118 CLIMAX FORMATIONS OF WESTERN NORTH AMERICA. 



"The map makes an acceptable working basis for outlining the vegetation 

 of the North American continent and remains still the best climatic chart that 

 has been published on forest and prairie distribution." 



It will suffice to point out that no climatic chart, no matter how accurate, 

 can hope to outline the vegetation of North America. The formations and 

 associations can never be outlined except as a result of painstaking recon- 

 naisance and survey, after which alone will it be possible to determine the 

 coincidence or correlation of the lines showing climatic factors or ratios. The 

 very general relation of the 60 per cent line of Transeau to the one-hundredth 

 meridian and the course of the upper Missouri River has led to the feeling 

 that this is the most important line climatically and vegetationally in North 

 America. It would seem that even the existence and location of this line must 

 be regarded as purely tentative at the present time. As to its vegetational 

 value, it can be stated unreservedly, after crossing and recrossing it repeatedly 

 from Saskatchewan to Texas during the past six years, that it does not exist. 

 While there can be no question of the interest and stimulus to be derived from 

 "trying on" all sorts of climatic correlations, this is certain to be unfortunate 

 if it does not lead to the conviction that causal relations between vegetation 

 and climate can only be discovered after we know exactly where plant com- 

 munities are and what they are doing. With this must also go a realization 

 of the fact that climax climates necessarily fall into subclimaxes, that a 

 climate may vary greatly and inconsistently within itself, that the variations 

 of one climate during a climatic cycle may be greater than the difference 

 between contiguous climates, and, finally, that it is the critical phases of a 

 climate which count most, and not its averages or sums. It must be more 

 fully understood that the growing season is the critical time for the vast 

 majority of species, and that some parts of this are more critical than others. 

 Furthermore, we must make more adequate use of our knowledge that plants 

 stand better conditions much more complacently than they do worse ones 

 (fig. 3). 



Relationship of associations. — The associations of the formation exhibit 

 relationships which may be considered from various angles. Geographically, 

 they are grouped in the Great Plains with a narrow interrupted band stretch- 

 ing across the north to the Palouse region of Washington, Idaho, and Oregon, 

 and a broader one at the south, reaching through New Mexico and Arizona 

 almost to California. Both of these connect the bunch-grass association with 

 the Great Plains mass. Climatically, the Andropogon subclimax is the wettest, 

 with a rainfall of 30 to 40 inches, largely as summer rain, and the short-grass 

 and the bunch-grass associations driest, with 10 to 20 inches. In the hotter 

 regions, where evaporation is great, such as Texas and California, the efficiency 

 of an inch of rainfall is naturally less. These are merely general correlations 

 which apply to the mass and not to its limits. In view of the ecological 

 requirements of grasses, the most suggestive correlation is with the line of 

 70 per cent of the annual precipitation occurring between April 1 and Septem- 

 ber 30 (fig. 4). With the exception of the winter rainfall region of the Pacific 

 Coast, the general agreement as to limits is good. There appears to be no 

 evident correlation as to temperature or altitude, as is well illustrated by the 

 range of Bouteloua gracilis from Mexico to Saskatchewan, and from 3,000 to 

 9,000 feet. 



