Page:Climatic Cycles and Tree-Growth - 1919.djvu/74

 In nearly every decade some are thus distinguished, and in each century there are usually 3 to 4 conspicuously small rings which give very important aid.

In the first work on the 2,200-year sequoia record, the identification was a laborious task involving all the writer's spare time for a year. The only real difficulty was with the ring for the year 1580. This was temporarily called 1580, but the material collected in 1919 showed it to represent a year and a final and complete renumbering included it as such. In the end the comparisons gave entire confidence as to the identity of every ring. Section No. 2 gave the most nearly perfect long record, beginning at 274 B. C, and is used as a standard with which to compare all new ones.

The most difficult parts to identify are the compressed rings. Over long periods, varying from 5 or 10 up to 100 years, the rings are sometimes so crowded together that large numbers of them seem to be merged into one and their identification becomes extremely difficult and in a few cases impossible. The great variations in sizes so produced also exaggerate effects. These groups of compressed rings are considered as of little value, and in fact in many trees their measurement is omitted altogether. Tree No. 12 of the sequoias obtained from the Indian Basin had such bad groups of compressed rings that it proved practically impossible to identify them without a large expenditure of time not then available. Tree No. 17, also, from Camp 7, was found so full of compressed rings in the last few hundred years that all measurements were omitted after the year 1130 A. D.

Fire-scars. — Most of the big trees show fire-scars at some time in their history, and the process of the tree's regeneration is very interesting to observe. If the scar is small the woody growth quickly comes in from each side and covers it. If the scar is very large, occupying perhaps one-quarter or one-third of the circumference, the tree is likely never to recover and the burnt place remains permanently on its side. In cases of less extensive burns, the wood from each side year by year grows toward and over the injured spot, and if the injury has not been too great the approaching sides may meet and imprison their own bark within the tree. Thus one often sees the tops of the stumps marked here and there by a hole as large as a foot in diameter, filled with bark in perfectly good condition.

No. 12 had several fire-scars that interfered with the identification of rings. No. 18 also had one or two fire-scars and in particular showed a fire in the year 1781. The latter evidently stopped the growth at that point completely, yet was not large enough to interfere with recovery. In the sample in the laboratory the usual reddish-colored heartwood changes about the year 1700 to the white sapwood, which ends with the ring 1781 and shows a surface that was once covered with bark. However, immediately outside of that surface, the red heart-