Tuatara: Volume 18, Issue 2, July 1970
Rain Forests and Gondwanaland
Rain Forests and Gondwanaland
In Recent Articles (Dawson 1966, 1967, Dawson and Sneddon 1969) I have compared certain lianes and epiphytes of the lowland New Zealand and montane rain forests of some tropical Pacific islands with ecologically similar growth forms of lowland rain forests in the Asian, African and American tropics.
Firstly, epiphytic species of the liliaceous genera Collospermum and Astelia in New Zealand and elsewhere can be compared with the “bird's nest” ferns, (species of Asplenium, Drynaria, Platycerium), of the lowland tropics of the Pacific, Australia, Asia and Africa (Old World tropics) and with the great array of bromeliaceous epiphytes in America (New World tropics); secondly, the strangling epiphytes of Metrosideros and the hemiepiphytes of Griselinia can be compared with the strangling species of Ficus and such hemiepiphytes as Fagraea and Brassaia in the Old World tropics and with Ficus again and species of such genera as Clusia in the New World tropics; lastly, the liane species of Metrosideros and Mearnsia (probably all should be referred to the latter genus), as well as species of other genera, can be compared with the liane species of Ficus, for example, in the Old World tropics and with yet other genera in the New World tropics.
These comparisons suggest that there has been a considerable amount of parallel evolution of these specialised rain forest growth forms in the New and Old World lowland tropics and in at least some lowland temperate and montane tropical areas of the southern hemisphere.
Such parallel evolution of growth forms and modes of life in taxonomically unrelated groups is not an isolated phenomenon among cither plants or animals. For example, marsupials in the isolation of Australia have paralleled the life forms of the placental mammals elsewhere, and the water storing succulents of the Euphorbiaceae in the South African deserts parallel the Cactaceae of the North American deserts. Such parallelism, however, implies long periods of isolation between the regions involved. With regard to rain forest epiphytes and lianes the New and Old World tropics are at present isolated from each other and probably have been for a very long time. Areas having the southern hemisphere type of rain forest, however, are not all isolated from the lowland rain forests of the Asian region of the Old World tropics at the present time. In New Caledonia, New Guinea, Fiji and other islands the montane forests have at least some of the southern liane and epiphyte genera, while the lowland forests have at least some of the corresponding genera of the Asian tropics. The altitudinal separation of the two forest types would not page 95 seem to provide sufficient isolation for the evolution of parallel growth forms.
Here the Gondwanaland concept may provide an explanation. According to this theory, recently lent support by evidence of sea floor spreading, the land areas of the southern hemisphere as well as India were once joined in a large continent, Gondwanaland, which was probably isolated from northern hemisphere land areas. When Gondwanaland broke up, Africa drifted into contact with Europe, India into contact with Asia, and Australia, New Guinea, New Zealand and the islands to the north into the proximity of the south-east Asian archipelagoes. For Australasia at least evidence from sea floor spreading suggests that the northward drift took place during the Tertiary. Thus the southern liane and epiphyte genera and the type of rain forest in which they occur could have evolved in isolation in some part of Gondwanaland. The occurrence together of “Gondwana” rain forest and Asian rain firest on certain Pacific islands could then be explained by invasion by the latter when drift made this possible.
The fact that the Gondwana rain forest favours higher altitudes in the tropics suggests that it evolved under mild, moist temperate rather than tropical conditions. This leads to the final point that tropical climates are not always necessary for the evolution of a rain forest type of vegetation characterised by an abundance of vascular epiphytes and lianes. What appears to be required is a more or less constantly moist climate without temperature extremes or as Axelrod and Bailey put it, a “highly temperate” climate (Axelrod and Bailey, 1969).
Axelrod, D. I., and Bailey, H. P., 1969: Paleotemperature Analysis of Tertiary Floras. Palaeogeography, Palaeoclimatology, Palaeoecology 6: 163-195.
Dawson, J. W., 1966: Vegetative Features of Griselinia lucida — A New Zealand shrub epiphyte. Tuatara 14: 121-129.
——, 1967: A growth habit comparison of Metrosideros and Ficus. Tuatara 15: 16-24.
——, and Sneddon, B. V., 1969: The New Zealand Rain Forest. A comparison with Tropical Rain Forest. Pacific Science 23: 131-147.