Page:Climatic Cycles and Tree-Growth - 1919.djvu/21

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In comparing the growth of trees with rainfall and other data, it is essential that the date of formation of any individual ring shall be certain. This depends directly on the yearly identity of the rings or the certainty with which one ring and only one is formed each year. The fundamental starting-point in all identification is the ring partially formed at the time of cutting the tree. This is usually found with ease and has led to no uncertainty in the pine. In the sequoia this partial ring is exceedingly soft and had been rubbed off in nearly all trees examined. It was found unmistakably in a tree cut on the date of visit. Superficial counting of rings is subject to errors due to omission and doubling of rings. In the first investigation of trees at Flagstaff it was supposed that the results were subject to an error of 2 per cent, most of which arose from double rings near the center of the tree. But the discovery and application of the method of cross-identification revolutionized the process of ring identification, and it was proved that the error of unchecked counting in the Arizona pines was 4 per cent and lay almost entirely in the recent years. It was due to the omission of rings or the fusion of several together.

Apart from cross-identification, confidence in the yearly identity of rings comes from the following sources :

(1) Belief that the well-marked seasons of the year cause absolute stoppage of growth in winter. The January mean temperature at Flagstaff is 29° F. and that of July is 65° F.

(2) The known time of cutting of nearly 100 different trees distributed through perhaps a dozen different years successfully and accurately checks cross-identification in the later years of the tree.

(3) The various identifications adopted for recent years check exactly with the neighboring rainfall records in Prescott and other places where such comparison can be made. This will have further illustration in connection with the chapter on rainfall and tree-growth.

(4) A check on the accuracy of the accepted identification of the Flagstaff trees was made by noting every statement of weather, freshets, or crop-failures mentioned by the historian Bancroft in his accounts of the settlements of Arizona and New Mexico. There were 14 cases in which the noted feature of the year agrees with the tree-record to one doubtful disagreement. The most striking correspondences occur with reference to the flood on the Rio Grande in 1680, the famines between 1680 and 1690, and the droughts in Arizona in 1748, 1780, and 1820-23.

The effect of the undetected omission or the doubling of the rings in individual trees is to lessen the intensity of the variations in the curve of growth obtained by the averaging of many trees. The