Popular Science Monthly/Volume 22/December 1882/Annual Growth of Trees
|ANNUAL GROWTH OF TREES.|
ARE the concentric rings of a tree a reliable record of its age in years? Such has been the conception—in fact, the undisputed knowledge—of the world, for all time past. I have no recollection of ever having seen or heard the authority of this record disputed till Désiré Charnay, in his "Ruins of Central America," said, when speaking of the age of the ruins as proved by such a record: "Unfortunately for the argument, it is altogether fallacious and proves nothing. I have put the evidence to a test. On examining a slice of wood of a shrub that I knew as a fact was only eighteen months old, I found that it had eighteen concentric rings. I thought it was an anomaly, but, in order to convince myself, I experimented upon trees of all kinds and sizes, and invariably found the like result produced in very nearly like proportions."
M. Charnay's statement was, in my estimation, rather loose, and lacking in the proof of his absolute knowledge of the age of the trees examined; and again, so far as applicable to the case, was only so in a tropical climate, where the conditions were entirely different from those surrounding us in a higher latitude, and altogether raised but little doubt on the subject.
In April of 1871 I planted a quantity of the seed of the common red maple (Acer rubrum). In transplanting, in 1873, they were placed too near each other, and it has become necessary to cut a part of them out. While cutting, I noticed that the concentric rings were very distinct, and it reminded me of M. Charnay's statement. I took sections from the butt-end of each tree (four of them) and dressed the ends off, at an angle of some 35 with the line of the body, thus largely increasing the exposure of each ring, and then counted them.
The situation, exposure, and condition of these four trees were, so far as I could see, identical. I had personal and positive knowledge that they had each twelve years' growth upon them, and I could count on each of the different sections from thirty-five to forty concentric rings. True, I could select twelve more distinct ones between which fainter and narrower, or sub-rings, appeared. Nine of these apparently annual rings on one section were peculiarly distinct, much more so than any of the sub-rings; yet, of the remaining, it was difficult to decide which were annual and which were not.
The thickness of these annual rings varied from two and one half millimetres to twenty-eight. This measure, of course, gave more than double the real thickness; but was preferable to a right-angled measure, as it gave better facilities for exactness, and yet preserved the proportion between the several rings unchanged.
Now, to ascertain what relation or connection there might be between the meteorology of
the several seasons and the growth made during the same, I selected from my meteorological records the maximum, minimum, and mean temperature, and the rain-fall, of the six growing months of spring and summer of each of the twelve years of growth. These extracts I have tabulated, and have also appended to each season the thickness of the ring formed, as measured on the oblique cut previously described.
An examination of this table shows a general relation of cause and effect between high temperature and large rain-fall, and greater growth. But it falls very far short of proving a general law of "so much heat and so much water during the growing season, to produce so much wood." For example, compare the years 1875 and 1878. The temperature of 1878 for the season is better than 4° in excess of the season of 1875, and the rain-fall only a little over four inches less; and yet the growth of 1875 is seven times what it was in 1878. This almost unparalleled growth of 1875—that is, as compared with the other years—can not be explained by the above general law. But I think the May and June record of that year throws light upon it. We see there a maximum heat in May of 96° (higher than I have ever known it in an observation and record of twenty-five years), and a mean temperature of the whole month, also unequaled, of 71°; and this great heat continued through the month of June, and no cold spells after the heat set in sufficient to check the growth. Then, in connection with this heat, the ground was well saturated with water when this heated term began (May 6th) by 1·62 inch of rain on the 4th. From this on, to the 26th of June, 15 inches more of rain fell, so apportioned over the time as to keep the ground saturated. This synchronous excess of heat and water evidently produced the abnormal growth. And probably, as this matter is further studied, it will be found that these agents, rightly proportioned, operating synchronously, produce these thicker rings; while as one or the other is in excess, or absent, the growth is checked, and thus has time to condense and harden, and form these sub-rings; and the more frequent these alternations, the greater the number of them.
- "North American Review," September, 1881, p. 401.