Geoffrey T. Creber’s research while affiliated with Birkbeck, University of London and other places

1. The wood of trees grown in temperate regions shows a periodicity in the form of rings which, with certain known exceptions, accurately reflects the annual cycle of the seasons. The wood thus has a built‐in dating system. 2. Tree rings are not always the same width in successive years; the widths show a positive correlation with variations in environmental factors. 3. Trees in a given geographical area influenced by the same environmental conditions show similar patterns in their ring sequences and may thus be cross‐dated. 4. Ring sequences from cross‐dated trees may be used to construct long, accurate chronologies. This practice, the science of dendrochronology, is now so reliable that it is widely used in archaeology and has also served to calibrate the ¹⁴ dating method. 5. The extent of the variation in a ring sequence may be expressed in the form of a coefficient known as the mean sensitivity. 6. The climatic effects recorded in the growth rings are those which operate at certain crucial periods, such as the growing season. The effects are primarily those of rainfall and temperature; humidity is usually only secondary. 7. Damage by insects or fire may be dated by examination of ring sequences. 8. As tree‐ring sequences correlate positively with contemporaneous meteorological data, they may be used in the reconstruction of past climates. This is the basis of extrapolatory dendroclimatology. 9. New isotope techniques have been developed which indicate the possibility of deducing the ambient temperature of the tree's environment whilst its wood was being formed. 10. Interpretative dendroclimatology aims to deduce from the features of a sample, or preferably from a number of samples of wood, the nature of the climate which influenced their development. 11. The wood of most growth rings is divisible into two zones, earlywood and late‐wood. The former consists of larger, thin‐walled cells while in the latter the cells are smaller and thick‐walled. 12. Beams of light, X‐rays and β‐rays have been used in various instruments to determine the earlywood‐latewood ratio. 13. The earlywood‐latewood ratio is dependent upon genetic as well as climatic factors. Of the latter, summer rainfall is the most potent in producing latewood. 14. The ring‐width data from a number of Recent and fossil wood specimens have been analysed to show that certain deductions may be made about the climate in which they were formed. 15. A histogram technique has been devised to show differences between wood specimens with otherwise similar coefficients. 16. Evidence has been produced in support of earlier work to the effect that ring‐width sequences from different radii of a tree trunk have fundamentally similar features, thus demonstrating ‘circuit uniformity’. 17. The interpretation of the features of tropical woods is more complex. The mode of development of such a wood is the resultant of the interplay of genetical factors, endogenous rhythms and slight variations in environmental conditions. 18. There is evidence that endogenous rhythms are also involved in the development of woods in temperate as well as tropical regions.

SUMMARY Specimens of Woodworthia arizonica Jeffrey trees from the Late Triassic of Arizona, U.S.A. and the Permian of Brazil, typically have horizontal vascular traces that have extremely close contacts with the tracheids of the secondary xylem. In modern gymnospermous and angiospermous trees, such traces terminate on preventitious buds deeply embedded in their bark. Such buds develop into epicormic shoots, after substantial loss of foliage through fire or other damage.

Two fossil tree species, both with unusual characteristics, occur in the Upper Triassic of the Petrified Forest National Park, Arizona, USA and adjacent areas. The first, Schilderia adamanica, has a highly idiosyncratic secondary xylem structure which contains normal uniseriate and broad complex multiseriate ‘herring-bone’ rays. The trunk cross-section of the secondary xylem may be either of a normal pycnoxylic type with a central pith and rays radiating from that or it may consist of appressed ‘xylem masses’ with rays curving towards one another at their extremities. The second, Woodworthia arizonica, has narrow, horizontal vascular traces traversing the entire radial width of the secondary xylem. By analogy with extant tree species, these traces would have terminated on preventitious buds deeply embedded in the bark which, in the case of these fossil trunks, have failed to be silicified. Such buds have the capacity to develop into epicormic shoots when the crown foliage of the tree is damaged. A further specimen of W. arizonica is recorded for the first time from the Permian of southern Brazil. Reconstruction drawings of both trees are produced.

Examination and measurement of many of the trunks attributed to Araucarioxylon arizonicum Knowlton eroded from the Late Triassic Chinle Formation in the Petrified Forest National Park, Arizona demonstrate that the living tree did not closely resemble any of the present-day Araucaria trees of the southern hemisphere as postulated in past reconstructions. The research indicates that it was a tall monopodial tree with branches occurring in a disordered manner on the trunk from the base to the crown. Calculations using the allometric method of Niklas indicate that the trees were of considerable size. The largest recorded trunk has a basal diameter of nearly 3 m and may represent a tree 59 m high, when living. The root system of the A. arizonicum tree consisted of a ring of four to six steeply inclined lateral roots and a massive, vertically directed tap root. Many of the trunks still have their root systems attached, a circumstance that indicates their felling by the cut-bank operations of the local river system. The massive roots of these trunks, particularly the large tap root, are consistent with growth in soft, deep, alluvial soil, and the thin scale bark is to be expected in a tropical climate free from frost.

Typically, the fossil woods in the Upper Triassic Chinle Formation in Petrified Forest National Park, Arizona, USA do not show annual growth rings but contain irregular growth interruptions similar to those found in trees now growing in the humid tropics. These interruptions could be due to endogenous hormonal effects or to occasional local variations in water supply Therefore, it is conclucded that the Late Triassic fossil forests in the park lived in conditions that permitted continuous tree growth. Limited data from areas outside of the park suggests that similar conditions prevailed in adjacent areas in the southwestern United States during the Late Triassic. Although this region is reported to have been under the influence of a monsoonal circulation pattern during the Late Triassic, the growth of the trees does not appear to have been greatly affected.

Certain Upper Triassic tree trunks in the southwestern U.S.A. show evidence of damage similar to that caused in tree trunks at the present day by pocket rot fungi such as Polyporus amarus Hedgc. and Heterobasidion annosum (Fries) Bref. The damaged trunks occur in a relatively thin stratum at the same horizon over a wide area in the Petrified Forest National Park, Arizona and elsewhere in the southwestern United States duting the Late Triassic. If the peculiar pattern of damage was caused, as it seems, by a pathogenic fungus then it can be compared in scale with the extensive destruction of Ulmus trees by Dutch Elm Disease [Ophiostoma (Ceratocystis) ulmi (Buisman)] in recent years in Europe and North America.

The agreement in the pattern of major biomes with that of climatic zonation of the Earth gives a strong indication that climate is the overriding influence controlling the distribution of plant communities. Three aspects of plants which may be preserved in the fossil state, give a signal of the climatic conditions under which they grew: (1) the present climatic association of their "nearest living relative'; (2) leaf physiognomy of arborescent plants; (3) the character of their secondary xylem ("wood' of ordinary usage) reflecting, by the presence or absence of growth rings, the seasonality (or lack of it) in their environment and the potential for tree growth that it offered. The significance and limitations of these "palaeoclimatic signals' as they may be read from the fossil plant record are reviewed and evaluated. The recent demonstration that stomatal frequency of leaves is responsive to changes in ambient carbon dioxide partial pressure offers promise for direct palaeobotanical evidence for past changes in the level of this climatically significant atmospheric constituent. -Authors

The solar energy input into the very high southern latitudes determines the maximum productivity level that could have been achièyed by Antarctic ecosystems in the geological past when there was not a major glaciation. The input of energy must supply all that is needed the primary producers (the green plants) to carry out photosynthesis.Finally, the carnivorous animals feed either on the herbivorous ones or on other carnivorous ones. Parasites feed either on living plants or animals and scavengers feed on the latter2019s dead remains. Every organism in the ecosystem is thus dependent on the energy input either directly or indirectly. The solar input of about 3500 megajoules/m2/yr (Farman and Hamilton 1987; LaGrange 1963) for high latitudes in Antartica will determine for the continent a temperature ecosystem.

The existence of a temperate Antarctic flora in the Permian, Mesozoic and early Tertiary poses a number of problems that are not soluble by reference to any environmental situation obtaining at the present day (i.e. in which plants live in a regime where a warm summer is followed by a winter without sunlight). Certain characteristics of the polar climate during a number of geological periods can be deduced from the remains of forests which grew at very high latitudes where tree growth cannot take place at the present day. The demands of tree growth of the order of 3 to 4 mm increase in trunk radius each year are such that an input of light energy of at least 3000 MJ m ⁻² a ⁻¹ would be necessary. Light energy inputs of this magnitude have been measured at very high latitudes in both North and South polar regions and it would appear that polar forest growth would be possible today if the ambient temperatures were sufficiently raised above their present levels. It is also significant that the forest productivity of about 10 t ha ⁻¹ as estimated for the Lower Cretaceous forest on Alexander Island (Palaeolat. 70°S), is a terrestrial productivity that is only matched at the present day in a variety of temperate and subtropical regions of the world. The evolution of photoperiodic ecotypes in tree species must have facilitated the colonization of polar regions by tree varieties adapted to make all of their annual growth during the long summer light period. There is evidence also that these photoperiodic ecotypes have a high rate of cambial activity concentrating the more rapid growth of wood into the polar summer.

Fossil plant records from a late Carboniferous to early Permian time interval are reviewed for the data that they give on connections and migration routes between floristic provinces, and their palaeolatitudinal positions. Caution is urged in using fragmentary leaf remains as a basis for recognizing affinity between floristic provinces, as illustrated by reference to the genera Phyllotheca and Glossopteris. The distribution of some 38 plant genera from a range of late Palaeozoic areas and localities is made the basis for ordinating them in the form of a dendrogram. An early Permian plate reassembly, based on that of Lin et al. 1985, is assessed in terms of its compatibility with the palaeobotanical evidence.