Die Jahresringe der Holzgewächse. 3(37 



Im Jahre 1901 nahm Douglass (1909, 1914) in Arizona eine 

 Serie präliminarer Jahresringmessungen an Pinus ponderosa („yellow 

 pine") vor, welche den Anfang einer von ihm und Huntington 

 (1914) inganggesetzten großartigen und vielversprechenden Unter- 

 suchung bezeichnen. Nachdem die vermutete Relation zwischen Nieder- 

 schlag und Ringbreite für mehrere Bäume in den ariden Gegenden 

 im westlichen Nordamerika bestätigt gefunden war, und nachdem 

 Korrektionsmethoden für einige Fehlerquellen ausgearbeitet waren, 

 fand nämlich Huntington in dem kalifornischen Riesenbaume 

 (Sequoia gigantea) einen klimatischen Maßstab, der mehr als 3000 Jahre 

 in der Zeit zurückreicht. 



Nach der in „The Climatic Factor . . ." von Douglass und 

 Huntington (1914, p. 123) ausführlich beschriebenen Untersuchungs- 

 methode wurden die Ringmessungen an Stümpfen längs einem Mittel- 

 radius oder längs dem besten Radius vorgenommen. Bei älteren 

 Bäumen wurden je 10 und 10 Ringe, bei jüngeren jeder einzelne 

 Jahresring gemessen (1. c, p. 103, 123). Die so erhaltenen Ring- 

 breitenkurven bringen doch nicht unmittelbar die Klimavariationen 

 zum Ausdruck. Sie müssen unter verschiedenen Gesichtspunkten 

 korrigiert werden, und verschiedene Zufälligkeiten müssen berück- 

 sichtigt werden. — Sehr langsam wachsende Bäume sind weniger 

 geeignet (1. c, p. 106, 109). 



„The annual rate of growth of trees is", sagt Huntington (1914, p. 123), „subject 

 to variation for four chief i easons. In the first place, trees grow at very different rates 

 according to their age, young trees usually growing rapidly and old trees slowly. 

 In the second place, trees destined to have a long life usually make haste slowly, 

 being outstripped at first by their neighbours, which are to die much sooner. These 

 two types of variation can be calculated with mathematical precision, and by the 

 use of the proper formulae corrective factors can be obtained by means of which 

 errors due to them can be largely eliminated. The third reason for variation in the 

 annual rate of growth of trees is the occurrence of non-climatic accidents such as 

 shading in youth, the breaking of branches, the slipping of the soil, the ravages of 

 insects, or the devastation wrought by fire. At first sight these appear to be of 

 almost preponderating importance, but as a matter of fact they play by no means 

 so great a rôle as would be expected, for, as Professor Douglass has shown, two 

 or three trees, or even a single tree, under exceptionally favorable conditions, gives 

 a fairly accurate climatic record but little disturbed by accidents. Finally, the fourth 

 reason for variation is the changing conditions of weather and climate which 

 prevail from year to year. It is these which we wish to determine, and this can 

 be done only by eliminating variations due to the other three causes." 



„The elimination of the effect of non-climatic accidents upon the rate of growth 

 of trees is accomplished largely by the process of averaging. If a sufficient number 

 of trees is used, and if the trees are distributed over a wide and varied area, purely 

 individual accidents will disappear by the law of averages." 



„The elimination of the differences in rate of growth due to the fact that 

 young trees grow more rapidly than older ones is easily made." 



„Manifestly allowance must be made for this varying rate of growth. I have 

 called this allowance the 'corrective factor for age'. The method of obtaining it is 



24* 



