Climatic Cycles and Tree-Growth/Chapter 2

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Climatic Cycles and Tree-Growth by Andrew Ellicott Douglass
Chapter II



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 may be divided into two classes: first, local errors of identity in small groups of rings in a few individual trees, which simply flatten the curve without affecting the final count; second, cases in which a given ring, in spite of attempts at cross-identification, is still in doubt, showing clearly in perhaps half of the trees and not in the other half. Such cases affect the final count, but do not flatten the curve. They leave a question of one year in the dating of all the earlier portions of the curve. Only two cases of this latter kind have been noted. One was the year 1822 in the Flagstaff pines (of which there is very little doubt) and the other is the ring 1580 in the sequoias, which was finally decided by material gathered in the special trip of 1919.


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. A group of 12 sections from Central Sweden show such agreement that there is not a single questionable ring in the last 100 years or more. Especially marked combinations of rings can occasionally be traced across Europe between the groups hereafter mentioned. In Arizona the identification across 70 miles of country is unquestioned, and even at 200 miles the resemblance is apparent.

The value and accuracy of cross-identification was first observed in 1911 in connection with the Prescott trees. After measuring the first 18 sections, it became apparent that much the same succession of rings was occurring in each; therefore the other sections were examined and the appearance of some 60 or 70 rings memorized. All the sections were then reviewed and pinpricks placed in each against certain rings which had characteristics common to all. For example, the red ring of 1896 was nearly always double, while the rings of 1884 and 1885 were wider than their neighbors. In the 60 years investigated several obvious details in each decade appeared in every tree. After this success it was evident that the process should be applied to the Flagstaff trees which had been previously collected. Of the 25, however, only 19 had been preserved. A minute comparison was made between these with complete satisfaction. Since then this process has been applied with great care to every group.

After the Flagstaff set was finished, it was compared with the Prescott group. It was interesting to find that the Flagstaff ring records could be identified at once in terms of the rings at Prescott; the narrow ring of 1851 was seen to correspond to one in the Prescott series. The compressed series from 1879 to 1885 likewise had its counterpart at Prescott and formed the portion of the sections which gave the most difficulties in identification. On the whole, so far as can be judged without minute study, the Prescott trees from relatively high elevations approximating the elevation at Flagstaff have a considerably closer resemblance to the Flagstaff sections than do those growing at lower altitudes.

Cross-identification and climate. — The process of cross-identification appears to be applicable to areas far removed from one another, but as the distances increase the resemblances between tree-growth records decrease, due to climatic differences. The correspondence between trees in different regions thus becomes a test of climate and we note a possible field for the application of this process in the delineation of similar climatic areas or meteorological districts. It seems to the author that in this way the growth of vegetation may easily be made of fundamental value in practical meteorology.


It is evident that it must take some time for the transmutation of rain into an important part of the organic tissue. 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. In view of the uncertainty as to the exact month when the precipitation begins to have an influence upon the growth of the following season, and of probable variations in different years, it seemed wise to test the matter by a purely empirical method. The annual rainfall was ascertained for yearly periods beginning (1) with July 1 of the preceding year, (2) with August 1, and so on to (9) with March 1 of the current year. Another method involved a separating of the summer rains, one-half to apply on each adjacent winter, while a final method involved a similar division of the winter rains. This was done for 12 years at Flagstaff and 43 at Prescott. Part of the Flagstaff curves are given in the lower portion of figure 4, where the rainfall can be compared with the growth of the trees. The curves plotted from these tests were found to have substantial disagreements, although of course the smoothed curves of all of them would be practically identical. A comparison of the growth of the tree with these various curves showed that the use of the year beginning November 1 at Flagstaff and September 1 at Prescott gave the closest agreement between growth and rainfall. At Flagstaff the majority of the trees came from a thin clay soil derived in place from decomposed lava, and so there was little depth for the storage of moisture. At Prescott the sections of group 5, shown in the solid line of figure 7, came from trees growing in a porous soil of decomposed granite in a rather flat depression with reaarded drainage, so that conservation would have a greater influence. Perhaps this explains why the year beginning September 1 gives the best results.

In the region of the great sequoias nearly all the precipitation in the mountains (and quite all in the valleys where comparative rain records are found) comes in the winter months. For these trees, therefore, the winter precipitation is compared with the growth for the succeeding year and the month of beginning annual means is in the autumn.


Among the problems connected with the relation of the growth of trees and the amount of rainfall, one of the most interesting was suggested by Director R. H. Forbes, formerly of the Arizona Experiment Station. This was to determine the time of formation of the red or autumn portion of the rings and the causes for the formation of double rings, which were very numerous in the Prescott group. It seems evident at once that the growth of red cells is related to the decreased absorption of moisture as winter approaches. A number of tests were made on the Prescott group. The first was designed to determine the character of the rainfall in the years producing double rings. The half-dozen most persistent cases were selected and in each of these the red ring was found double in the following number of cases: 4 out of 10 in 1896; 5 out of 10 in 1891; 7 out of 10 in 1881; 4 out of 10 in 1878, 1872, and 1871. The average width of all the rings was 1.55 mm. The mean rainfall by months for the years above selected was found and is plotted in the solid line of the upper diagram of figure 1. Six other rings showing one double in 10 trees in 1898, but no doubles in 1897, 1885, 1884, 1876, and 1874, and averaging 1.54 mm. in thickness, were then selected and the curve of rainfall by months for the year during which they grew has been plotted as the upper dotted line in figure 1. In each curve the 6 months preceding and the 2 months following the year are included. 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. If, however, the preceding winter precipitation has not been entirely adequate, the spring drought taxes the resources of the tree and some red tissue is formed because of deficient absorption in the early summer before the rains begin. When these rains come the tree continues its growth. It appears further that if not only the winter snows are lacking, but the spring rains are unusually scanty, then the tree may close up shop for the year and produce its final red tissue in midsummer, gaining no immediate benefit from the summer rains. This appears to be the interpretation of the lower diagram of figure 1. Here the same 6 big

Climatic Cycles and Tree-Growth Fig 1.jpg

Fig. 1. — Effect of monthly distribution of precipitation on thickness of rings of growth; Prescott, Arizona.

doubles mentioned above are plotted, together with a selected list of 6 small singles particularly deficient in red tissues. They are, 1904 double once in 10, 1902 double once in 10, 1899 single, 1895 single, 1894 single, and 1880 double once in 10. In these it is evident that drought in the spring stops the growth of the tree. The double ring, therefore, seems to be an intermediate form between the large normal single ring, growing through the warm parts of the year, and the small, deficient ring, ending its growth by midsummer. This occasional failure to benefit by the summer rains probably explains why the Prescott trees do not show an agreement of more than about 70 per cent between growth and rainfall. It suggests also that the Flagstaff trees, which grow under conditions of more rainfall and have very few double rings, give a more accurate record than those of Prescott.

Consistent with this view of the doubling is the condition of the outer ring in the Prescott sections collected by Mr. Hinderer. These trees were cut during various months from May to November. Naturally, those cut in May are in the midst of their most rapid growth, while those cut in summer may or may not show the double ring just forming. The conditions are shown in table 2.

Table 2.

1 feet.
1911 May, June 9 out of 10 show white tissue only.
2 and 4 6420 1909 July to Sept. 30 out of 33 show red ring just forming, probably a doubling.
5 5,800 1909 Summer 3 or 4 out of 10 show red ring just forming, probably a doubling.
3 6,800 1910 Oct. and Nov. All 12 show white without red, probably a large single.

By reference to figure 1, showing the curves of monthly rainfall for 1909 and 1910, it will be seen that 1910 would be likely to carry its growth through the year and produce a single line, as in group 3 above. The year 1909 is of intermediate character, having heavy winter precipitation and a severe spring drought of 3 months. In the groups cut at this time 33 out of 43 show a red ring forming in July, August, or September, doubtless the preliminary ring of a double. This lesser red ring is due to the spring drought, and its appearance at this time indicates a lag of a couple of months, more or less, in the response of the tree to rain. The whole matter of the relative thickness of the red and white portions of the rings is illustrated in figure 2. The heavy sinuous line shows the rainfall month by month at Prescott throughout the 43 years under consideration. The total rainfall for the year is indicated by the dotted rectangles while the size and character of the, rings is shown in the solid rectangles. In these the white portion indicates the white tissue and the shaded portion indicates red tissue.

Significance of subdivisions in rings.—The normal ring consists of a soft, light-colored tissue which forms in the spring, merging into a harder reddish portion which abruptly ends as the tree ceases growth for the year. The present subject (namely, the time of year of ring formation) indicates that the red tissue appears as the tree feels lack of sufficient moisture. Therefore, the great diversity in relative size of the red tissue and the occasional appearance of false rings undoubtedly has a real significance as to distribution of precipitation during the growing-season. This subject is a very promising one, but has received little attention in the present work. The trees of the Prescott group offer a few interesting examples of two or three false red rings in one year; they also have exceptionally many cases of omitted rings; both of these peculiarities are explained by the fact that these trees are close to the lowest elevation at which the climate permits them to live; they are therefore greatly affected by rainfall distribution and probably exaggerate its changes.


Climatic Cycles and Tree-Growth Fig 2.jpg

Fig. 2.—Monthly and yearly precipitation at Prescott and size and character of rings.

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. Averages were then obtained of the oldest 5, going back about 400 years, the oldest 10 (350 years), the oldest 15 (300 years), and the entire 19 reaching back only 200 years. Finally, the record of the oldest 2 was carried back fully 500 years. On plotting the groups of 15, 10, and 5 with its extension of 2, it became immediately evident that 5 trees gave almost the same growth as 15, even to small details. Between these 5 and the oldest 2 taken by themselves the agreement was not quite so perfect, yet was so close that errors thus introduced would not affect the curves. It must not be taken for granted without test that this remarkable agreement between very small groups of trees is true necessarily for other trees or even for this yellow pine under all conditions. Without doubt it is here due to homogeneous climatic conditions in a uniform topography and a tree sensitive to varying moisture-supply.

In a good many cases where the number of trees in a group has decreased in earlier years, it has been found (by carrying overlapping curves through a considerable period) that a few trees give essentially the same curves as a large number. From the entire experience I have been led to assign a minimum preferably of 5 trees in any one group, while in some groups (notably the yellow pine of Arizona and the sequoias of California, together with the Scotch pine in central Sweden and in north Germany), 2 trees would give a very excellent record. In only one group have 5 failed to give a satisfactory record, and that was the set of Scotch pines from the outskirts of Christiania. The cross-identification of this group was not felt to be satisfactory, and a double number of trees from that locality would have been an advantage. This failure was thought to be due in part to the rugged character of the region.

Direction of maximum growth.—The maximum trunk-growth was observed to occur a Uttle east of north. The average difference between the radii was 12 per cent. An explanation of this increased growth to the north is to be foimd in the increased amount of moisture on that side, due to the slower melting of snowand decreased evaporation in the shade. For nearly all these trees the ground had a gentle slope toward the south, so that moistm-e working down hill reaches the north side of the root system first.

Rate of growth and age.—The relation of average ring-width to radius was found to be intermediate between an inverse proportion to the radius and an inverse proportion to the square of the radius. If the tree merely increased in diameter without growing upward, the width should be roughly inversely proportional to the radius. If the tree is increasing in height at the same time, we should expect an inverse proportion to the square of the radius. We find the relation to be between these.

Growth and soil.—In early studies of 25 yellow pines at Flagstaff it was noticed that a certain subgroup of 6 trees dropped to its strong minima in 1780 and 1880 more promptly than the others. This appears to be conliected with the soil upon which the trees grew. This subgroup stood on a Umestone formation where the soil is porous and the rock below full of cracks. The other two subgroups grew on recent lavas, very compact and unbroken, covered with a rather thin layer of clayey soil. With the former, therefore, the rain passed quickly through the soil and away, and we do not find so much conservation of moisture as in the latter, where the water could find no convenient outlet. On the whole, the growth seems to be more rapidly influenced by changes of moisture on limestone than on volcanic rocks.