenance; (3) the "northern reproduc- 

 tive boundary" corresponding to the lowest 

 summer temperature permitting reproduction 

 over a period of several months; and (4-6) 

 the corresponding southern boundaries. 



These boundaries have not all been 

 investigated for Macrocystis pyrifera on 

 the west coast. The lethal southern 

 boundary for adult Macrocystis in 

 California is thought to be where 20°C 

 temperatures persist for at least two 

 weeks (North 1971b). An established 

 population of Macrocystis off San Diego 

 deteriorated during such a period. The 

 southernmost population of Macrocystis off 

 Baja California, Mexico is, however, 

 subject to higher temperatures, and yet 

 has generally persisted. This highlights 

 some of the problems in determining 

 tolerances of individual species. Some 

 populations and some individuals will be 

 more tolerant than others to environmental 

 stresses. Nevertheless, Van den Hoek 

 (1982) found broad correlations between 

 species tolerances along the coasts of the 

 Atlantic Ocean and their distributions 

 along isotherms for those tolerances. 



Because of the variability in 

 community composition among sites, it is 

 clear that testing for the effects of 

 temperature on geographic limits of 

 species can only be done indirectly. Two 

 approaches are possible: (1) laboratory- 

 based studies on tolerances, growth rates, 

 germination and survival of gametophytes 

 and young sporophytes; and (2) transplant 

 experiments of gametophytes and 

 sporophytes into similar habitats at 

 localities with different temperature 

 regimes. 



The main problems confronted in 

 comparing geographic localities are that 

 there are usually large differences in 

 biotic factors such as the presence of 

 other algal species and grazers (see 

 Chapters 3 and 4), and also that 

 temperature, nutrients, and light are 

 often correlated (Jackson 1977). It is 



difficult to separate these factors in the 

 field, but it is relatively 

 straightforward to measure them. Dean et 

 al. (1983) and Dean and Deysher (1983) 

 made continuous measurements of 

 temperature and irradiance in the San 

 Onofre kelp forest in southern California. 

 They found that temperature and irradiance 

 were positively correlated (Figure 27). 

 Successful natural recruitment of 



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 Required For Sporoohyte 

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JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC JAN 



Figure 27. Temperature and irradiance 

 recorded on the bottom in a Macrocystis 

 forest over a 4-year period. Lab and 

 field studies indicate that Macrocystis 

 will successfully recruit only when both 

 temperature and irradiance are above the 

 solid line. Note that temperature de- 

 creases going up the vertical scale (from 

 Dean et al. 1983). 



92 



