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The National Geographic Magazine 



a lower limit to the distribution of trees 

 do not make their influence conspicuous 

 over such a wide extent of territory as 

 those which determine the similar upper 

 boundar}^, yet when they are in control 

 the line established is strongly pro- 

 nounced and frequently passes through 

 an inhabited region and is of decided 

 economic importance. This last con- 

 sideration does not apply to the line de- 

 fining the upper limit of forests. Then, 

 too, the lower limit of the forests in arid 

 regions is also the border of treeless 

 valleys, prairies, and deserts, and hence 

 the most important of all the natural 

 boundaries as expressed in the flora and 

 fauna of arid and semi-humid countries. 

 These statements do not seem to be open 

 to objection, and, as may reasonably be 

 claimed, indicate the importance of the 

 boundary referred to and the desira- 

 bility of a brief and convenient method 

 of designating it. The boundary in 

 question is of the same general nature 

 as the line defining the upper limit of 

 tree growth — that is, it is a line across 

 which trees do not pass, and must either 

 be designated in a corresponding man- 

 ner or be given an entirely new name. 

 Of these two alternatives the first cer- 

 tainly seems the more logical. By using 

 timberline as a generic term, as many 

 species of the ' ' genus ' ' can be recog- 

 nized as the conditions warrant. By 

 doing this we are not detracting from 

 the ' ' fixed and definite value ' ' of the 

 generic word, but adding to the precision 

 with which it may be employed and 

 enlarging its usefulness. 



In criticism of the term ' ' dry timber- 

 line," Dr Merriam states "that the 

 lower limit of tree growth in many parts 

 of the West is determined by tempera- 

 ture rather than aridity, though in some 

 cases aridity is the controlling factor." 

 If temperature — i. e. , too great a degree 

 of heat — is sufficiently important in this 

 connection to be frequently mentioned 

 or discussed, it would indicate the de- 

 sirability of recognizing a ' ' hot timber- 



line, ' ' but I do not understand that such 

 is the case. In proposing the term ' ' dry 

 timberline," I sought to express in the 

 name the prevailing and most important 

 reason for the sharp demarcation of for- 

 ests at their lower limit in arid regions. 

 Other factors, as is well known, enter 

 into the problem and locally assume con- 

 trol; such as soil texture, hot winds, 

 fires, etc., but the prevailing and domi- 

 nant condition governing the advance of 

 forests into arid regions, to which all 

 others must be considered as secondary, 

 is dearth of soil moisture due to aridity 

 of climate. In this connection it is in- 

 structive to note that timberline, in the 

 narrow sense to which Dr Merriam 

 would restrict the use of the word, is, to 

 use his own definition, "the upper or 

 boreal limit of tree growth as determined 

 by temperature. " Is this more exact 

 than referring the lower limit of tree 

 growth to aridity ? There are other fac- 

 tors besides temperature, as observation 

 has shown, which are effective in deter- 

 mining the upper limit of forests, such 

 as the severity of winter storms (a very 

 important and frequently controlling 

 factor), slope of surface, degree of ex- 

 posure to the sun, depth of snow, etc. , 

 each of which in certain instances lo- 

 cally determines the position of the 

 boundary referred to. The most com- 

 mon and the dominant cause, however, 

 to which the others are secondary or 

 local, is a low mean annual tempera- 

 ture. Taking the dominant cause in 

 each instance for the specific designa- 

 tion, we have the terms dry timberline and 

 cold timberline. In a similar manner the 

 use of the term wet timberline might be 

 justified. 



This discussion, however, is not really 

 to the point, since the main objection 

 advanced is reverence for precedent. 

 The word timberline has been employed 

 for a long time to designate the upper 

 limit of tree growth on high mountains 

 and no other similar line recognized. 

 This custom may be said to be fixed in 



