The obtruding difficulty which now meets us is, Where are we to measure the girth? Sir Robert Christison thought the ground level best, but, wherever possible, he also measured the tree at five feet from the base[961]. Yews, however, frequently thicken upwards; there may be swellings under the branches; the stem is often encumbered by bunches of young sprays; aged trees are usually deformed, and are studded with knobs and excrescences. To follow Christison’s plan with the trees at Dryburgh, Roseneath, and Sanderstead, and other yews which have protuberances at the base, would result in too liberal an estimate. The reaction of root-pressure tends to make trees “lift themselves out of the ground.” Moreover, in exposed situations, scraping animals, such as rabbits and foxes, lay bare the roots of trees, and increase the apparent girth. Rain tends to wash away the soil, and so aids in the deception.
On the other hand, the level of the soil in churchyards is gradually rising (cf. [p. 90] supra), and it is primarily with churchyard yews that we are concerned. Hence it is proposed that the measurement should be taken at the base, and also at a height of three feet from the ground. De Candolle recommended a height of two feet for exogenous trees. Those who have had practical experience in measuring yews, will, while employing the tape at both these levels, recognize that there must be a slightly varied treatment for each individual tree. Perhaps, like Sir R. Christison[962], the investigator will be impelled, where the trunk is short, to take the girth at the narrowest part.
Unfortunately few actual records have been kept of measurements of yew trees taken at intervals. One or two cases may, however, be instructive. At Hurstbourne Tarrant, near Andover (Hants), there are two churchyard yews, which, the parish register informs us, were planted in 1693 and 1743 respectively. Now, if we accept Dr Lowe’s mean rate of one foot for 65 years, the first tree, when re-measured in 1896, should have been about 9´ 10´´ in circumference, and the second 7´ 6´´. The actual measurements were[963]:
| 1st tree. 8´ 4´´ at base; 6´ 8´´ at a height of five feet. |
| 2nd” 7´ 3´´””; 7´ 3´´”” ”” |
Hence Dr Lowe’s rate of growth would be too high for these trees; in the first example, markedly, yet the trees were still only of moderate age, and growth must have been active.
Other good records refer to two yews in the churchyard of Basildon, Berkshire. Details of the measurement of the first tree were entered in the parish register in 1780, and of the second in 1834, though both trees had been planted by Lord Fane at a considerably earlier date than the year 1780. In 1889 the trees were again measured by Mr Walter Money, and the results thus compared[964]:
| 1st tree. From 1780-1889, an increase from 6´ 3´´ to 9´ 10´´; a gain of 3´ 7´´. |
| 2nd”From 1834-1889, an increase from 9´ 2½´´ to 9´ 6´´; a gain of 3½´´. |
Now, if it be true, as Dr Lowe’s rule supposes, that a mean period of 65 years represents a gain of one foot in diameter, the first tree should have increased 5´ 3´´ in circumference, and the second 2´ 8´´. But here, again, the postulated rate of growth is far too high; in the second example, nine times too high. Even if we grant that “old trees grow, at intervals, much more rapidly than young ones,” Dr Lowe’s main rule is not verified. For it happens that measurements of the first tree were made at two intermediate dates, 1796 and 1834. During the first period, 1780-1796, the increase was indeed three times as great as Dr Lowe’s formula would demand, but in the second, and longer period, 1796-1889, the growth was less than a quarter of the estimated amount. These measurements of yews, dealing with odd inches, truly suggest some degree of error, due to the substitution of one observer for another, nevertheless, we may assume the figures to be roughly correct. At the same time, these rather surprising results indicate the wisdom of calling in the aid of the total ring-estimate as a supplementary witness, since the increase of girth tends to be so variable.
Particulars respecting the growth of yews at Wrexham, as given by John Timbs, also show that an increase of one foot in 65 years is somewhat over the limit[965]. More records are desirable, yet the facts generally seem to favour Dr Lowe’s higher limit of 70 years for each foot of growth, or even the 75 years proposed by the Christisons. On the whole, Dr Lowe himself seems to sanction the last-named estimate, for, while he thinks that one foot in 75 years is “below the average rate of growth,” yet for purposes of calculation he prefers to adopt that scale[966].
The basis of 75 years, then, shall be taken in the present chapter. Two reservations, dependent upon what has been said previously, must, however, be borne in mind. First, seeing that young trees have, proportionately, a more extended leaf surface than old ones, larger rings are formed and the 75-years rule will make the trees too old. Secondly, and more important, there seems to come a period when aged trees are practically at a standstill; they make no more increase, but linger until disease and decay set in and slowly drain off the vitality. Of the yew at Aldworth, Berkshire, it is asserted that it has not increased in circumference since 1760[967]. I feel sure that this arrest, though not usually absolute, is very common; hence, for aged trees, additions to the age must be made after applying the 75-years rule. Much stress should be laid on this point.