132 THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



curve, going back 3,000 years and based on 450 trees, and shall discuss it in detail. The 

 total number of curves available for 300 years, more or less, is 17, which are distributed 

 from Maine to California. They are shown in figure 31. In most cases there is only one 

 curve for a species, although the yellow pine, red fir, white oak, and spruce have two 

 curves each, but from distinctly different localities. An attempt has been made to correct 

 all the curves by the same method employed in the case of the yellow pine. In certain 

 cases, however, the number of specimens is too small to allow of accuracy in the determina- 

 tion of the correction for longevity, and hence it has been omitted. In other cases, no 

 correction for longevity appears to be required. Among the trees which do not require 

 tills correction only one, the short-leaved pine of Arkansas, is a conifer. The others — 

 namely, the beech of New York, the white oak of Missouri, the white oak of West Virginia, 

 and the tulip poplar of West Virginia — are deciduous. The number of trees varies from 

 26 in the case of the white oak of Missouri and 29 in the case of the Douglas fir of Idaho 

 to 728 in the case of the white oak from West Virginia, Kentucky, and Tennessee. The 

 figures for each tree are given in the diagrams and in Table H, pages 325-327. 



The full significance of these curves can not j'et be determined, but they are published 

 here for the benefit of future investigators and because they enable us to arrive at greater 

 certainty in some of our conclusions. The various groups are arranged partly according 

 to place, but chiefly according to similarity, the curves at one extreme being quite diverse 

 from those at the other. In general no curve is wholly different from those placed immedi- 

 ately beside it, but the first and second groups, although quite dissimilar, have been placed 

 beside one another for the sake of contrast. All are characterized by pronounced fluctu- 

 ations, having a periodicity of 100 to 200 years or more. In each group (excepting the last 

 curve, that of the beech) the general form of the major fluctuations is similar, although 

 details differ widely. I shall at present make no attempt to interpret the curves as a whole, 

 for that is impossible in view of the absence of any exact, specific measurements of the 

 growth year by year, such as Professor Douglass has obtained for the yellow pine, and 

 such as I shall shortly present for the sequoia. Until these are obtained it would be rash 

 to use the curves as the basis of an attempt to reconstruct the cUmate of the United States 

 during the past 200 or 300 years ; for different species of trees or the same species in different 

 habitats may be stimulated by very different combinations of temperature and moisture, 

 and a given species may find itself equally stimulated by two diverse combinations of 

 these two factors. For example, to quote certain facts for which I am indebted to Mr. 

 Raphael Zon, chief of silvics in the United States Forest Service, in regions like Idaho 

 having two seasons of precipitation, winter and summer, the dry spring is the critical period. 

 Therefore the fall of snow late in the winter is especially beneficial. In regions like Cali- 

 fornia, having no rain whatever in summer, the same is true except that the entire amount 

 of snowfall for the whole winter assumes a greater importance. In regions having precipi- 

 tation at all seasons, on the other hand, the amount of winter snow makes little differ- 

 ence, provided the rains of summer, and especially spring, are abundant. But all trees are 

 not equally stimulated by such rains, for some, such as the white oak and tulip poplar, 

 require warmth as well as moisture, while others, like the beech, seem to demand only 

 moisture and care little whether the temperature is above or below normal. 



Before attempting to draw anj^ conclusions from the curves, let us return to the question 

 of how far their sinuosities are due to climatic variations, even though for most species 

 the nature of those variations can not yet be determined. We have already spoken of 

 the extent to which accidents, such as fu-es, the shading of trees during youth, the ravages 

 of insects, and the change of conditions brought about by the white man's occupation of 

 the country, may have influenced the shape of the cui'ves. Now that we have them before 

 us in their final corrected form, it will be profitable to consider these matters once more. 

 Unquestionably, extensive forest fires not only kill trees by the thousand, but markedly 



