Alexander von Humboldt, in his work entitled Views of Nature (pp. 220. 268-276. ed. Bohn), has some interesting remarks on the age of trees.
"In vegetable forms (he says) massive size is indicative of age; and in the vegetable kingdom alone are age and the manifestation of an ever-renewed vigour linked together."
Following up this remark, he refers to specimens of the Baobab (Adansonia digitata), with trunks measuring more than thirty feet in diameter, the age of which is estimated by Adanson at 5150 years. All calculations of the age of a tree, founded merely on the size of its trunk, are, however, uncertain, unless the law of its growth, and the limits of the variation producible by peculiar circumstances, are ascertained, which, in the case of the Adansonia, have not been determined. For the same reason, the calculation of 2,500 years for a gigantic cypress in Persia, mentioned by Evelyn in his Silva, is of no value.
Humboldt afterwards refers to "the more certain estimations yielded by annular rings, and by the relation found to exist between the thickness of the layer of wood and the duration of growth;" which, he adds, give us shorter periods for our temperate northern zone. The calculation of the age of a tree, founded on its successive rings, appears to be quite certain; and whenever these can be counted, the age of a tree can be determined without risk of error. Humboldt quotes a statement from Endlicher, that "in Lithuania linden (or lime) trees have been felled which measured 87 feet round, and in which 815 annular rings have been counted." The section of a trunk of a silver fir, which grew near Barr, is preserved in the Museum at Strasburg: its diameter was eight feet close to the ground, and the number of rings is said to amount to several hundreds.
Unfortunately this mode of determining a tree's age cannot be applied to a living tree; and it is only certain where the tree is sound at the heart. Where a tree has become hollow from old age, the rings near the centre, which constitute a part of the evidence of its duration, no longer exist. Hence the age of the great oak of Saintes, in the department of the Charente Inférieure, which measures twenty-three feet in diameter five feet from the ground, and is large enough to contain a small chamber, can only be estimated; and the antiquity of 1800 or 2000 years, which is assigned to it, must rest on an uncertain conjecture.
Decandolle lays it down that, of all European trees, the yew attains the greatest age; and he assigns an antiquity of thirty centuries to the Taxus baccata of Braburn in Kent; from twenty-five to thirty centuries to the Scotch yew of Fortingal; and fourteen and a half and twelve centuries respectively to those of Crowhurst in Surrey and Ripon (Fountains Abbey) in Yorkshire. These ages are fixed by a conjecture founded on the size, which can lead to no certain result.
Can any of your correspondents state what is the greatest number of rings which have been actually counted in any yew, or other tree, which has grown in the British Isles, or elsewhere? It Is only by actual enumeration that vegetable chronology can be satisfactorily determined: but if the rings in many trees were counted, some relation between the number of rings and the diameter of the trunk, for each species, might probably be laid down within certain limits. These rings, being annually deposited, form a natural chronicle of time, by which the age of a tree is determined with as much precision as the lapse of human events is determined by the cotemporaneous registration of annalists. Hence Milton speaks of "monumental oak." Evelyn, who has devoted a long chapter of his Silva to an investigation of the age of trees (b. iii. c. iii.), founds his inferences chiefly on their size; but he cites the following remark from Dr. Goddard:
"It is commonly and very probably asserted, that a tree gains a new ring every year. In the body of a great oak in the New Forest, cut transversely even, (where many of the trees are accounted to be some hundreds of years old) three and four hundred have been distinguished."—Vol. ii. p. 202. ed. Hunter.
A delineation and description of the largest and most celebrated trees of Great Britain may be seen in the interesting work of Jacob George Strutt, entitled Sylva Britannica, or Portraits of Forest Trees, distinguished for their Antiquity, Magnitude, or Beauty: London, 1822, folio.
The age of some trees is determined by historical records, in the same manner that we know the age of an ancient building, as the Parthenon, the Colosseum, or the Tower of London. It is, however, important that such historical evidence should be carefully scrutinised; for trees which are known to be of great antiquity sometimes give rise to fabulous legends, destitute of any foundation in fact. Such, for example, was the plane-tree near Caphyæ, in Arcadia, seen by Pausanias in the second century after Christ, which was reported by the inhabitants to have been planted by Menelaus when he was collecting the army for the expedition against Troy. (Paus. VIII. 23.) Such too, doubtless, was the oak of Mamre, where the angels were said to have appeared to Abraham. (Sozomen, ii. 3.) A rose-tree growing in the crypt of the cathedral of Hildesheim is referred, by a church-legend, to a date anterior to 1061; which would imply an age of more than 800 years, but the evidence adduced seems scarcely sufficient to identify the existing rose-tree with the rose-tree of 1061. (See Humboldt, p. 275.)