HEAT 



115 



country, the diflFerent zones made a com- 

 plex system. The zones and their supposed 

 temperature limits are bsted in Table 9. 



The transition zone and the two zones 

 south of it were later divided into a dryer 

 western and a moister eastern region at 

 the 20 inch line of equal annual rainfall 

 (isohyet) which, in these latitudes, happens 

 to approximate the 100th meridian. Mer- 

 riam planned to use 6 degrees as the base 

 for temperature summation. Through a mis- 

 understanding, the temperatures were 

 summed with zero Centigrade as a base, 

 although only for those days on which the 

 mean temperature was above 6° C. The 

 values for summed temperature are there- 

 fore much too high, and the agreement be- 

 tween Merriam's isotherms and the bound- 

 aries of his life zones becomes a proof of 

 the incorrectness of his use of temperature 

 in setting up the scheme. Corrected tables 

 have never been published. 



at what purports to be the root of 

 Merriam's life zone concept have actually 

 affected its usefulness very little. This para- 

 doxical situation results from the fact that, 

 from the very beginning, the life zones 

 have been mainly delimited by the ob- 

 served distribution of life-zone indicators 

 rather than by temperatures. The life-zone 

 concept owes its considerable vitality to the 

 soundness of the work of the early natural- 

 ists on which it was really based. The 

 weakness of the false fagade of tempera- 

 ture relations has been a factor in prevent- 

 ing ecologists generally from taking the life 

 zones seriously. Also, their studies of the 

 distribution of biotic communities have 

 shown that, south of the Canadian forests, 

 temperature is only one of a number of 

 environmental factors that regulate bio- 

 tic distribution (Sanderson, 1908; Shreve, 

 1914; Allee, 1923; Kendeigh, 1932; and 

 Shelford, 1932). 



Table 9. The Temperature Values in Degrees Centigrade Assigned to 

 Merriam's Life Zones 



Zone 



Arctic* 



Hudsonian 



Canadian 



Transition 



Upper austral or upper Sonoran 

 TjOwer austral or lower Sonoran . 

 Tropical t 



Northern Limit 



in Accumulated 



Day-Degrees 



5500 



6400 



10,000 



14,500 



Southern Limit; 



Mean Temp, of 



6° C. Hottest 



Summer Weeks 



in Degrees, 



Latitude 



10 

 14 

 18 

 22 



2(i 



* Northward, ground never thaws, 

 f Southward, no freezing. 



This application of temperature "laws" 

 to latitudinal distribution zones overlooks 

 entirely the phenomenon of winter killing 

 and cold hardiness, an oversight that biolo- 

 gists soon emphasized. Further, in hot 

 weather, daily maxima are more important 

 limiting factors than are daily means, and 

 still further, high temperatures may be an 

 effective limiting agent for the distribution 

 of organisms at other times besides the six 

 hottest weeks of summer. 



These inadequacies and others that strike 



Students of the distribution of birds and 

 mammals in the Rocky Mountains and 

 westward in North America continue to 

 find this scheme of hfe zones useful in a 

 descriptive sense. The distribution of verte- 

 brates in the Yosemite region of California 

 is given in Figure 19 in terms of the local 

 life zones. Even in these vertical distribu- 

 tions, the temperature implications of the 

 life-zone system present an oversimplified 

 picture. In northern California, for example, 

 the following environmental factors are 



