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Tuatara: Volume 3, Issue 2, August 1950
Climate Relations of Fossil and Recent Floras
Climate Relations of Fossil and Recent Floras
1—Introduction: The Record of Climate Change
Studies of climate change and of the dating of pre-historic events are closely inter-related and have reached their highest development in the northern hemisphere. Fluctuations of temperature and of humidity have left their record in layered sediments, in pollen deposition and in the growth rings of trees. In soils, in peat bogs and in areas of living forest evidence of change may be found. Fiords and lakes, U-shaped valleys, mounds of debris, striated rocks and huge boulders lying far from their place of origin bear eloquent witness to the eroding power of the great ice tongues of which our glaciers are the dwarfed and dwindling vestiges. So thick were the ice sheets formed in high latitudes during the last glaciation that areas as large as Scandinavia were weighed down by their burden and have been slowly rising again in post-glacial time.
How living conditions were affected when the fourth and last Pleistocene glaciation reached its maximum and the long task of melting away the glacier ice was begun, is graphically described by E. M. van Zinderen Bakker in his account of the peat areas of West Holland. The last glacial stage, he says, was characterised by a polar climate with severe cold. The ice-mantle which reached from Scandinavia across Germany to the river Elbe left the west of Holland uncovered, and this region supported a tundra vegetation wherein lived typical arctic animals—reindeer and giant mammoths.
Very slowly the climate became milder. The summers could have been quite warm in the tundra but bitterly cold winters continued still for thousands of years. In the woodless region, where in favourable seasons wandering hunters followed the reindeer herds, the soil remained permanently frozen throughout the winter cold. In the summer superficial layers thawed and water collected in the deeper parts, which could not find an outlet through the ice-floor. In these swamps lived a species-poor flora comprising the water trefoil and a few sedges and mosses, while around them when the cold was less severe grew dwarf birches.
The climate of this period was characterised by warm summers and cold winters and, moreover, by long continuing snowstorms. The accumulating continental ice locked up much moisture which normally drains off the land into the sea. The result was a lowering of ocean levels and withdrawal of the sea from the continental margin. The west of Holland in this period is described by van Zinderen Bakker as a flat sloping terrain that descended toward the great dry plain of page 54 the North Sea, a monotonous plain from which eminences, now islands, stood out as solitary hills. The shore of the North Sea was then some two hundred kilometres from the present day coast while a land bridge linked England with the continent of Europe. He goes on to describe the effects of alternate freezing and thawing, producing stresses whereby the soil still shows peculiar foldings.
Postglacial time is reckoned by various authors as beginning 20,000 years ago, when the withdrawal of the ice sheet began, or 10,000 years ago when the last traces of the influence of the ice-time on the flora and fauna disappeared and the cold times were at an end. More recently the date at which the ice left the region of Ragunda in North-central Sweden and the European ice-sheet split in two parts has been arbitrarily fixed upon as the end of the Pleistocene and the beginning of recent geologic time. This was about 6,500 years B.C.
Dating these past events is a subject fully dealt with in a recent book by Zeuner called “Dating the Past.” A very readable account of varve chronology is given by Brewster in “This Puzzling Planet” and a brief quotation may serve to bring out more vividly the relation between climate and chronology. The method of dating by means of varves began with de Geer in Sweden. Sweden has a heavy winter snowfall, and long summer days to melt the snow away. Its summer-winter cycle is therefore well marked. The result is that the clay deposits both at the heads of the fiords and in the fresh-water lakes tend to show distinct lamination. The annual layers caused by seasonal deposition of coarse and fine silt are called varves. Describing studies by Antevs of an old glacial pond in Connecticut, Brewster says:
“It really takes one's breath away to see what can be made out by this method, concerning events of ten millennia ago. Varve 3001, for example, of date in the ten thousand B.C., was a rather cool year. That particular pond got seven millimeters of winter clay. The next year had about average weather, and the pond filled up twenty millimeters altogether, the clay again a little ahead of the silt, twenty-seven millimeters in all. Next year, the summer was correspondingly cold, the ice melted less than usual, the freshets were small and only five millimeters of silt washed in. In general, the finer winter clay is about the same in amount year after year, becoming finer and more regular as the ice front gets farther and farther away. The difference between different years shows mostly in the summer wash.”
With many gaps, it is true, but often in surprising detail, glimpses of living conditions in the past can be gleaned from the record of the rocks and the embedded remains of prehistoric plants and animals.
2.—Evolution and Climate Change
Glaciations appear to have been of rare occurrence in the geological past and conditions of greater warmth even than the present have page 55 prevailed over such long periods as to have been called the earth's “normal” climates. According to W. D. Matthew, author of a book on climate and evolution which has been recently reprinted, “Theories of alternations of moist and uniform with arid and zonal climates, as elaborated by Chamberlin, are in exact accord with the course of evolution of land vertebrates, when interpreted with due allowance for the probable gaps in the record.”
There may be a fundamental rhythm underlying climate changes, the discovery of which would assist in the elucidation of causes. The period of the larger cycles, however, is not easy to demonstrate. It is possible that a swing to cooler conditions does not always produce extensive glaciation and even that traces of former glaciations have not been preserved on present-day land surfaces. Many theories have been advanced to account for the past occurrence of warmer conditions in polar regions, the recurrence of glacial periods at long intervals, and the less marked differentiation of climate zones in former times. Changes in the circulation of ocean water, in the distribution of land and sea and in the cloud envelope may have produced conditions in the past of which we have no experience.
We look to the continents for the great centres of origin, though isolation on small islands favours the survival of variants. The great upheavals which bring new continents into being initiate new cycles of climate and evolution. The reduction by erosion of an original high relief gradually removes a watershed and produces great plains, a wide continental shelf and shallow seas. Thus climate and erosion cycles are closely related. With new areas for colonisation and new climatic conditions, a new flora and fauna appear.
The distribution of living organism reflects the occurrence of suitable conditions. Plants may survive adverse changes for a time under natural conditions and may be maintained outside their natural range under cultivation. However, for a form to endure over long periods of geological time it must be in harmony with its environment. Plants are less mobile than animals and therefore less fitted to escape the onset of unfavourable conditions. They are said, therefore, to be good thermometers of the past.
For interpreting climate from fossil floras the methods used are to assess the affinities of the largest number of species and to consider the living conditions of a similar assemblage of plants. When the living Metasequoia was discovered in China it was found associated with the same assemblage with which it occurred in high northern latitudes in Eocene time as shown by the fossil deposits of Alaska. “The survival of all these trees with Metasequoia in the valleys of Central China is one of the most remarkable records of forest continuity known in earth history,” says Chaney. “Judging from their page 56 leaves,” he adds, “there has been no great change in any of these trees during several tens of millions of years.”
Besides the remains of plants and animals, including insects, natural deposits and sand or ice eroded rocks have helped to tell the story of past climates.
3.—Mesozoic Climates
(a) Land and sea.
The earliest plant fossils in New Zealand go back something over 142 million years when, according to the most recently published determination, the Jurassic period began. The great continent to the north and west which linked parts of Australia, Malaya, India, Madagascar, Africa and also the north of South America, probably presented its maximum relief in the earlier Permian period, when it was the scene of a great ice age. At the time selected for our starting point (the Rhaetic) it must have ben already reduced to a low-lying area and subject to extensive flooding by the sea. “The Australian region,” says Walkom, “was a continental area of low land covered by wide-spreading fresh-water lakes, and this extended across to New Zealand, which lay on the coastal shelf, and was sometimes covered by sea-water and at other times raised slightly above the sea.”
The subsequent geological history of our island region was discussed by C. A. Fleming in an article in Vol. II, No. 2 of this Journal. For the geological time divisions and their New Zealand equivalents his article may be referred to. As a sequel an attempt is here made to discuss climate mainly in retrospect and from a botanical point of view. No effort is made to provide a balanced treatment of the successive periods. The main object is to discuss climate relationships with a view to finding a basis for future work and to review some of the more recent conrtibutions to the subject.
Interest in times so remote from our own centres round the appearance of the angiosperms ancestral to our modern and distinctive southern flora. Studying our vegetation as we must, under the influence of postglacial climate flux, we have all the more reason to consider parentage and former environments in order to understand adjustments to contemporary conditions. Our survey should begin with a glance, however brief and imperfect, at the old Mesozoic floras, the living descendants of which are now much reduced in area.
(b) Rhaetic (Late or post-Triassic)
Fossil floras of Triassic age are rather poor in the earlier stages and there is evidence from salt and gypsum deposits and from sand erosion, of desert climates. Halle remarks that among the remains of the Mesozoic flora the high percentage of xeromorphic plants is very striking, adding that the Gymnosperms as a group are characterised by a high degree of xeromorphism (i.e., morphological adaptation to withstand drought). As might be expected, however, some Triassic page 57 floras indicate favourable conditions for plant growth. The evidence has been interpreted as indicating a general aridity of Triassic climates though it may be that the desert belts which today lie mainly between 20 and 40 degrees of latitude then existed in higher latitudes. Some of the tree trunks of the Triassic forest of Arizona show well marked rings of growth.
The largest Rhaetic floras are in Sweden and Greenland. These are similar in character and, of course, belong to higher latitudes than New Zealand. Both floras contain numerous ferns, cycadophytes, ginkgos and conifers as well as some scouring rushes and lycopods. A similar type of flora seems to have reached New Zealand. A small number of New Zealand fossils were referred by Arber to the Rhaetic including a ginkgo, a scouring rush, conifers and fern-like plants. They belong to a widespread Mesozoic flora and some of them occur in New Zealand Jurassic deposits. The remains of a variety of aquatic reptiles, some of gigantic size, which have been found in the deposits of Canterbury may indicate warm seas in our latitude, particularly as coral reefs in the Upper Triassic are known to have extended from California into Alaska (60 deg. north latitude). On the whole, climates in this early period appear to have been warmer than now.
(c) Jurassic
The fossils from the New Zealand Rhaetic and Jurassic represent similar kinds of plants to those found in northern latitudes. A Jurassic fern (Cladophlebis), is mentioned by Chaney as having been found in the rocks of northern Alaska, arctic Siberia, Spitzbergen, northern Norway, western Europe from England to Spain and east across France and Germany, northern Africa, from southern Russia across Asia to China and Japan, India, Australia. New Zealand, Patagonia, and Antarctica. “Other wide species,” says this author, “make the Jurassic flora one of the most cosmopolitan known during the history of plant life.” Seward also remarks that although among widely scattered floras there were unquestionably regional peculiarities deeper and greater than the incomplete and fragmentary records appear to indicate, yet such data as we possess lead us to conclude that the Jurassic vegetation was less diversified and less affected by geographical position than that of any other stage in geological history. The significance of the relative lack of regional differences in floras may be partly that the vegetation was somewhat less specialised in adaptations to climate, or in other words showed wide tolerance. Seward warns that the apparent uniformity of the Jurassic period may be due in part to our regarding as contemporary, floras that may have been widely separated in time. In a discussion on the climatic significance of annual rings in fossil woods Antevs says:
page 58“Taking into consideration all the facts, we can say with certainty that the occurrence of very marked zones in Jurassic woods from Spitzbergen and the lack of rings in Jurassic woods from British East Africa indicate marked climatic zones and pronounced annual periodicity in Jurassic times. The occurrence of distinct climatic zones at this time is also indicated in the distribution of the conifers, with Abietineae in the highest latitudes (Spitzbergen and King Charles Land) and a varying percentage of Araucarieae in mid-latitudes.”
A Jurassic flora from Graham Land on the borders of Antarctica supports the view that during the Jurassic warmer climates were experienced than at present. It contains species found also in the fossil beds of England, Europe and North America and is said to show no evidence of stunted growth. Further light might be thrown on the distritubion of Jurassic vegetation in relation to climate from an investigation of the question of elevation and relief. Periods of high relief are indicated by the Permian glaciation, the Triassic deserts and also elevation in the early Jurassic may be indicated by the evidence of fossils for cooler climates, particularly by the reduction of the Ammonites. By the same criteria (Schuchert 1915 p. 852) later Jurassic climates were warm—the insects were large, coral reefs were built, and the ammonites became abundant and varied again. Much of the present New Zealand became submerged during the Jurassic to be uplifted early in the Cretaceous period.
(d) Cretaceous
A rich fossil flora in Greenland indicates a continuance of warm conditions from the Jurassic. It is not surprising, then, that vegetation of the Jurassic type survived in some localities well into the Cretaceous period. Many of the earliest Cretaceous plants were cosmopolitan, and in both hemispheres there is a general absence of angiosperms in the lowest rocks although both in the north and in the sourth rare fossils or microfossils representing flowering plants have been reported from Jurassic strata. Gradual uplift in the New Zealand region during this period must have favoured extensive recolonisation and, as pointed out by Fleming (p.75) it was likely to be a time of active evolution in our flora. Invasion of our forests by angiospers laid the foundations of a modern vegetation and hence the Cretaceous period is one of special biologic interest.
4.—Cainozoic Climates
(a) Tertiary
Eocene. The later Cretaceous climates may have been cool, but parts of the Eocene at least were decidedly warmer, for forests of distinctly tropical aspect and affinities inhabited what are now temperate regions. The London Clay beds yielded a rich harvest of fossil page 59 fruiting bodies and in a monographic study Reid and Chandler showed that the flora contained a large proportion of ligneous species (in contrast to the present British flora!) and that the majority of genera were related to Indo-Malayan plants. They concluded that the flora undoubtedly represented a tropical climate, the mean annual temperature being 70 deg. F. A rich Eocene flora from Aix in the south of France agrees in indicating warmer conditions. The occurrence of such warm conditions in these latitudes is attributed in part at least to the influence of the Tethys sea, the ancestor of the later and deeper Mediterranean, whose waters, warmed in the tropics, then lapped a more northerly coast line than the present sea. It is to be noted in this connection that repeated changes in the flora during the formation of the coal seams in Thuringia (Germany) were demonstrated by Hunger in 1947. In Oregon, however, in latitudes similar to our own, there was a vegetation resembling the tropical rain-forests of Panama.
Oliogocene. A trend to cooler climates seems to have led to a great migration of forest associations. Evidence for this comes from the western United States where, it is claimed, the most complete Tertiary sequence of fossils is to be found. Chaney relates that at Bridge Greek (Oregon), in 1923, he quarried 98 cubic feet of leaf-bearing shale from three pits and on splitting the slabs of ashy shale obtained a total yield of 20,611 specimens, or an average of more than two hundred to a cubic foot.
This flora was much less diverse, however, than the Eocene flora. About twenty-five different kinds of plants were represented, more than half the specimens being of birches and alders. Besides conifers there were oaks, elm, sycamore, basswood, hornbeam and maple. These belong to the Arcto-Tertiary flora, a flora of temperate regions, represented in Alaska in Eocene times, and which, by the end of the Oligocene, seems to have completely displaced the former tropical rainforest of Oregon. Chaney discusses the rate at which species of the walnut family, with heavy fruits, spread, and shows that this southward migration, even if continuous, must have occupied a great lapse of time—some millions of years. Evidence of a southward migration of the temperate forest in eastern Asia corresponding to that from Alaska to Oregon between Eocene and Oliogocene time is afforded by the Fushun flora of southern Manchuria. According to Rott the Upper Oligocene climate of Germany was warm temperate, humid, not tropical.
The possibility, suggested by Fleming (p.77) that a late Oligocene pulsation of the New Zealand part of the Pacific margin could have opened fresh paths for immigration, should also be viewed, in the light of the migrations of the Arcto-Tertiary flora, as an opportunity for a northward extension of the range of Antarctic plant genera, including page 60 Nothofagus! It is interesting in this connection that Dusen assigned an Oligocene age to the Fagus flora of South America. The contrast between this Fagus flora and the succeeding Lower Miocene flora of South America is described in the next section.
Miocene. Climates appear to have been warmer, even tropical in the Middle Miocene. A study of the floras from the high plains of central U.S.A. east of the Rocky Mountains, from Idaho west of the Rockies and from Oregon and California crossing the Cascade Mountains to the Pacific Coast, supports the theory of gradual cooling accompanied by a steady decrease in annual precipitation. Temperature conditions were moderate in the Lower Miocene, with few extremes of high or low. In the succeeding epochs up to the present, temperature means have been lower but with progressively greater extremes of heat and cold. The changes in this region have no doubt been largely influenced by the rise of the Pacific Ranges along the west coast, which thus became effective rainfall barriers and eliminated the moderating effects of the Pacific Ocean from the regions of the interior.
In the Old World many and widely scattered Miocene floras have been studied, the most extensive being that of Switzerland, described by Oswald Heer. This flora includes some 920 species of which 164 were herbaceous flowering plants. Heer concluded that the full Miocene flora of Switzerland could hardly have been less than 3,000 species, whch is far in excess of the number now living there. Associated with the plants were numerous species of insects, and from their combined study Heer was able to work out many interesting details relating to the succession of seasons and to climate requirements. The conclusion reached was that the Swiss Lower Miocene climate was similar to that now prevailing in Louisiana, the Canaries, North Africa and South China, with a mean annual temperature of approximately 60 deg. F. In the Upper Miocene the climate resembled that of Madeira, the south of Sicily, southern Japan and New Georgia, with an annual temperature of 64-66 deg. F.
South American Tertiary floras were reviewed by Berry in 1938. Climate considerations have to take into account the influence of mountain ranges and periods of uplift in Miocene, late Pliocene and quaternary times. The Lower Miocene of the Argentine was warmer and more humid than at present. He describes a fossil flora intermediate in time between the so-called Fagus and Araucaria floras of South America and quite dissimilar in character and appearance from the former. The leaves in the Fagus flora are of small size, tend to be coriaceous in texture, and have a facies difficult to characterise, he says, but recognisable at a glance. Those of the Lower Miocene are much larger and less coriaceous and are for the most part very similar to existing genera. As far as can be judged from the literature the page 61 evidence of South American floras agrees with a cooler Oligocene and warmer Miocene.
Pliocene. According to Axelrod middle Pliocene climates are considered to have been a major factor in the final break up and segregation of major continental Tertiary floras into restricted areas, and in their evolution into modern plant communities. Mid-Pliocene climates of Eupore and North America are thought to have been warm while there is evidence of semi aridity in North America. In New Zealand Oliver concludes, from plant fossils considered to be of Lower Pliocene age, that “a climate warmer than that at present prevailing in Dunedin obtained in that locality when the Kaikorai plant remains were embedded.” Similar conclusions were reached in regard to a fossil flora of somewhat younger age from the Waipaoa Series near Gisborne. Other evidence is discussed by Fleming (pp. 65-86) with the hint that the Pliocene-Pleistocene climate boundary should perhaps go before the Nukumaruan so that two stages hitherto regarded as Pliocene would be included in the Pleistocene. Pollen results from Nukumaruan lignites recently investigated by Couper and Harris suggest the following climates, the lower Maxwell being, of course, the oldest.
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Upper Maxwell
- Lower warm temperate, moderately humid.
- Upper warm temperate, semi-humid.
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Middle Maxwell
- Upper warm temperature, moderately humid.
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Lower Maxwell
- Lower warm temperate, semi-humid.
- Lower warm temperate, moderately humid.
- Upper warm temperate, moderately humid.
It will be seen that there were sample-series from six different levels in the Maxwell formation and that fluctuations in temperature and humidity are indicated. A more detailed description is being prepared for publication. The temperature belts are those of Zotov, and imply that temperatures near sea-level in the Wanganui region may have been at times as cool as now experienced in the southern part of New Zealand. Geologists consider that conditions of high relief were unlikely near the site of deposition so that if the deposits were contemporary with the mid-Pliocene of Europe, the lower temperatures indicated would not be expected.
(b) Pleistocene
A good deal more is known of the Pleistocene or glacial period in the northern hemisphere, where ice sheets advanced over great continental areas, than in the south where there is less land and more sea in cool temperate latitudes.
page 62The glacial period is generally considered to have lasted a little less than a million years. During this time there appear to have been four glaciations with three interglacial periods, the greatest of which was mild and lasted for tens of thousands of years.
Little is known of the New Zealand Pleistocene and although some lignites have been examined and reported on, results are still too fragmentary for comparisons with the northern hemisphere. European floras are known to have been depleted by Pleistocene climates owing to mountain barriers to the south, and evidence is already available that the New Zealand flora suffered casualties.
(c) Recent or Holocene
In the northern hemisphere, generalisation from a very large number of investigations gives a postglacial climate succession which can be closely related to a time scale and the work of Auer on South American peat deposits indicates that the main fluctuations were of world-wide significance. For New Zealand a three-period cycle was worked out by Cranwell and von Post from pollen studies and there seems every justification for applying the northern time scale.
Period I. The first postglacial period corresponds to the northern Boreal, a period of increasing warmth and of comparative aridity which lasted until about 5,000 B.C.
Period II. The period of maximum warmth, referred to as the postglacial climate optimum, lasted for some 4,000 years, ending five centuries or more before the Christian era. An intermediate zone between this and the next, known as the Grenzhorizont, is well developed in some localities and not in others and seems to represent a transition period between seven and five centuries B.C. The first and greater portion of this warm period was more humid, the latter portion drier again and these are the Atlantic and Sub-boreal periods of the Blytt-Sernander scheme.
Period III. This corresponds to the Sub-atlantic period with cooler and moister climates lasting up to the present day. The three or four main periods are sometimes divided into six or eight zones according to local forest succession and indications are reported that rainfall was higher than now until a few centuries ago. There appears to be some evidence of this in New Zealand.
Though our region is well provided with peat deposits, the elucidation of postglacial history presents problems peculiar to our conditions. Our forests contain a large proportion of woody species which do not, like the northern deciduous forests, have wind-borne pollen. As compared with the deciduous forests our vegetation is rich in ferns. There is therefore a large pollen and spore flora to investigate. Much preliminary work is involved in pollen and spore studies, in a general survey of the peat areas, and in collection of materials for sample peat and pollen profiles from different types of area.
5.—The Influence of Past Climates on the Vegetation of New Zealand.
Events in the evolution of New Zealand vegetation of climatic significance are the gradual replacement of the Jurassic flora by an angiosperm flora comprising a deciduous element and a broadleaf evergreen element, loss of the deciduous habit, evolution of a mountain flora, segregation of rainforest of a temperate character, and reduction of area and extinction of former species and genera.
In the foregoing account it has been shown that in the northern hemisphere conditions favouring the most northerly extension of the flowering plants existed during the Eocene, when tropical climates and vegetation occurred in Oregon and in the south of England and France. The deciduous belt was further north than at present and a fossil flora of this character is preserved in the Eocene beds of Alaska. This arcto-Tertiary flora has subsequently migrated southward along three main routes — through North America, through Europe and through Siberia and eastern Asia. Extensive migration appears to have occurred during the Oliogocene but warmer climates in the mid-Miocene may have temporarily halted the southward march. A tendency to greater segregation set in during the Pliocene and with the onset of the Pleistocene cold further migration took place. During the postglacial period forests have moved extensively northward again.
In the south the same general pattern might be looked for, but with some differences due to the different distribution of land and sea. There is not the extensive development of tundra nor is there evidence of a southern conifer belt of the needle-leaf type as found in the north. Our first angiosperm flora appears to have contained a deciduous element which apparently arrived during the Cretaceous. The deciduous habit is favoured by the marked seasonal rhythm of continental climates in high latitudes where long warm days are experienced during the growing season. Since our only Jurassic flowering plant was of tropical affinity if we may trust the name, and in view of warm climates attested by the Jurassic Graham Land flora, we might picture an immigration from the south during cooler Cretaceous climates to explain the deciduous element. But whence the affinity of these plants with northern genera? The question of the origin of the angiosperms is still undecided. Their early development could have been on the Mesozoic continent which is believed to have linked Australasia with lands to the north and west, whence they could have spread north and south as a mixed flora (with less segregation perhaps than in present climates).
The presence of a broadleaved evergreen element in our Cretaceous and early Tertiary flora suggests an early colonisation by an element which should have found its climatic optimum in the Eocene if climates page 64 were as much warmer here as they appear to have ben in the north. Possibly the shallow Tethys sea produced climatic conditions in the north which were not reflected in the south. During the long Oligocene migration of the Arcto-Tertiary flora, southern genera should have extended their northward range along Australasian and South American migration routes. In New Zealand we should have had an influx of plants from the south, whence we might expect the herbaceous element from which our mountain flora evolved. A Graham Land Oligocene flora shows relationships with living plants of Patagonia, Australia and New Zealand.
A return to warmer conditions appears to have been general during the Miocene and our mid-Tertiary flora may have developed many tropical features. It is perhaps to the Miocene that our fossil coconuts belong and also the southern occurrence of the kauris. Careful dating of materials is important. From a study of wood sections in lignites Evans concluded that “kauris, totaras and beeches, and close relatives of our rimus and celery pines made up a large portion of the lignite forming forests of the Tertiary period in the South Island” and he remarked on the apparently anomalous association of beeches and kauris. However, a single dominant species, or a dominant group of allied species, is much commoner in the temperate rainforest than in the tropical, and the segregation of forest types with which we are familiar may well have been a later development.
A recent visitor interested in plant geography remarked on the number of small leaved species and on the “mediterranean” appearance of our “scrub”—features which suggest the influence of rigorous and perhaps more arid conditions. That many species and even genera failed to survive the climates of the glacial period is confirmed by plant fossils in the kaikorai and Waipaoa deposits and the occurrence in Nukumaruan lignites of pollens which could not be matched from the living flora.
These few comments are included not as proposed solutions to problems of the climate history of our vegetation, a subject that demands much further study, but to draw attention to points of general interest and to the need for assistance of geologists in the careful dating of materials.
6.—Methods and Future Work.
The need for accurate determinations of fossils and microfossils cannot be over-emphasised and if palaeo-taxonomy is to be progressive the building up of suitable collections for the identification of plant fragments, seeds and spores should not be neglected. The task of blocking out the broad outlines of the history of our flora and fauna has been attempted by various writers and from different points of view and should be under constant review. Problems of change of page 65 relative land and sea levels are complex but broader features such as the uplift of mountain chains and peneplanation have bio-climatic significance. The conditions of deposition of our lignites and coals may afford valuable indications and in this connection the study of recent bogs and their rates of growth will help. Lake and even sea-bottom sediments await future study.
Palaeo-botanical studies can be supplemented by the application of pollen and spore techniques and a start has been made in this direction. The relation between pollen deposition and the proportion of wind pollinated species in our forest requires study. In other countries studies of surface samples have been resorted to in order to find how living forests are represented in the pollen statistics, the most promising results being obtained from moss cushions, which act as pollen traps. In Switzerland work has been progressing over the last two decades on the pollen-analysis of glacier ice, and has yielded evidence of seasonal and annual deposition. Surveys of atmospheric pollen afford useful indications on pollen and spore dispersal and the effect of wind carriage. If our peat bogs were still in their virgin condition the most profitable course would be to obtain pollen spectra from the upper layers of these bogs for comparison. Many of our peats, however, have been deeply burned. It is all the more important therefore, to seek and utilize any remaining areas where pollen deposition within the last century or two can be studied. This is without doubt the most urgent task confronting the pollen worker.
An important step would be the adoption of a suitable climate classification. Zotov's definition of temperature belts and his characterisation of species common on the Tararuas on this basis has been found useful, but does not take into account the precipitation evaporation factor. The formation of a palaeo-climatology group deserves consideration, not with the immediate object of launching new projects but of securing better co-ordination of various lines of work already in progress. Team work has obvious advantages and a better exchange of information might be effected, leading to a saving of duplication of effort.
Bibliography
Antevs, E. (1925)—The climatological significance of annual rings in fossil wood. Am.J.Sci. 209.
Axelrod, D.I. (1948)—Climate and Evolution in Western North America during middle Pliocene time. Evolution II (2) 127-144.
Bakker, E. M. van Zinreren (1947)—“De West-Nederlandsche Veenplassen, Biologisch-Natuurhistorisch. Amsterdam: Albert de Lange 1947.
Berry, E. W. (1938—Tertiary Flora from Rio Pichileufu, Argentina. Geor.Soc.Amer.Special Papers No. 12 149 pp.
Brewster, E. T. (1942)—“This Puzzling Planet” New Home Library.
Brooks, C. E. P. (1949)—Climate through the ages. Lond. Ernest Benn Ltd. 395. pp.
Chaney, R. W. (1948)—The Ancient Forests of Oregon. Condon Lectures. Eugene, Oregon. 56 pp.
Evans, W. P. (1947)—Note on the Flora which yielded the Tertiary Lignites of Canterbury, Otago and Southland. N.Z.Jour.Sci. & Techn. 19(3). 188-193.
Fleming, C. A. (1944)—Molluscan Evidence of Pliocene Climatic Change in New Zealand. Trans.Roy.Soc.N.Z. 74 (3) 207-220.
Fleming, C. A. (1949)—The Geological History of New Zealand. Tuatara II (2) 72-90.
Garnier, B. J. (1946)—The Climates of New Zealand: according to Thorn-thwaite's Classification. Ann. Assoc.Amer. Geographers XXXVI (3) 151-177.
Gentilli, J. (1948)—Foundations of Australian Bird Geography. Melbourne: The Emu 49 (2) 85-129.
Godwin, H. (1949)—Pollen Analysis of Glaciers in Special Relation to the Formation of various types of glacier bands .Jour. Glaciology. 1 (6) 325-329.
Halle, T. G. (1915)—Some Xerophytic leaf-structures in Mesozoic plants. Geol.Foren.Forhandl.Bd. 37 (5) 493-520. Stockholm.
Harris, W. F. (1949)—Palynology as a Key to the History of New Zealand Vegetation. Proc. VII Pac. Sci. Congr.
Kelle, R. A. (1947)—Notes on Australian Quaternary Climates and Migration. Memoirs Nat. Mus. Victoria 15 28-81.
Knowlton, F. H. (1919)—Evolution of geologic climates. Bull.Geol.Soc. of Amer. 3 499-565.
Matthew, W. D. (1939—Climate and Evolution. N.Y.Acad.Sci.Ann. 24 171-318.
Oliver, W. R. B. (1928)—The Flora of the Waipaoa Series (later Pliocene) of New Zealand .Trans.N.Z.Inst. 59 (2) 287-303.
Oliver, W. R. B. (1936)—The Tertiary Flora of the Kaikorai Valley, Otago. N.Z.Trans.N.Z.Inst. 66 (3) 284-304.
Oliver, W. R. B. (1949)—Geological History of the New Zealand Flora. Proc. VII. Pac.Sci.Congr.
Pettersson, H. (1949)—Exploring the bed of the Ocean. Nature 164 (4168) 468-470.
Raeside, J. D. (1948)—Some Post-Glacial Climate Changes in Canterbury and their effect on Soil Formation. Trans.Roy.Soc.N.Z. 77 (1) 153-171.
Reid, E. M. and Chandler, M. E. J., (1933)—The London Clay Flora. London.
Schuchert, C. (1915)—Historical Geology. Lond. Chapman & Hall.
Seward, A. C. (1941)—Plant Life through the ages. Cambridge Univ. Press. 607 pp.
Thornthwaite, C. W. (1948)—An approach towards a rational classification of Climate. Geographical Review. New York 55-94.
Willett, R. W. (1940)—Pleistocene Glaciation Epochs: Their number, sequence and correlation. Proc. Sixth Pac.Sci.Congr. II 823-827. See also pp. 815-822 (King).
Willett, R. W. (1949)—The Pleistocene snow-line, climatic conditions, and glacial control in New Zealand. Presidential address, Geological Section, Roy.Soc. of N.Z.
Zotov, V. D. (1938)—Some correlations between Vegetation and Climate in New Zealand. N.Z. Jour.Sci. & Tech. XIX (8) 474-487.