Andrew Ellicott Douglass

“ It is possible that the Flagstaff trees with their higher elevation, more certain rainfall, and more central location in the zone occupied by this species give somewhat more accurate records. They are probably much less often subjected to extremes of dryness, which throw the tree out of its equilibrium and cause it to produce an abnormally small set of rings. It seems likely, also, that the less compact soil, combined with a more abundant precipitation, produces a yearly growth more nearly proportional to the rainfall than at Prescott. ”

“ Apart from care in measuring the rings, the details of which will be given in Chapter IV, the most fundamental and essential feature of the method of studying tree-growth is the cross-identification of rings among a group of trees. The ease and accuracy with which this can be done in a fairly homogeneous forest is remarkable. A group of 13 tree sections collected along a distance of a quarter of a mile in the forest of Eberswalde, near Berlin, show almost identical records. Two to ten rings in every decade have enough individuality to make them recognizable in every tree. ”

“ The ordinary plot and the ordinary averaging seem extremely good and appropriate when the variations are small in comparison with the mean values, but when the variations are large in comparison with the mean values it does not seem to the writer certain that the usual plotting conveys an accurate idea or gives a suitable basis for further work. This may be illustrated by the plotting of rainfall. If the rainfall doubles in some unusual year, it produces an immensely greater change in the area of the curve than when it goes to one-half of the mean. ”

“ Certain areas of wet-climate trees in northern Europe give an admirable record of the sunspot numbers and some American wet-climate trees give a similar record, but with their maxima 1 to 3 years in advance of the solar maxima. It is possible to identify living trees giving this remarkable record and to ascertain the exact conditions under which they grow.
(7) Practically all the groups of trees investigated show the sunspot cycle or its multiples; the solar cycle becomes more certain and accurate as the area of homogeneous region increases or the time of a tree record extends farther back ”

“ There is evidence, as will be shown later, that the summer rains often have a prompt effect. The winter precipitation, however, is necessarily more remote in its action. Much of the first growth in the spring must come from the melting of the autumn and winter snows. It seems reasonable, therefore, to consider any snowfall as applying to the following yearly ring. At Flagstaff the precipitation of November is almost always in the form of snow, and therefore that month should certainly be considered as falling after the arboreal New Year of that locality. ”

“ The fundamental data tabulated in the appendix are the means of the actual measures of the various groups. They, therefore, contain the effects of the two chief arboreal constants, which are (1) the nearly universal big growth at the center of the tree and (2) the increased size in some entire trees due to specially favorable environment. In producing a perfectly normal record of tree-growth over long periods, one desires to have it expressed throughout in terms of the normal adult growth of an average tree. ”

“ It would be possible to apply correlation formulas to the Arizona tree records and perhaps to the sequoias and construct a probable rainfall record for long periods of time, but apart from Huntington's study of the "Climatic Factor in History," the chief use of such a record would be in studying the laws which govern rainfall; and this is best done through cycles. We shall find that the sunspot cycle plays an important rôle in rainfall. ”

“ In seeking the best curve of tree-growth which a given locality can supply, it might be thought at first that a very large number of trees must be obtained in order to get an average, but experience has shown that the number may be very small. In order to test the accuracy obtained from a small number of trees, a comparison was made between large groups and small. Of the original 25 trees in the first Flagstaff group, 19 were subjected to very careful cross-identification. ”

“ Yet, gross rings are not universal at any one time, and while one epoch may show gross rings in half the trees of a group it does not show it in the other half, judging by the groups examined. It is considered best to allow the ring values to enter the curves just as they are found, for while the gross rings disturb very greatly the size of a series of 10 to 20 rings, they do not seriously disturb the relation in size between a ring and its immediate neighbors. ”

“ The curves seem to indicate clearly that the chief cause of doubling is a deficiency of snowfall in the winter months, December to March. This appears to mean that if the winter precipitation is sufficient to bridge over the usual spring drought, the growth continues through the season, giving a large single ring which ends only in the usual red growth as the severity of winter comes on. ”

“ In order to get a long range of periods, the diagram was mounted on an axis with clock-work and slowly rotated in front of a camera with a cylindrical lens for objective, a horizontal slit in the focal plane, and a sensitive plate passing slowly downward across the slit by clock mechanism. In this way a full range of possible periods come under the summing process, and when a real period is vertical the crests of the curves form vertical lines which come down as a series of dots or beads in the slit. When no period is in the vertical the light coming through the slit is uniform. ”

“ The unsmoothed representation consists of what one may call a columnar plot. Examples of plots of that type may be found in connection with some rainfall records published by the United States Weather Bureau and in a representation of the London rainfall for more than 100 years published by the British Rainfall Association, and elsewhere. In this kind of plot the rain for a year is not represented by a dot, but by a block column which extends from the base-line up to the required amount and it has a width equal to the interval of one year according to the scale of the plot. ”

“ The European groups. — For better comparison, the nine European groups have been corrected for change of growth-rate with age, reduced to percentages of their own means, smoothed by Hann's formula, and plotted in figures 23 and 24 together with the sunspot curve. They do not all follow the sunspot numbers with equal accuracy, and the six groups showing best agreement are segregated in the first of the two figures. The north German and south Sweden groups around the Baltic Sea are the most satisfactory; the group from the west coast of Norway is almost as good. ”

“ The 11-year period is plainly evident through most of the record and for some centuries is the predominant cycle, but for long periods other slightly differing cycles, such as 10 years, 12.6 years, or 13 years, are more evident. It is as yet impossible to say whether at these times there was a real change in the sunspot period, whether some subordinate period is operating in the sun, or whether only local conditions of some kind are the controlling factor.
The yellow pines of northern Arizona are dry-climate trees. They have a modified winter precipitation of the temperate zone. ”

“ So botanical aid was sought in order to extend our knowledge of weather changes over hundreds and even thousands of years by making use of the dependence of the annual rings of trees in dry climates on the annual rainfall. If the relationship sought proves to be real, the rings in the trunks of trees give us not only a means of studying climatic changes through long periods of years, but perhaps also of tracing changes in solar activity during the same time. ”

“ Throughout the whole study it was desired to get as long records as possible and old trees were therefore selected. In nearly every case this meant large trees also. Apart from this no special selection of trees was made at any time, save only in the Christiania group, in which so many of the logs showed a "complacent" habit, with long successions of equal rings rather large in size, that some effort was there made to find the logs which showed variations in ring-size. A complacent ring-record without doubt means that the environment of the trees was well adapted for its best development. ”

“ Number of cycles covered by the given curve. (2) Shortness of maxima in relation to length of cycle; if the maximum is sudden and sharp, as in rainfall, the accuracy may be very great; if the maximum is long, as in a sine curve, the accuracy is less. (3) Regularity in the maxima and freedom from interference. These features all appear in the differential pattern and hence the accuracy of any period is its most evident feature and all observers can judge it equally well. ”

“ In nearly all parts of the yellow-pine curve there are suggestions of an 11-year cycle. By tracing this throughout the record, the period is found to have a length of about 11.4 years, which is sufficiently close to the length of the sunspot cycle to be considered identical with it. This exact figure is not yet considered final, as future intensive study of the short-period variations in the trees may throw more light upon it. Taking 11.4 years as the probable length, the average total variation is found to be some 16 per cent of the mean growth. ”

“ The wet-climate trees near the Baltic Sea show variations following almost perfectly, the curve of sunspot numbers. The Scotch pines just south of the sea have had good care since they were planted about 90 years ago. This care has prevented the excessive competition between individuals which characterize natural forests, and perhaps for that reason they give this remarkable record of external conditions. The trees to the north of the Baltic include spruces as well as Scotch pines, and show the same reaction. ”

“ The effect of Jupiter with its substantial variations in distance between perihelion and aphelion predominates, and we have a marked resemblance between the sunspot curves since 1770 and the differential planetary effect. One notices that this interchange of energy would presumably affect all parts of the sun alike and that therefore we could not expect an excess of sunspots on the side facing Jupiter. H. H. Turner (1913; cf. Sampson, 1914) has worked out an hypothesis which is stimulating, even if not yet acceptable. ”

“ The rainfall in this region shows the solar period also, but it is roughly inverted with respect to the tree curve. The Oregon group must be considered as in the wet climate of the temperate zone. It is near the Pacific coast and has abundant rain or snow. The solar cycle is probably in it, but it is not so conspicuous as other short cycles. When these trees are summed up on the 11-year period, they show about 10 per cent total variation with maximum and minimum coinciding with the Vermont group and therefore anticipating the sunspot maximum by 3 years. ”

“ On the first examination of these sections in November 1912, it was evident that their growth follows with fidelity the sunspot curve since 1830. This may be traced in the curves below and in the accompanying photographs of two of the sections in plate 8. It will be seen at once that there is a rhythmic sway in the growth, groups of large rings alternating with small ones. The arrows placed in the photographs mark the years of maximum sunspots. Taking the group as a whole, the maximum growth comes within 0.6 year of the sunspot maximum. ”

“ Application to length of sunspot period. — The interest in the sunspot period makes a special consideration of plate 9, c, worth while. This figure is a photograph of plate 9, b, taken out of focus for the purpose of calling attention to certain general features. In b the eye naturally turns to the sharp outlines and notes its minute details. In c the crests of b are changed into large blotches connecting somewhat with their nearest neighbors and varying in intensity. The alinement which they form in a nearly vertical direction is a graphic representation of the period. ”

“ In the very beginning of the work it was expected that only in large averages would a relationship be found between tree-growth and climate. Accordingly, something like 10,000 measures had been made on the pines of northern Arizona and the results all tabulated, when it occurred to the writer to compare the annual growth of Flagstaff trees directly with the 8 or 10 years of rainfall records taken at the United States Weather Bureau station recently established there. ”

“ The sequoias grew often within a few feet of each other, and even between them were pines, firs, and cedars. Lumbermen often point out the bottom of a basin and say that such a place ran over 1,000,000 board feet to the acre. To-day nearly all the trees are gone and debris and rubbish are scattered about everywhere. The constant supply of water in the basin made the trees less dependent upon the annual precipitation and they show, in fact, large rings with very slight variation from year to year. ”

“ Fundamental periodic changes in the body of the sun have been suggested and, in the absence of better explanations, some such statement hazily indicating the direction in which explanation is to be sought, is perhaps the best that we can do. Planetary influence, however, has often been proposed as the cause. The near agreement between the revolution period of Jupiter and the sunspot period has naturally attracted attention. ”

“ Each group supplies an approximate curve of its decreasing growth with age. So, after plotting the means, a long average line as nearly straight as possible is drawn through them. This gives the factor by which individual rings may be reduced to the standard adult growth ”

“ Of these 6 troublesome sections, the first 5 were very slow growers. Hence it would seem advisable not to use extremely slow-growing trees any more than is necessary. It may be urged that trees do not grow continuously at the slow or fast rate and that we can not tell how much of the change is due to rainfall. On the whole, however, it seems advisable to exclude trees or parts of trees whose identification is extremely difficult. The inner rings if well identified may be extremely useful in carrying back early records, as the slow-growing trees are likely to be among the oldest. ”

“ Any one cycle together with others close to it may be studied more minutely on the differential pattern produced when the instrument is set at the desired period. Plates 12 and 10, b, are arranged to illustrate this and at the same time show the solar cycles in several of the groups. A periodic effect equal in length to the setting of the instrument is indicated by a vertical row of light crests or dark spaces. These rows may be seen in any of the patterns. If the row of crests points downward to the right, its period is greater than the setting of the instrument ”

“ GROUP.
These 13 trees were cut and sections prepared for me by the kindness of Professor A. Schwappach of Eberswalde. They were all Pinus silvestris planted about 1820 to 1830, exactly alike in height and size, with tall, straight, clear trunks about 10 inches in diameter and bushy tops. The land is a gently rolling country with a slight northerly slope, leaf-covered ground, a sandy soil with loam on top, and an elevation above the sea of 200 to 300 meters. The height above the city level was 200 feet or so; the locality was south and west of Eberswalde station. ”