Byways in British Archaeology

by Walter Johnson

The errors to which De Candolle’s method is liable are thus epitomized. A yew which has been maltreated by lopping or injured by the browsing of animals may thicken and form bosses, and so increase the apparent girth. Trunks may be fused together. Wounds may prevent the formation of rings. Small shoots may be enveloped by the spread of the “bark,” and thus a vast number of rings may be formed which eventually become concentric[956].

Nevertheless, since we cannot always cut a tree down, some other method of estimating the age must be followed. And if the actual circumference be not always the true measure of the normal free-growing parent stem, our observations should, by way of counterpoise, be correspondingly numerous. Whoever has attempted to measure a yew, with the object of employing De Candolle’s rule, will recognize the force of Dr Lowe’s contentions, yet one cannot but think that the risk of error has been exaggerated.

We are told, again, that there may be a difference in the number of rings on the two sides of a tree, even at the same level, in other words, a given ring may not be everywhere of the same thickness. This is noticeable in the specimen, [Fig. 69]. In the famous yew of Darley Dale, Derbyshire, it is asserted that the number of rings varied from 33 to 66 in an inch of radius, taken horizontally—a curiously neat ratio[957]. Michel Montaigne, so far back as 1581, pointed out that the rings were narrower on the north side of the tree. Yet these occurrences do not hopelessly affect our conclusions. The writer possesses a section of a branch of yew from Offchurch, Warwickshire, which was over-developed on one side, the result of proximity to a stream, and of a sunny aspect. The consequent curvature of the heart wood, though certainly disappointing to the bowyer, who bought the tree, did not prove very troublesome even to the amateur ring-counter. Taking the average of a long series of years, and examining a large number of specimens, the inequalities in such specimens would be found to cancel each other. In an aged tree like that of Darley Dale, the coalescence of the rings would be far more perplexing than the unsymmetrical growth.

With respect to one source of error, Dr Lowe seems to answer his own objection. “A tree may have died on one side,” says he, “or may have ceased forming, while the other side is growing vigorously.” Yes, but in that case the error would be one of under-calculation; the yew would be credited with fewer years than the measurements warranted.

Once again, it is argued that not only may a particular ring have inequalities of width, but the different rings vary among themselves in thickness. And we have seen that growth does not increase uniformly with age. An oak of 50 years had a circumference equal to another which was four times that age. Nor is the rate of growth always uniformly diminished as the tree becomes older. De Candolle found an oak, 333 years old, which showed as great an increase between the rings of 320 and 330 as between those of 90 and 100 years. Now it is not mere captiousness to remark that these figures refer to an oak, not to a steady-growing yew, though Dr Lowe claims that the observation would apply equally well to the latter tree[958]. We may repeat: while variations in rainfall, temperature, and food-supply, correspondingly affect the rate of growth, it is a matter of common knowledge that our seasons tend to follow ill-defined cycles, and that a series of such cycles may be expected to equalize each other with a fair approximation to exactitude.

Fig. 71. Yew at West end of Tandridge churchyard, Surrey. Though hollow, it is one of the finest specimens in England.

A far graver indictment of De Candolle’s figures is contained in the insinuation that his selected trees were stunted and ill-grown, so that the rate of growth was made to appear too slow[959]. The supposition, if well grounded, would severely shake De Candolle’s rule, but at present it is a supposition merely, as readers of the Physiologie Végétale may learn for themselves. De Candolle did indeed believe that trees die from accident or disease rather than from old age, but how could the bias resulting from such an opinion make the age of a yew greater than it actually was? An injured tree whose development had been arrested would be credited with too few years rather than with too many. Take the case of the Tandridge yew, in Surrey ([Fig. 71]). Aubrey found that this tree had a girth of 30 feet at a height of five feet from the ground. Manning and Bray, about 130 years later, gave the corresponding measurement as 32 feet 9 inches. To-day the reading is only 32 feet 4 inches. Allowing for some discrepancies in the modes of measurement, the results are striking. The explanation is that, though the tree is still vigorous, it has long been hollow, and growth must have been slight, if indeed there has not been an actual arrest for the past century.

The method, adopted by the two Christisons, of measuring a tree at known intervals of time, is perhaps open to less objection than that of assuming a mean rate of growth, based on the enumeration of the rings of selected specimens. A combination of both systems would be better, if not ideal. The “interval method,” nevertheless, overlooks the objection that growth is not quite uniform. De Candolle urged, as already noticed, that the rate diminishes in aged trees, and gave several reasons. The roots are farther from the air, and they are also working in competition with those of neighbouring trees. Should the soil be rocky or otherwise uncongenial the lessened elasticity of the bark retards growth. Add to these factors the likelihood of oncoming disease, and the slackened development would be appreciable. Against these considerations, Dr Lowe boldly affirms that “there is abundant evidence to show that old trees grow, at intervals, much more rapidly than young ones”; but he makes this concession: “they do not, as I have said, grow uniformly, but have periods of comparative arrest of growth[960].” These pauses, he believes, are due to the overshadowing head of the tree. Were the head to be broken every half century or so, rapid growth would again commence. But to what degree does such a pollarding occur in nature? Does not the head continue, in the main, to overshadow the trunk and roots? (The lopping of yews for making bows, as apart from true pollarding, will be discussed later.) One reiterates, all systems are liable to error, but some systems are more accurate than others. And the “interval method,” especially if supplemented by estimating the total number of rings, according to an ascertained standard rate of increase, still awaits the coming of a better system to supersede it.