TREES AND SHRUBS FOR SHADE AND ORNAMENT
SOME MISCELLANEOUS TREE EXPERIMENTS
Doubtless the most interesting tree in the world is the Sequoia. The mere fact that this is the most gigantic of all existing trees gives it distinction. But it has added interest because it represents a link with the remote past. Of course it might be said that any existing vegetable represents a link with the past, since every race has its lines of ancestry tracing back to primordial times. But the Sequoia represents the past in a somewhat different sense, inasmuch as it has maintained more fixedly the traits of its remote ancestors than has been done by any other tree, probably, that now grows in the northern hemisphere, with the possible exception of the tulip tree, which represents a quite different type of vegetation. The story of the Sequoia's fight for life during the remote geological ages when the climate of the northern hemisphere was changing, has been outlined in an earlier chapter. Could we know the details of the story, we should doubtless find that the ancestors of the Sequoia migrated southward before the chilling blasts of successive glacial epochs, and made their way northward again in the intervening periods. And of course the present age may represent merely another of these interglacial epochs, during which the Sequoia has carried its return march along the coast to about the fortieth parallel of latitude. It maintains in this location its proud position as the one champion of the ancient traditions. And perhaps it will still maintain them in some remote epoch of the future when another ice age has driven man from the northern hemisphere and reduced the civilization of the twentieth century to a half-forgotten tradition. Be that as it may, the Sequoia and its daughter, the redwood, stand to-day as sister giants in an age of pigmies. Individual trees that are still young according to the reckoning of their tribe were gigantic centurions according to human estimates when Columbus discovered America. And Sequoias that are moderately old have witnessed the ceaseless change of the seasons since the period, perhaps, when Moor and Christian were battling for supremacy in Europe in the dark age that preceded the segregation of the modern nations of Europe. The patriarchs of the race were living in the days that saw the building of the Egyptian pyramids. A tree with such racial traditions and with such individual representatives is surely entitled to be considered the most interesting tree in the world. Whoever has camped in a primeval forest of Sequoias or redwoods will attest that merely to enter into the presence of these colossal antediluvians is to experience an almost overwhelming sense of their grandeur. And it is the common experience that this feeling of awe grows day by day and becomes overpowering if you linger like a lost pigmy in the shadow of the giants. From our present standpoint the interest in the Sequoias hinges on the possibility of growing seedlings or transplanting saplings for ornamental purposes in the parks and fields. It is rather strange that the attempt to do this has not been carried out more extensively. Curiously enough, the redwoods are grown more in England than they are anywhere in America outside the regions where they are indigenous. But doubtless the climatic conditions account for this. The trees thrive fairly well in the relatively mild climate of England, but they find the winters of the North Central and the Northeastern United States prohibitive. A tree that has weathered successive ice ages should not mind the winters of the present era, even at the northern boundaries of the United States, one might suppose. But such an inference misses the chief point of the Sequoia's ancestral story. In point of fact, the giant trees are alive today in something like their pristine form because they migrated before the ice sheets and finally found a place of refuge west of the Sierras where they were sheltered from the northern blasts and given protection by the tempered breezes of the Pacific. As compared with the other conifers-pines, spruces, hemlocks, cedars, and the rest-the Sequoias are really tender trees. They are hardy indeed in contrast with their ancestors of still remoter geological times. But they have never developed that extreme hardiness that characterizes their modified and stunted cousins. Nevertheless it has been found possible to raise the Sequoia gigantia as far north as Central New York. But the tree does not really thrive in regions so inhospitable, and the redwood is even more tender. In central and south-central regions of the United States, however, the giant trees can be grown to better advantage, and here they should find a place as ornamental trees that has not hitherto been accorded them. In the region of Washington, D. C., the Sequoia has proved altogether hardy, and of course it may be grown readily anywhere along the Atlantic Coast south of this region. It is a tree of extremely rapid growth, almost equalling the eucalyptus. The redwood also is of such rapid growth under cultivation that it soon overshadows most other trees. Indeed, it grows so rapidly and requires so much room that it is hardly adapted to use as an ornamental tree except in large grounds. I have raised the giant Sequoia (it is known technically as Sequoia gigantia) in the nursery from seed, and the redwood (Sequoia semper-virens) from cuttings as well as from seed. The cuttings do fairly well if started in the fall and treated like cuttings of other conifers. As to the matter of selection and development, the redwood itself may probably be regarded as a comparatively recent variation from the form of the giant Sequoia. The ancestors of the redwood took up their location in the valleys nearer the ocean and were modified until they are considered to rank as distinct species. But the similarity of the two forms is obvious, and the two species stand in a class by themselves-obviously allied to other conifers in the form of leaf and cone and manner of growth, yet so far outranking all others as to be properly thought of as representatives of a unique order of vegetation. Whether further modifications in the giant trees could be wrought by hybridizing the two forms or by selection among variant seedlings is a question of interest. Presumably, such modifications could be brought about were there time for it. But in dealing with a tree that is a mere child when it has outlived half a dozen generations of men, the plant developer feels himself in the presence of forces that lie almost beyond his ken. Moreover the attempt to deal experimentally with the redwood is made difficult by the fact that the tree seldom bears seed. Some of the woodmen claim that it bears once in seven years, but this is doubtless a mere guess, instigated by the popular superstition connected with the number seven. On one occasion, some thirty years ago, I was informed that the redwoods were loaded with seed. I went out with some helpers and gathered a dozen grain sacks or more of the cones, which could be obtained in any desired quantity. On drying the cones I found that the seeds themselves made up half the total weight. There was a good deal of variation in the cones themselves and in the seed from different trees. The seed when dried kept its germinating quality for seven or eight years. But only a very small proportion of the seeds will germinate under any circumstances, even when fresh. This seems to be especially true of seeds collected from the younger trees-a fact that accentuates the already sufficient difficulties that confront the plant developer who cares to undertake the rather discouraging task of experimental breeding with these antique giants. Nevertheless, it should be recorded that a certain amount of work has been done with the redwood, particularly in the way of selecting trees that bear weeping branches. It has been observed that seedlings usually show the characteristic drooping branches of the parent form. In my experience there is less variation among seedlings of this type than among the normal ones. The latter show a rather wide range of variation of foliage, particularly where seed from different localities is sown. Some are much lighter in color than others, and there are various interesting characteristics that may be noted by a close observer, leaving no doubt that there is sufficient material for the purposes of the plant developer. Doubtless anyone who has patience to undertake the task will be able to produce various types of redwoods that will reveal interesting characteristics of the remote racial strains that now are so blended in the existing representatives of the family as to be scarcely observable. I must not attempt to speak except in a general way of the other members of the great tribe of conifers, the merits of most of which, as ornamental trees, are familiar to every garden and landscape architect. There are some scores of genera and some hundreds of species of conifers but the varieties are too numerous and too intricately blended for accurate computation. No other single region has so many forms of evergreens, and ones that show such wide range of variation, as the Pacific Coast region. It has been estimated, indeed, that there are as many species of conifers in California as in all the rest of the world. But the conifers of one kind and another grow everywhere throughout the colder regions of the northern hemisphere, some of them making their way also to parts of the South. Every one of them is an object lesson in the possibility of plant variation; for as a class they represent a modification of leaf form of the most striking character to meet the exigencies of a changing environment. Time was, doubtless, when the ancestors of the conifers had flat, spreading leaves like the leaves of other forms of vegetation. But when the climatic conditions changed, the pampering influences of warmth and moisture being supplanted by the chill and drought that presaged the onset of perpetual winter, a premium was put on the conservation of plant energies. Whereas before the elements favored the tree that could raise its head highest and thrust out the most luxuriant growth of spreading leaves to absorb the carbon from the heavily laden atmosphere, the time now came when the tree that had a smaller system of branches to nourish and a less expansive leaf system had better chance of maintaining existence. So in the lapse of ages, the conditions becoming more and more hard, the trees that varied in the direction of smaller size and narrower leaves had an ever-increasing advantage. These survived where their more rank-growing and luxuriant-leaved fellows perished. Thus generation after generation natural selection operated to modify the size of the trees and to develop a race of trees with narrow leaves, which ultimately were reduced to the form of needles. Such leaves, offering the largest possible surface in proportion to their bulk, could gain nourishment from an impoverished atmosphere, and at the same time would obstruct the rays of the sun but little, so that the entire foliage of the tree might secure a share of the all-essential light which now, age on age, became less and less bright as the earth changed the direction of its axis. Of course there were other trees that did not undergo this modification. But these were forced either to make more rapid migrations to the south or to give up the fight altogether and to submit to extermination. The only ones that were able to maintain existence in the regions where the climate became exceedingly cold were those that had developed the new type of leaf-form, and had learned to conserve their energies to the last degree. But of course the trees that took on this new habit varied among themselves, and as they spread to different regions such variations were developed and fixed under the influence of different environments, until many tribes of needle-leaved trees were developed so differently as to constitute the races that the modern botanist terms pine and spruce and cypress and juniper and hemlock and yew and cedar. Representatives of all the chief genera of conifers have recognized a place among ornamental trees and are everywhere popular in cold climates. The variations among the different species are so obvious as to attract the attention of the least observant. And the opportunity to develop any fixed new form is correspondingly good. I have raised large numbers of conifers of many species, and have experimented with them in the way of selection, producing in some cases varieties of considerable interest. I have, for example, developed several beautiful varieties of the spruce, including some very conspicuous forms with weeping foliage; also some that grew very compactly, being strikingly different in appearance from the usual spruce with its long, graceful branches. Variations in the color of foliage have also been given attention, especially in the case of the Colorado yew cypress (Abies nordmanniana). I have observed variations from budding sprouts in the case of this cypress that were of interest. In particular I have seen a branch in a wild species (a bud sport) that would droop several feet below the other branches. Such a branch may generally be propagated by grafting or from cuttings, and a race of trees having this habit may thus be developed. There are corresponding variations in cypress and other conifers grown from the seed. The Douglas Spruce is a common California form that is quite variable, and in this also the variations sometimes appear as bud sports. The Douglas Spruce has exceptional interest, because it is a tree of very rapid growth. In many cases where a tract of land has been burned over or the trees have been cut off, there will spring up what at first appears to be a growth of oaks alone. But in fifteen or twenty years the growth of Douglas Spruce will entirely overshadow the oaks, ultimately kiling them off altogether, and presenting yet another illustration of the practical operation of natural selection. But there is very great variation among the different species of conifers as to rapidity of growth. So there is fine opportunity for the experimenter to select the more rapid-growing trees, and thus to develop a race of timber trees of exceptional value. The experiment is not difficult because the Douglas Spruce bears seed while quite young, particularly when the trees stand by themselves. The seed remains in the cones for some time, to mature, so that it may be collected at any season of the year. The seeds germinate readily, the seedlings may be easily transplanted, and in general this is one of the easiest conifers with which to work. The hardiness of the tree and its adaptation to all soils and climates are further merits that commend it to the attention of the plant developer, whether he have in mind a tree for ornament or for reforestration. The experimenter should know, however, that the seed of the spruce, unlike that of the redwood and some other conifers, retains its vitality for a short time only. If attention is given to the securing.of fresh seed, the experiments can scarcely fail to go forward successfully. There are, of course, almost numberless other species and varieties of conifers that hold out inviting opportunities for the plant developer. A beginning may be made with almost any varieties that chance to grow in your dooryard, and the facility with which the different varieties may be reproduced, together with the wide range of variation, offer opportunity for selection and insure interesting developments, provided you have patience to wait for them.
SOME DECIDUOUs FAVORITES
But if there are no broad-leaved trees that quite equal the hardiest of the conifers in capacity to withstand cold and to draw nourishment from rocky soils under disheartening conditions, there are a few tribes of deciduous trees that make at least a commendable effort to rival them. Notable among these is the birch. But the beech and oak and maple and hickory and walnut also have representatives that are able to withstand the winter in regions where the mercury freezes. All of these have a certain importance as ornamental trees, but in the main they are valued rather for their timber, and we have dealt with them when we spoke of forest trees. There is a considerable company of trees of less hardy character that nevertheless are resistant enough to thrive in the streets, parks and gardens of our northern States if given a certain amount of protection, even though some of them could not make their way in the wilds in competition with the hardy tribes just mentioned. These trees are less hardy than the others, presumably because they migrated a little more rapidly in the old days of changing climates, and kept far enough away from the ice sheet to be able to retain something of their taste for tropical conditions. They not only retained the broad leaf system, but some of them also retained or developed the habit of bearing handsome flowers-a habit that would have served small purpose for the conifers, since insects could not thrive in cold regions where they remained to battle with the elements. Doubtless the most interesting of these trees that escaped destruction by flight, and the one that has maintained most fixedly the traditions of the Mesozoic era is the tulip tree (Liriodendron). This beautiful tree, with its unique broad glossy leaves and handsome flowers is now the lone representative of its genus. One species alone survives as the remnant of a tribe that flourished abundantly in the Mesozoic age. This species made its way to what is now the southern part of the United States, and has kept up its aristocratic traditions throughout intervening ages of such vast extent that it staggers the mind to attempt to grasp their significance. The thoughtful person cannot well escape a feeling of awe as he stands in the presence of this representative of a race that in the main was gathered to its fathers at a time when the ancestors of man were perhaps still progressing on all fours. But, traditions aside, the tulip tree of today is a thing of beauty, prized for itself, regardless of its ancestry. It makes a fine tree for avenue, dooryard, or park, and it may be grown as far north as New York and New England. Being a monotypic tree, one would not expect it to show very great variation. But no very keen powers of observation are required to see that the tulip trees are not identical, and doubtless their variation is enough to afford opportunities for interesting experiments, though there is nothing on the earth at the present time with which to combine them. Exceptional interest should attach to a line of experiment in which the plant developer is dealing with racial traditions of such antiquity and such fixity. Meantime, the fact that the tree has a beautiful flower gives opportunity for a line of experiment that is usually possible only among herbs and bushes, inasmuch as most of our trees, as the reader is well aware, are wind-fertilized, and hence do not bear conspicuous blossoms. There are several other trees, however, that resemble the tulip tree in the matter of blossom bearing, and that are not altogether unlike it in general appearance, some of which have corresponding interest, being representatives of ancient forms, even if not quite rivaling the tulip tree in the length of their unmodified pedigrees. The catalpa and the magnolia may be named as perhaps the chief representatives of these flowering trees. Both of these are represented by several species, and the representatives of each are subject to considerable variation. There are at least two distinct hybrid catalpas, involving three species, and I have noted great difference in the rapidity of growth of seedlings; also variation in color and abundance of flowers, in length of seed-pods, and in manner of growth of the trees themselves, some being much more upright that others. I have seen magnolia hybrids also, and have thought it matter for surprise that there are not more of them, for the trees are readily cross-fertilized. Doubtless the fact that different species bloom at different seasons largely accounts for the relative infrequency of natural crossing. There is an opportunity to work with the catalpa, and I could scarcely mention a plant that seems to me to give better promise for experiments in crossing and selection than the great family of magnolias. If the seeds are planted while fresh, they germinate readily. The seedlings are easily raised-almost as easily as apples or pears. Among the magnolia seedlings now growing on my grounds, there are some that will grow three or four feet the first season, while others grow as many inches. Some have a branching habit, and others form an upright front. The leaf varies in breadth and length and in general appearance. Some are early bloomers and some are late bloomers. There are different shades of flowers. All in all, there is abundant opportunity for interesting experiments in selective breeding. Among other interesting deciduous trees, all of which afford ready opportunity for experimentation, are the acacia and its relative the locust (the seeds of which may best be made ready for germination by boiling), the alder, which is extremely variable and with which I have made interesting experiments; the ash, which affords excellent chances for hybridization, and is especially promising for timber; and the hawthorne, which has attractive flowers and fruit that are subject to a wide range of variation, and which has exceptional interest because of its not very remote relationship with the great tribe of trees that furnish our chief orchard fruits. The names of the dogwood, the pepper tree, the tree cranberry and numerous others might be added, but regarding each of them substantially the same thing might be said. All offer excellent opportunities for selective breeding; but few or none of them have been extensively worked with hitherto.
THE FINEST OF ORNAMENTAL TREES
There is one peerless tree, however, that I must single out for a few added words of special mention in concluding this brief summary of the more notable among the ornamental trees. This is the elm, a tree that occupies a place apart, having scarcely a rival when we consider the ensemble of qualities that go to make up an ideal ornamental and shade tree. Whoever has visited an old New England village, and has walked through the corridors of elms or looked down the vista of streets arched over by the interlocking branches of the rows of trees on either side, will not be likely to challenge the preeminence of this tree. Nothing could more admirably meet the purposes of a shade and avenue tree. The English elm, which is a more compact grower than the American species, has been widely planted in California. But the American elm thrives here also, although not native to the coast, and it is much less subject to insect pests than is the European species; also the English elm is stiff, and quite lacking in the graceful lines that the American elms so naturally assume. There is a considerable range of variation among American elms, notably in the size of the leaves, and the openness or compactness of growth. Indeed the variation is so great that it is never wise to plant a row of seedling elms along a street or roadside. It is much better in the interest of uniformity to secure good roots and then graft them with cions of a single variety. The slippery elm, which grows in the same regions with the common American species, is a tree of more compact growth, but on the whole not to be compared with the other species. There are natural hybrids, however, between the American elm and the slippery elm that exceed either parent in size, and sometimes are of surpassing beauty. The largest tree that I have ever seen in New England was an elm that grew in Lancaster, my boyhood home, and which I have reason to believe was a hybrid. As I was born and brought up under the elm I have naturally an affection for them greater perhaps that for any other tree. On a visit to my old home I secured branches of the gigantic hybrid and brought them to California, and grafted them on roots of a seedling of the American elm on my place at Santa Rosa. When this grafted tree was only fifteen years old, it was two and a half feet in diameter. Its hybrid character was obvious to all botanists who examined it. Doubtless this accounts for its extraordinary rapidity of growth. This was of course a natural hybrid, but it stands today as an object lesson in the possibility of hybridizing various species of elms and thus producing a tree of extraordinary rapid growth. I have not experimented further with the elm in this direction; but the hybrid tree that thus reproduced the personality of a giant elm in the shadows of which I passed my boyhood-a souvenir that links the home of my mature years with the home of my ancestors-is a source of perpetual pleasure.
This text is from: Luther Burbank: his methods and discoveries and their practical application. Volume 11 Chapter 9