Popular Science Monthly/Volume 24/November 1883/The Age of Trees
|THE AGE OF TREES.|
SINCE De Candolle, the celebrated Swiss botanist, propagated the idea that a tree has no limits set by nature in its constitution to the term of its existence, the question of the age attainable by trees has never ceased to be debated with considerable interest. De Candolle's argument was to the effect that whereas animals have, by the physiological construction of their vessels, a set limit to the duration of their lives, trees have no such natural termination; and that although their decay and death are so familiar to us that we commonly speak of this or that species as living for a given period like two hundred years, yet such decay is rather the result of accident or disease than of any law inherent in their nature such as in our own case we designate as death by old age. Whence, the same botanist inferred, there is no reason why trees under perfectly favorable conditions should ever perish; and he proceeded to adduce in favor of that theory instances of trees which even then were in the enjoyment of no contemptible moment of eternity.
Until accurate observations have been made for hundreds or perhaps thousands of years, it would seem impossible to arrive at even an approximate solution of so wide a problem as this. Under the best conditions we could never eliminate those causes of tree mortality which De Candolle fairly enough calls accidental, but which are contained in the invariable laws of the elements. The largest, and therefore probably the oldest, trees are the special sport of the lightning; and the storm which has so often felled trees of the most prodigious size will, even if it spare the trunk, break off boughs, thus admitting at the point of fracture that caries into the trunk which will ultimately reduce it to a mere shell, similar to one of those bull-oaks wherein the bull loves to hide himself. These causes of disease and decay can never be absent, since they evidently belong to the permanent order of nature.
Again, De Candolle accounts with great probability for the diminished rate of tree-growth after a certain period by such considerations as the greater distance of the roots from the air, their coming into contact with the roots of other trees, or with a rocky or otherwise unsuitable substratum, or the diminished elasticity of the bark; and though it is possible that trees might continue to grow in their fifth century at the same rate as in their first, if the conditions remained equally favorable, yet, since the proviso can never be insured, a further difficulty, amounting to insuperability, occurs, to prevent such an hypothesis from being brought to the test of either observation or experiment.
Whether, therefore, a tree might possibly continue living and growing forever is a question of less entertainment than the question of its possible duration in the common state of nature and under the irreversible conditions of climate, soil, and the elements. What age may we ascribe to some of our largest specimens, either still existing or recorded in trustworthy history? Is the period of one thousand years, the favorite figure of tradition, a common or probable period of arboreal longevity, or have our proudest forest giants attained their present size in half the time that is commonly claimed for them?
In the discussion of this question we have but few known data to guide us, since statistics of the rate of growth, as afforded by careful measurement, date only from about the beginning of the eighteenth century. Of such statistics we may dismiss at once measurement of height or of the spread of a tree's boughs, the measurement of girth being far easier and more conclusive. But it is unfortunate that no standard of distance from the ground has yet been adopted for measurement, so that the needless perplexity might be avoided which is derived from giving the circumference now at the ground and now at two, or three, or six feet above it.
The counting of the rings added by exogenous trees every year to their circumferences can only, without risk of great error, be applied to trees cut down in their prime, and hence is useless for the older trees which are hollow and decayed. Trees, moreover, often develop themselves so unequally from their center that, as in the case of a specimen in the museum at Kew, there may be about two hundred and fifty rings on one side to fifty on the other. Perhaps the largest number of rings that has ever been counted was in the case of an oak felled in 1812, where they amounted to seven hundred and ten; but De Candolle, who mentions this, adds that three hundred years were added to this number as probably covering the remaining rings which it was no longer possible to count. This instance may be taken to illustrate how unsatisfactory this mode of reckoning really is for all but trees of comparatively youthful age.
The external girth measurement is for these reasons the best we can have, being especially applicable where the date of a tree's introduction into a country or of its planting is definitely fixed, since it enables us to argue from the individual specimen or from a number of specimens, not with certainty, but within certain limits of variability, to the rate of growth of that tree as a species. In these measurements of trees of a century or more in age, such as are given abundantly in Loudon's "Arboretum," lies our best guide, though even then the growth in subsequent ages must remain matter of conjecture. The difficulty is to reduce this conjectural quantity to the limits of probability; for, given the ascertained growth of the first century, how shall we estimate the diminished growth of later centuries? The best way would seem to be to take the ascertained growth of the first century, and then to make, say, the third of it the average growth of every century. Thus, if we were to take twelve feet as the ascertained growth of an oak in its first century, four feet would be its constant average rate, and we might conjecture that an oak of forty feet was about a thousand years old. But clearly it might be much less; for the reason for taking the third is not so much that it is a more probable average than the half, as that it is obviously less likely to err on the side of excess of rapidity.
The cypress affords an instance where the approximate certainty of its introduction into England enables us to form some conclusions with regard to its attainable age. The fact of its being first mentioned in Turner's "Names of Herbs," published in 1548, makes it probable that it was not introduced into England before the beginning of that century. But, at all events, the cypress at Fulham, which in 1793 was two feet five inches at three feet from the ground, can not have been planted there before 1674, the year that Compton, the great introducer of foreign trees into England in the seventeenth century, became Bishop of London. That gives a growth of about two feet in the first century; but sometimes it attains a higher rate, as in the case of the cypress planted by Michael Angelo at Chartreux, which was thirteen feet round in 1817, giving the average rate of over four feet in the first three centuries. Now, the cypress at Somma, between Lake Maggiore and Milan, for whose sake Napoleon bent the road out of the straight line, is not more than twenty-three feet in girth, so that the tradition which makes its planting coeval with Christianity would seem doubtful; though if we take three feet as the first century's growth, and take the third as the average, it may evidently have been standing in the time of Cæsar, as an old chronicle of Milan is averred to attest.
The Lebanon cedar first planted at Lambeth in 1683 was only seven feet nine inches (girth measurements alone need be given) one hundred and ten years later. Dr. Uvedale's cedar, planted at Enfield not earlier than 1670, was fifteen feet eight inches when measured in 1835, i. e., one hundred and sixty-five years afterward. And the large cedar at Uxbridge, which was blown down in 1790, was one hundred and eighteen years old when Gilpin measured it in 1776, and found it to be fifteen feet and a half. We should therefore be justified in assuming twelve feet as the possible first century's growth of a cedar even in England; whence we may test the probability of the oldest cedars now on Mount Lebanon having been growing there in the days of King Solomon. In the year 1696 the traveler Maundrell measured one of the largest of them and found it to be twelve yards six inches. Four feet a century being the average rate, the cedar measured by Maundrell would have required only nine centuries to have attained its dimensions of thirty-six feet; so that it need have been no older than the time of Charlemagne, and, allowing for a more rapid growth on a site where it is indigenous, may probably have been considerably younger.
From the claims to antiquity of the cedars of Lebanon let us pass to those of the Tortworth Spanish chestnut in Gloucestershire, which sometimes boasts to be the oldest tree in England, and bears an inscription to the effect that King John held a Parliament beneath it. Sir Robert Atkyns, whose history of that county was published in 1712, usually bears the responsibility of connecting the tree with King John; but he only speaks of it as said by tradition "to have been growing there in the reign of King John. It is nineteen yards in compass, and seems to be several trees incorporated together, and young ones are still growing up which may in time be joined to the old body." It was also probably on hearsay evidence that Evelyn spoke of it as standing on record that a chestnut (at Tamworth) formed a boundary tree in the reign of Stephen. We may assume Evelyn to have meant the tree in question; we may pass the hesitation of tradition between two kings not remote from one another in time; and we may accept fifty-seven feet as the maximum measurement, though no subsequent measurement gives so high dimensions. Now, that a chestnut may attain seventeen feet in its first century is proved by the fact that a chestnut at Nettlecombe, planted within the recollection, and therefore within the life, of Sir John Trevelyan, who died in 1828, was over seventeen feet. But we may be content with fifteen feet for the first century. Then, on the principle of the third as the average, we should require a period of eleven centuries for fifty-seven feet; but that this average would be too low is evident from the fact that in seventy-one years—i. e., between 1766 and 1837—it was proved to have increased two feet in girth. Therefore we should have a diminishing series between, say, fifteen feet a century at one end and a little over two feet a century at the other. This might be at the following rate, taking each figure for the growth of a century: 15 13 10 8 6 3 2 57. By which calculation seven centuries would have been the tree's age when Sir Robert Atkyns declared it to be fifty-seven feet in 1712, an antiquity that would amply satisfy tradition, but could not remove the probability that the tree is not a single trunk, but really a number of different trees that have become incorporated together.
A somewhat similar theory may be applied to the famous Castagna di Cento Cavalli on Mount Etna, so called because a Queen of Aragon and one hundred followers on horseback are said to have taken shelter beneath it from a shower of rain. Brydone, in 1790, measured the circumference to be two hundred and four feet, but it seemed to him that the tree in question, of which only separate trunks remain, was really five separate trees; and though he professed to have found no bark on the insides of the stumps nor on the sides opposite to one another, yet a more recent traveler states, in Murray's guide-book, that this is only true of the southernmost stem, and that one of the masses still standing does show bark all round it, which would of course prove it to be a separate tree. Of the other large chestnuts on Etna the Castagna del Nave is rather larger than the Tortworth specimen, while the Castagna della Galea is seventy-six feet at two feet from the ground. The rich soil of pulverized volcanic ash combined with decomposed vegetable matter probably enabled them to attain their present size within a shorter period than would be implied by such dimensions elsewhere; but whether they are five centuries or ten it is absolutely impossible to conjecture.
The great variability in the rate of growth in trees of the same species is perhaps the most remarkable thing afforded by statistics. We say, for instance, roughly, that the beech grows twice as fast as an oak; but take four beeches mentioned by Loudon, placing their years in one column and their circumference in another:
|One in King's County||at||60||years was||17||feet.|
|One at Foster Hall||"||100||""||12||"|
|One at Courtachy Castle||"||102||""||18||"|
|One in Callendar Park||"||200||""||17||"|
So that of three beeches nearly the same in size one was only sixty, another one hundred and two, and another as much as two hundred. And this variability of rate is still more conspicuous in the oak. De Candolle, who counted the rings of several oaks that had been felled, found one that at two hundred years had only the same circumference that another had attained at fifty. Some had grown slowly at first, and then rapidly; others, like bad racers, had begun fast and ended slowly. And even the diminished rate of growth would not seem to be an invariable rule, for one oak of three hundred and thirty-three years was shown to have increased as much between three hundred and twenty and three hundred and thirty as it had between ninety and one hundred.
This reduces the computation of the age of an oak to little more than guess-work. The Cowthorpe oak, the largest existing in England, reached at one time seventy-eight feet in circumference. Damory's oak, in Dorsetshire, was only ten feet less when it was so decayed that it was cut up and sold for fire-wood in 1755; and the Boddington oak, in the vale of Gloucester, was fifty-four feet at the base when it was burned down in 1790. It is needless to mention other English oaks which are also claimants to a remote antiquity; but it is obvious, from the very variable rate of the growth of oaks, that size establishes no indisputable title, and that the Cowthorpe oak need not therefore be the oldest English oak because it is the largest recorded. From Loudon's statistics of oaks are extracted the following notices of trees, according to their age and girth:
This table not only shows the great variability of growth, but, if we take the three specimens of one hundred years old, gives us the high average of seventeen feet as that of only the first century. Taking, then, as usual, the third as the average growth, we should require rather more than eight centuries for an oak of fifty feet, which reduces to a very small number the oaks in England that can claim a thousand years.
When, therefore, Gilpin, in his "Forest Scenery," speaks of nine hundred years as of no great age for an oak, it must be said that very few oaks can be named which by measurement would sustain their title to that age. Tradition, which is always sentimental, leans naturally to the side of exaggerated longevity. William of Wainfleet gave directions for Magdalen College, Oxford, to be built near the great oak which fell suddenly in the year 1788, and out of which the president's chair was made, in memory of the tree. Gilpin assumes that for the tree to have been called great it must have been five hundred years old, and, therefore, perhaps standing in the time of King Alfred. But it is clear that it need not have been a century old to have fairly earned the title of great, and that, therefore, a period of six centuries may have covered its whole term of existence.
We are certainly apt to underrate the possible rate of growth where a tree meets with altogether favorable conditions. The silver fir was only introduced into England in the seventeenth century by Sergeant Newdigate; and one tree of his planting was thirteen feet round when Evelyn measured it eighty-one years afterward. A comparison of the statistics of growth, as above collected with reference to the oak, indicates with respect to most trees a more rapid rate than is commonly supposed. Let us test the claims of some of the oldest limes. The Swiss used often to commemorate a victory by planting a lime-tree, so that it may be true that the lime still in the square of Fribourg was planted on the day of their victory over Charles the Bold at Morat in 1476. A youth, they say, bore it as a twig into the town, and arriving breathless and exhausted from the battle had only strength to utter the word "Victory!" before he fell down dead. But this tree was only thirteen feet nine inches in 1831, i. e., three hundred and fifty-five years afterward, and it would be extraordinary if a lime had not attained in that period greater bulk than even an oak might have reached in a century. The large lime at Neustadt, in Würtemberg, mentioned by Evelyn as having its boughs supported by columns of stone, was twenty-seven feet when he wrote (1664), and in 1837 it was fifty-four, so that within a period of one hundred and seventy-three years it had gained as much as twenty-seven feet. Consequently, making allowance for diminished growth, we may fairly assume that two hundred years would have been more than enough for the attainment of the circumference of the first twenty-seven feet which it had reached in the time of Evelyn. No English lime appears to have reached such dimensions as would imply a growth of more than three centuries, though the lime at Depeham, near Norwich, which was forty-six feet when Sir Thomas Browne sent his account of it to Evelyn, sufficiently dispels the legend that all limes in this country have come from two plants brought over by Sir John Spelman, who introduced the manufacture of paper into England from Germany, and to whom Queen Elizabeth granted the manor of Portbridge.
It would be natural to expect the greatest longevity in indigenous trees, and, though it has been much disputed what kinds are native to the English soil, etymology alone would indicate that the following trees were of Roman importation: the elm (idmus), the plane (platanus), the poplar (populus), the box (uxus), the chestnut (castanea). The yew, on the contrary, is probably indigenous, though its opponents find some reason for their skepticism in the fact that its larger specimens are chiefly found in church-yards and artificial plantations. In favor of its claim is the fact that its pretensions to longevity seem to be better founded than those of any other English tree, not even excluding the oak. A yew that was dug up from a bog in Queen's County was proved by its rings to have been five hundred and forty-five years of age; yet for the last three hundred years of its life it had grown so slowly that near the circumference one hundred rings were traceable within an inch. Some great and sudden change for the worse in the external conditions may have accounted for so slow a rate; but it would hardly be safe, with such evidence before us, to allow more than three feet a century as the normal growth of a yew, in which case the Fortingal yew in Scotland, fifty-six feet round in 1769, may have lived more than eighteen centuries; and a longevity in proportion must be accorded to the yews at Fountain's Abbey, or to the Tisbury yew in Dorsetshire, which boasts of thirty-seven feet in circumference. Hence tradition in this case would seem to contain nothing incredible when it asserts that the yews on Kingley Bottom, near Chichester, were on their present site when the sea-kings from the North landed on the coast of Sussex.
It is, however, but seldom that any real aid can be derived from tradition in estimating the longevity of trees. We have even to be on our guard against it, especially when it associates the general claim to antiquity by a specific name or event. In the classical period the tendency was as strong as it is still; and we should look to our own legends when tempted to smile at the Delian palm mentioned by Pliny as coeval with Apollo, or at the two oaks at Heraclea as planted by Hercules himself. Pausanias, traveling in Greece in the second century of our era, saw a plane-tree which was said to have been planted by Menelaus when collecting forces for the Trojan war, whence Gilpin gravely inferred that the tree must have been thirteen centuries old when Pausanias saw it. Tacitus calculated that a fig-tree was eight hundred and forty years old because tradition accounted it the tree where under the wolf nursed Romulus and Remus. Nor was Pliny's inference more satisfactory, that three hollies still standing in his day on the site of Tibur must have been older than Rome itself, inasmuch as Tibur was older than Rome, and they were the very trees on which Tiburtus, the founder of the former, saw the flight of birds descend which decided him on the site of his city. There is of course no more reason to believe in the reality of Tiburtus than of Francion, the mythical forefather of France, or of Brute the Trojan, the reputed founder of the British Empire.
These things suffice to justify suspicion of trees associated with particular names, such as Wallace's Oak, or trees claiming to have been planted by St. Dominic or Thomas Aquinas. Our only safe guide is measurement, applied year by year to trees alike of known and of unknown age, of insignificant as of vast dimensions, and recorded in some central annual of botanical information, facilitating the work of comparison and the arrival at something like trustworthy averages. The experiment, moreover, has not been sufficiently tried whether our oldest trees are capable of an increased rate of growth by the application of fresh earth round their roots, favorable though the case of the Tortworth chestnut is to the probability of such a result. Until, therefore, such statistics are more numerous than at present, we must be content to rest in the uncertainty with regard to the ages of trees which the preceding attempt to estimate them makes sufficiently manifest, and to arrive at no more definite conclusion than was long ago arrived at by Pliny, that "vita arborum quarundam immensa credi potest" ("The life of some trees may be believed to be prodigious").—Longman's Magazine.
- Jesse, "Gleanings in Natural History," i, 341.
- Selby, "Forest Trees," 334 (1842).