Art. VII.—The Fall of the Leaf.
[Read before the Philosophical Institute of Canterbury, 6th September, 1888.]
Why deciduous trees prevail in the north temperate zone and evergreens in the south is a question which can scarcely fail to intrude itself on any one, however uninquiring, whose experience enables him to institute a comparison between the forests of the two regions in their respective winter periods.
To the emigrant from Britain, whose home is amidst the wilds of the New Zealand bush, the contrast is most apparent. Surrounding himself as he generally does with the plants of his native country, he sees them unaltered in their habit side by side with the indigenous productions, whilst he is constantly reminded by the cold winter nights that the elimatic conditions of his old and new homes do not differ very widely. Hence the reason why this difference is forced upon and kept ever before him.
The mere fact of northern species remaining unaltered when removed to corresponding southern latitudes, and vice versá, is sufficient to show that an answer to this question is not to be found in the existing conditions of soil or climate, while the exceptions that occur in both hemispheres prove that the deciduous and non-deciduous habit cannot be entirely due to the nature of the plants that constitute the respective floras.
For example: We have in Europe the guelder-rose (Viburnum opulus) and the laurustinus (Viburnum tinus), belonging to a purely northern genus, the former deciduous, the latter evergreen; and in New Zealand the ribbon-woods (Plagianthus betulinus and P. divaricatus), evergreen or deciduous according to the situation in which they grow, the only other species of this genus being Australian.
As the fall of the leaf, which is the subject of this inquiry, always takes place at the approach of or during winter, we are justified in concluding that a lowering temperature is the immediate or most important cause of it. The problem we have then before us is, why are not the effects more uniform?
For the examination of these effects there are probably few places that offer such facilities as the Pelorus District, wherein I now write. In the narrow bush-valleys and their numerous branches, with encircling hills running into peaks of 3,000ft. elevation, and in the long, narrow, sheltered sound, though extremely limited in extent, we meet with a variety of
climatic conditions stamped so plainly on the surrounding vegetation that even the most incurious cannot fail to notice them.
That plants cease to grow or increase in volume when the temperature of the air and soil fall below a certain point is too well known to require more than mention; but the degree of cold at which growth ceases varies extremely when many species are observed. For instance, some plants grow only during the warm summer months; others, again, lie dormant during that season, as many of our cultivated bulbs, putting forth their leaves and flowers in autumn, in spring, and even in winter.
That there is a degree of cold which will put each species to rest, and another which would terminate its existence, may, I think, be safely assumed. Between these two effects—the suspension of growth, and death—other effects—the fall of the leaf, and the destruction of the overground portion or ascending axis of the plant—sometimes take place.
Evidently some species are incapable of exhibiting the lastmentioned phenomena, death immediately following the suspension of growth.
Taking again our former examples—the laurustinus and the guelder-rose—we commonly see those shrubs growing together in our gardens, the former in full leaf when the latter is quite bare. A further diminution of temperature, instead of assimilating the appearance of these species, would destroy the laurustinus, leaving the guelder-rose uninjured.
We thus arrive at the conclusion that the evergreen habit may be absolute or conditional; and, secondly, that, though a species may remain unaltered when exposed to a temperature which deprives others of their leaves, the latter may be the hardier, or capable of surviving through the greatest degree of cold.
Of the absolute and conditional evergreens this district furnishes some instructive examples. Thus, the Olearia, hectori,* evergreen in other parts of the colony, is a deciduous shrub in the Pelorus Valley, where it grows in low situations outside the bush, associated with Plagianthus betulinus, Sophora tetraptera, and Fuchsia colensoi, which are also deciduous.
Again, along the shores of the Pelorus Sound the karaka (Corynocarpus lævigatus) is very plentiful; but, finding that it did not occur in the inland valleys, I some years ago raised plants from seed, which were immediately cut to the ground by the first severe frost, and thus destroyed. In this case the
[Footnote] * For the identification of this and many other species I am indebted to the kindness of Mr. T. Kirk, F.L.S.
deep, cold soil of the Pelorus Valley assisted, I have no doubt, in producing the results; still, the fact remains that death followed the suspension of growth without the intermediate effects. From this example we also learn how very close to the verge of their climatic range species are sometimes to be found.
Among deciduous trees the time at which the leaf falls varies considerably when a comparison of species is made, and slightly in the case of individuals of the same species. Thus, in this part of these islands the walnut, ash, Lombardy poplar, and others begin to shed in April, and are quite bare early in May. The weeping willow (Salix babylonica), on the other hand, retains its leaves till June—sometimes to the shortest day. But the retention of its leaves by the latter species depends to a certain extent on the situation in which the trees grow: for instance, where the roots are in contact with running water the leaves turn yellow and fall earlier than do the leaves of trees occupying warm ground. We can thus see that the fall of the leaf is hastened, in species capable of assuming the deciduous habit, by the condition of the soil, just as death is hastened in the case of the absolute evergreen species.
As might naturally be expected, species which lose their leaves early in autumn resume them late in spring, and vice versá. To this general rule I have observed a marked exception—the lemon-scented verbena (Aloysia citrodora), which is here deciduous. Though this tree never sheds its leaves before the end of May, and sometimes retains them till the middle of June, it is, of all my cultivated species, the last to come back into leaf. From the peculiar behaviour of this plant I long suspected that in a slightly milder climate it would become an evergreen: this conjecture I found to be correct, for at The Rocks, Queen Charlotte Sound, a specimen during several winters remained in leaf. The distance between the Pelorus Valley and The Rocks being less than twenty miles in a direct line, the climatic difference must be very small; still, it is sufficient to turn the scale from a deciduous to an evergreen habit, and, as we shall presently see, from a herbaceous to a deciduous or semi-evergreen habit.
This last example shows how the deciduous habit may be reversed as well as induced, and, though exact experiments are wanting, there is good reason for believing that the process of reversion, or converting deciduous into evergreen plants, is possible in all cases, though the capacity for assuming the deciduous habit is confined to a limited number of evergreen species. From this we might conclude that the evergreen is the original form, a conclusion which is strengthened by the fact that certain deciduous species are evergreen when young.
The passage of the deciduous plant into the herbaceous.
under the influence of cold can be well observed amongst our cultivated fuchsias, some of which in this climate merely lose their leaves, while others have their stems wholly or partly cut off. Of several varieties I find Fuchsia fulgens the most susceptible of cold. It is the first to shed its leaves on the approach of winter, during which the stems are invariably destroyed, even when the plants are kept within the shelter of a verandah; yet a specimen taken from here to the Rocks, before mentioned, retains not only its stems, but a portion of its leaves, and on one occasion I noticed it in flower during the winter months.
One more effect of cold—the dwarfing-down of vegetable forms—requires mention: This is well illustrated by our Alseuosmia macrophylla, which in the North Island is a bushy shrub 4ft. to 5ft. in height, but here rarely exceeds 1ft., the stem being seldom branched.
From the foregoing considerations and examples we seem to arrive at the following general conclusions: That, though the majority of existing evergreen plants would at once perish were the temperature of the air and soil in which they grow reduced below a certain point, a few, owing to some structural peculiarity, would shed their leaves or suffer the loss of their stems before they finally succumbed, and that in this latter fact we have the immediate origin of the deciduous tree.
The effects of cold consequent on the changing seasons and the effects produced by the artificial removal of species from their proper habitat to higher latitudes, come within the scope of observation and experiment; but we have now to consider a portion of our subject of a more speculative character—namely, the effects of a lowering temperature through climatic changes.
That various portions of the earth's surface have, during different periods of their history, been subjected to very different climatic conditions is one of the most important facts made known by the researches of geologists. In Europe, where the grape now ripens the reindeer once roamed; and here, in the Wairau Valley, within a few miles of where I write, are the moraines of an ancient glacier shown in the Upper and Lower Travers hills, the latter being only about 1,000ft. above sea-level.
The climatic changes which the earth has undergone are plainly referable to two causes. One, alteration in the elevation of the land, converting low land into alpine climates, is easily understood; the other, though its operation has been on a much more extensive scale, remains, as far as I am aware, yet undiscovered. I refer to the Glacial Period, that followed after the deposition of the tertiary rocks in the northern hemisphere.
We cannot from observation explain the formation of alpine floras, but wherever they exist we see that they correspond in their general aspect with the flora of the surrounding region. Thus, the mountains of Australia and Tasmania are clothed with plants essentially Australian, evergreen trees ascending to the limits of arboreal vegetation.
In like manner, typical New Zealand species—such as Veronicas, Coprosmas, and Pittosporums—are found in our alpine heights. Nor do the few deciduous trees we possess particularly affect those elevated regions. One only seems to owe its deciduous habit to the alpine cold, Plagianthus lyalli, which the late Sir J. Haast observed as “a deciduous tree at and above 3,000ft., but evergreen below that line.”
The two allied species, Plagianthus betulinus and P. divaricata, both deciduous, belong to the lowland country; the former, in this district, invariably growing on the low rich land of the inland valleys, the latter fringing the tideway throughout the sound. A careful scrutiny of the flora of any extensive and varied region will at once reveal the fact that the horizontal and vertical range of species do not always coincide, for it is not the plants of the highest latitude which are invariably found closest to the summits of high mountains. The reason of this becomes apparent when we examine the distribution of the plants within any limited mountainous district traversed by deep valleys, bearing in mind as we do so what has been so clearly pointed out by Liebig in his “Natural Laws of Husbandry”—viz., that “all plants which give landscapes their peculiar character, clothing the plains and mountain-slopes with perennial green, have an underground development, according to the geological or physical condition of the soil, admirably adapted to their perennial existence and propagation.” In this district, for example, we have two very distinct classes of land, the low alluvial flats of the valleys, and the steep hill-sides, with an intermediate class consisting of level terraces, composed of rock-fragments imbedded in gravel and clay.
Now, in the alluvial flats and hill-sides respectively we find certain species of plants which never encroach on each other's territory, though most of them intermingle on the terrace lands. Amongst those which most strictly adhere to their proper habitat are the deciduous Plagianthus betulinus and P. divaricata before mentioned. As both species range to the southern portion of the island we are forced to conclude that it is not inability to withstand cold, but an inability to adapt themselves to any other than the deep soil in which they invariably grow, that determines their vertical range.
It can thus be seen how, in the process of selection which must take place through the elevation of a portion of a
hitherto low-lying region clothed with evergreen vegetation, and the consequent climatic changes, other causes beside the mere lowering of temperature would assist in determining the upland flora, and that amongst the species thus selected those capable of assuming the deciduous habit might be only in part included, or might be wholly omitted.
We may now turn to the second class of climatic changes to which portions of the earth have been subjected. Though the causes which operated to bring about the cold of the European glacial period are unknown, there is abundant evidence that prior to its commencement that continent enjoyed a climate as mild as, or milder than, at present prevails. Whether during this pre-glacial epoch an evergreen vegetation clothed the plains and mountains cannot well be determined, but the fact that Australian forms are found amongst the fossil-plants of the Swiss miocene rocks at least suggests that the flora of the northern and southern hemispheres did not differ as widely then as now.
Assuming that Europe at the close of the tertiary epoch supported a vegetation similar to what is now found in corresponding southern latitudes—that evergreen trees, with a small percentage of deciduous forms, then prevailed—let us ask what would have been the effect of the coming-on and subsequent passing-away of the glacial cold.
Before approaching this question it may be well to review what is known of the glacial period. That movements of upheaval and subsidence on a considerable scale took place in Europe during the continuance of the glacial cold is evidenced by the deposits of the period and the present distribution of animal life on the continent and adjacent islands.
Thus, in the commencement of the period when the cold was most intense, the continent extended westward of the area now occupied by the Hebrides and Ireland, the land being generally higher than the existing islands.
Towards the middle of the period a movement of subsidence commenced, and continued until only what are now the tops of the highest British mountains remained above water, forming an archipelago of small islets, amongst which drifted masses of floating ice, depositing where they grounded quantities of débris, which still remain on the mountain-sides in the form of stratified deposits, containing marine shells. This movement of subsidence was followed by one of elevation which brought the British Islands to their present level. As the land rose, the cold, which had abated during the downward movement, again increased, though not to what it had previously been. Although the greatest cold of the glacial period in Europe was coincident with the greatest elevation of the land, we cannot in any way ascribe the great climatic
changes to alterations of level. Indeed, the most with which we can credit the movements referred to is that they may have helped to bring about the fluctuations of temperature which took place, the minor acting in conjunction with the major cause.
By picturing to ourselves an extension southward of the Arctic regions, and a consequent narrowing of the temperate zone, with a subsequent movement in the opposite direction, we shall probably obtain a fairly correct idea of the climatic changes which took place in Europe between the commencement of the quaternary epoch and our own time. Obviously the effect of those changes, as selective and differentiating agencies, on the vegetation of the region must have been very great. As the temperature lessened, the power of endurance in every species would be tested to the utmost, and all the phenomena displayed by plants under the influence of cold would be exhibited, such as the dwarfing - down and the assumption of the deciduous habit, or, to borrow a term from the zoologist, passing into a state of chill coma.
During this ordeal many species would inevitably perish, and some hitherto evergreen would become deciduous; thus the proportion of the latter to the former would be increased.
Here the question naturally arises, would the deciduous tree have any advantage over the evergreen while these climatic changes were taking place? Or, to put the question in another form, beside the mere loss of vital energy, would the evergreen tree suffer in any way through the lowering temperature while the deciduous form escaped? This, I think, may be answered in the affirmative; for in the winter of 1860 a light fall of snow which visited the Pelorus Valley destroyed numbers of trees throughout the bush, owing to their not being able to sustain the unusual weight. Judging by what then happened, I am satisfied that, were a tithe of our winter rainfall converted into snow, the mixed forests in this district would completely disappear in a few years. On the destruction of the larger trees would follow the death of the undergrowth to which they afford shelter, and which in turn protects the all-pervading surface-roots of its protectors.
Once broken into, and the surrounding conditions permanently changed, our mixed forests are doomed: this the effects of the removal of timber for commercial purposes and the running of cattle in the forests daily teach us.
None of our deciduous trees except the generally-diffused Fuchsia excorticata being found within the precincts of the bush suffered from the fall of snow referred to. Their slender, naked branches could not accumulate a sufficient weight to cause breakage; while, not being overshadowed by loftier trees, they did not incur danger from falling timber. Our
Plagianthus betulinus and Sophora tetraptera, growing as they do in small groves, the trees far apart and interspersed with brambles—Rubus australis and other tangled bushes—invariably recall the woods of the mother-country when observed in winter.
That these deciduous trees would soon overspread much of the low land in this valley were the mixed forest removed, and thus give a more boreal appearance to the district, is shown by the abandoned clearings made by the natives prior to the introduction of cattle and foreign weeds.
Whatever may have been the nature of the European flora at the commencement of the glacial period, when the cold attained its maximum the aspect of the region would be completely altered. What was previously, and is now, the north temperate would be included in the frozen zone, and the limit of arborescent vegetation removed far south.
Having evidence that the reindeer then roamed throughout central France, we cannot give to the shores of the Mediterranean a climate milder than is at present enjoyed by the British Islands. Thus shorn, as it were, of the most genial portion of its climate, the vegetable production of Europe would be correspondingly reduced. Only those species capable of withstanding severe cold and of adapting themselves to every variety of soil could survive the southward movement of the flora enforced by the climatic changes.
As we have seen that the tendency of the lowering temperature would be to induce the deciduous habit, and that in the struggle for existence during the increasing cold the deciduous would have a certain advantage over the evergreen tree, I think we may reasonably conclude that, however largely the latter preponderated in the upper tertiary flora, in the remnant that survived the glacial period the proportion would be greatly altered, perhaps reversed.
A comparison of the British flora with that of New Zealand reveals, besides the deciduous habit of the arborescent species, so frequently referred to, other general differences that demand explanation. For instance, though the phanerogamic plants of the British Islands exceed those found within the New Zealand group, the latter flora contains a much larger number of arborescent forms, while, again, the British species belong to fewer genera and fewer orders than are represented in New Zealand. In the northern flora we seem to have the more or less altered descendants of a few original types, in the southern flora the waifs and strays from some rich and varied botanical region.
As the glacial cold abated, as the snow disappeared from the mountain-heights and the plains were freed from their
icy bondage, the soil capable of supporting vegetable life would demand forms suitable to its varying condition. To supply this demand there would be the scanty remnants of the tertiary flora before referred to—for Europe, cut off from the warmer regions of Africa by the waters of the Mediterranean, would be more dependent on these resources than if it were a portion of an uninterrupted mass of land stretching southward to the torrid zone.
When commenting on the unavoidable destruction of the tertiary species by the glacial cold, I pointed out that deciduous trees would probably form a conspicuous feature in the surviving remnant. Another peculiarity of this remnant would be the preponderance of herbaceous plants; for wherever we observe the effects of a low temperature, whether in high altitudes or high latitudes, we find a larger number of herbaceous than arborescent species.
As Europe, when the glacial cold was at its height, lay entirely within the colder regions of the earth, we may fairly conclude that this was one of the characteristics of its vegetation.
If we now suppose the European continent, as its climate gradually improved, to be reclothed with vegetation by its glacial flora, through a mere multiplication of the individual plants, we can readily imagine how monotonous would be the effect. But such a result would be impossible, for in so diversified a region, and with constantly changing climatic conditions, expansion could not take place without giving rise to variation. New varieties and species would thus appear, the flora being thereby enriched in forms specifically distinct, but belonging to a few generic and ordinal types.
Returning to the British flora, we can now see how the various points in which it differs from the corresponding southern flora—the prevalence of deciduous trees, the preponderance of herbaceous plants, and the comparatively few orders represented—seem capable of explanation by the effects of the glacial cold.
If the prevalence of deciduous trees in the north temperate zone is the result of certain former climatic conditions, it necessarily follows that the vegetation of the southern zone has not been subjected to similar conditions.
To what, then, must we refer the evidence of ice-action in parts of these islands where light falls of snow now rarely occur?
To this question I shall not venture a reply, not having a sufficient knowledge of the geology of the southern hemisphere. I may, however, state that the glacial moraines of the Wairau Valley before mentioned might be accounted for by a former elevation of the land. That the land throughout
this portion of the globe was at some former period more elevated than at present is proved by the distribution of the fauna. For example, Tierra del Fuego and Tasmania must have formed integral portions of the adjacent continents, and the various islands of the New Zealand Archipelago, including the Chathams and Lord Howe Island, were directly or indirectly connected. Still, even if we could show that the evidences of ice-action now observed were due to the period of elevation, it would not in any way prove that the southern hemisphere has not undergone climatic changes similar to those which took place in the north. There is no reason to look for such a difference in the histories of the two regions; for a careful consideration will enable us to perceive that, if the distribution of land and water during that time was analogous to what it is now, the glacial cold may have been simultaneously experienced in both hemispheres, though its traces cannot be readily discovered in the southern vegetation.
For instance, were the island of South Georgia, situated in a latitude corresponding with the north of Scotland, to become capable of supporting vegetable life, it would require stocking from entirely foreign sources. Being at present enveloped in ice to sea-level, it is, of course, destitute of vegetation. A flora having such an origin could afford no direct clue to the former history of its region. If during the glacial period the climates of the northern and southern hemispheres bore the same relation to each other as at present, the condition of the New Zealand Islands, supposing they existed, must have been analogous to that of South Georgia. At most, a scanty vegetation might have been found in the low lands in the northern portion of the group. Under these circumstances the present flora must be chiefly of recent foreign origin, and we discover in it certain general characteristics that seem to favour this view. For instance, the tropical nature of the forest-vegetation, so frequently remarked, has been always accepted as evidence of a former distribution of land which enabled a more northerly flora to extend its range southward. On the other hand, our open lands and mountains furnish numerous species which might belong to colder climes in a colder period. Nor is the presence of plant-remains allied to our forest-vegetation in deposits older than the glacial period necessarily opposed to the recent introduction of this portion of the flora, for there is good evidence that a portion of the vegetation removed by the glacial cold in Europe returned to its old habitat. Thus, in an upper cretaceous deposit at Aix-la-Chapelle, associated with fossil remains of Pandanaceæ and Proteaceæ, are species of the genera Quercus and Juglans now proper to that part of Europe. Though a rigorous process of selection and much differentiation has taken place between
the present and the pre-glacial flora, a connection is clearly traceable.
Whether we confine our attention to limited regions or extend our observations over the earth, we find everywhere that the main features of the vegetation are due to climatic influences. Thus, in these islands we meet with contiguous and sharply-defined districts clothed with dense bush or associated grasses denoting differences in the rainfall. These features we see repeated on a grand scale in the vast forests of the torrid zone and in the broad savannahs and prairies of colder latitudes.
Again, we have wide tracts, subject to long periods of drought, tenanted almost exclusively by annuals, the adaptation to climate being therein evident; and in hot countries where very dry and wet seasons alternate, we have the curious phenomenon of heat-coma, displaying itself by the shedding of leaves.
Among climatic causes, then, it seems as if a reason for the distribution of deciduous and evergreen trees must be found; but, no existing conditions throwing any light on the matter, I have sought an explanation in such records as we have of the past—with what success I must let others, with more knowledge, judge, for as I close these pages I am fully sensible of having undertaken a task for which I was very inadequately prepared. The solution of the question having been, as far as I am aware, hitherto unattempted, if I merely succeed in directing attention to it I shall consider I have done well.