126 THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



line in question will be straight. Thus the process under discussion reduces the smoothed 

 curve of growth to a straight line. It does not, however, eliminate the u-regular idio- 

 syncrasies of the curve of an individual tree, although it changes their relative importance. 

 In the example under consideration the average growth during the fu-st year is 0.10. Let 

 us take this as the standard or normal growth. During the fortieth year the growth is 0.20, 

 or twice as much; to reduce 20 to 10 means simply dividing by 2. Similarly during the 

 two-hundredth year the growth amounts to 0.05; to reduce 0.05 to 0.10 means inultiplying 

 by two. In other words, the corrective factor for age during the fortieth year is one-half, 

 or 0.50, while that during the two-hundredth is 2. Having this corrective factor for 

 each year of the tree's life we must apply it to the curves of individual trees. In this way the 

 dot-and-dash Une shown in the diagram is reduced to the form shown by the dash line above 

 it. The sinuosities occur at the same time as before, but are less marked than previously 

 during the early years of the tree's growth and more marked during old age, when the tree 

 was growing so slowly that the original curve became very flat. In this final curve the differ- 

 ence in rate of growth between old trees and young has no effect. The variations that remain 

 are due either to accidents, to climate, or to another factor which we shall now consider. 



In the original investigation whose results are here being set forth, a puzzling feature 

 appeared when the correction for age was applied to the first thi-ee or four species. In the 

 earlier portion of each curve— that is, in the part where only the oldest trees could be 

 used— there was a systematic lowering of position. This appeared to indicate markedly 

 drier conditions in the past than in the present, but the apparent difference was greater 

 than could possibly have existed, and it occurred at different times in different trees, being 

 dependent apparently on the age of that special species. Moreover, it occurred at times 

 when other hnes of evidence seem to point to exactly the opposite state of affairs. In 

 attempting to ascertain the cause of this, it was soon discovered that, other things being 

 equal, trees which are destined to live to a ripe old age grow in their youth more slowly than 

 do those of the same species and in the same locality which are destined to die young or 

 to live only until maturity. If the normal life of a species is 200 years, the individuals 

 which are to attain an age or 300 or 400 years, almost without regard to climatic conditions, 

 grow on an average more slowly than do those which in the natural course of events are to 

 die at the age of 200 years or earlier. Slow growth in youth is apparently one of the 

 essential conditions of a prolonged and vigorous old age. This is a well-known fact when 

 different species of trees are compared. The fast-growing horse chestnut does not live 

 to anything like so great an age as the slow-growing oak, but that this same law holds good 

 within the species is not generally realized. It is probably not a universal law, however. 

 Among the conifers whose analyses were obtained from the Forest Service almost all the 

 species show this feature, the only possible exceptions being species for which the data 

 were insufficient to allow of its calculation. Among deciduous trees, on the contrary, 

 judging from the few species yet investigated, there seems to be little difference m the 

 rate of growth of trees which live to be old and of those which die young. 



The apparent cUfference in rates of growth between the old trees and the young ones 

 is by no means a matter of climate or of shelter of the trees during youth. This is proved 

 by the fact that it applies to trees of all ages. That is, it makes no difference whether the 

 average hfe of the species is 100 or 300 years. In the one case trees which live to be 150 

 years old are characterized by slow growth in youth, while in the other case the slow- 

 growang trees are those 500 years old. Moreover, in some cases the difference m rate of 

 growth decreases by regular stages. It is most marked in the first decade, less marked in 

 the second, etc., and it commonly disappears or even is reversed by the time the trees attain 

 approximately the average age of maturity. That is, if the ordinary age of a certain species, 

 its "three-score years and ten," is 200 years, the rate of growth of the trees destined to 

 live much longer is, at that time, no less than that of the others, and in many cases more. 



