A tectonic and climatic framework for plant evolution in New Zealand

The main thrust of this project is to provide a comprehensive framework for the study of plant evolution in New Zealand through interpreting and evaluating the significance to the modern flora of the major geological, geographic and climatic changes since New Zealand separated from Gondwana.

The promise of detailed knowledge on the course and timing of plant evolution, and in particular, species radiations, provided by improved molecular phylogenies raises the possibility of a comprehensive approach to the interaction of climate and landscape change on one hand, and evolutionary change on the other. New Zealand is particularly suited for this sort of study because of the limited land area, its isolation, its well understood flora, and the excellent geological record. Although there have been a number of previous syntheses (1-4), as well as numerous detailed studies on the New Zealand biota, until recently biological and geological information was insufficient for all but the most general of conclusions.

 

Geological background

New Zealand is an extraordinarily dynamic landmass. Unlike the major continental landmasses, it does not have a stable craton as its core, but largely owes its very existence to its position athwart a major plate boundary. For millions of years after the nascent Tasman Sea forced it apart from Antarctica and Australia, the New Zealand landmass slowly subsided until it nearly vanished (reduced to 20% of its current size) in the Oligocene. Uplift in the later Tertiary, associated with the creation of the transcurrent Alpine Fault system, increased the land area and created the first substantial mountains. Tectonic uplift continued to increase in tempo, peaking at around 2 million years ago with the formation of the high Southern Alps. Many areas of the country are still subject to rapid uplift and frequent earthquakes.

The generalised uplift that began roughly 10 million years ago along the Alpine Fault fundamentally changed the nature of the New Zealand landscape. First of all, it created a series of new adaptive zones in the form of permanently ice covered terrain, and alpine and subalpine habitats; second, it promoted the formation of rock-falls, screes, alluvium choked river valleys and plains constructed of coarse debris; third, the taller mountains created a high rainfall, mild, westerly zone and a more continental, drier and more variable eastern zone. The uplift markedly increased the rate at which soils and regolith were turned over, except in the relatively stable northern third of the North Island and eastern Otago-Southland.

In paleogeographic terms, New Zealand has undergone very marked changes, ranging from being a scatter of low-lying islands in the early Oligocene to one massive mountainous mainland island with some small off shore islands during the Pleistocene glaciations. The glacial-interglacial cycles accompanied by fluctuations of sea level by 150 m, tectonism, dune building and offshore volcanism in the north, have resulted in repetitive formation and recoalescene of varying combinations of mainland islands, and size changes of the terrestrial landmass by 50%.

 

Climate background

Although isolated in mid latitude oceans, New Zealand has also had a tumultuous climate history. During the late Cretaceous and early Tertiary, the area that was to become New Zealand lay close to the Antarctic circle, and thus had long dark winters and brief summers. Despite the near polar climate, the oceans were ice-free and relatively warm. In the course of the Tertiary, New Zealand moved equatorward and into warmer oceans, experiencing at times sub-tropical climates. Establishment of the circumpolar ocean with its strong persistent westerly airflow and ocean currents in the mid Tertiary and global cooling intensification in the late Tertiary which culminated in the early Pleistocene, created the present highly variable but oceanic climate regime.

It appears now that fundamental changes in the Earth's geography sparked off reciprocal alterations of the Earth's climate. The massing of land around the North Pole appears to have provided the basis for the giant but unstable ice sheets of the northern hemisphere, while the thermal isolation of Antarctica at the South Pole gave rise to the hypercold but much more stable Antarctic icesheets. More recently the narrowing of the seaways north of Australia promoted the formation of the West Pacific warm pool, heat engine and the source of most of the atmospheric water vapour for the globe.

 

Milankovitch cycles and global climate change

As long as the Earth has existed it has been subject to the pull of other massive planets which cause it to tilt and wobble around its axis, and its orbit to oscillate between more and less elliptical. These orbital changes (known as Milankovitch cycles) affect the amount of solar radiation falling at any particular latitude, but not to any extent total global radiation. The southern and northern hemispheres are essentially out of phase. It has long been known that fluctuations in solar radiation received at high northern hemisphere latitudes affect the waxing and waning of the northern hemisphere ice sheets. The glaciations, that is the regular ebb and flow of ice sheets at the pole and the marked increase and decrease of global temperatures, is in phase with the changing solar radiation at high northern latitudes. One result of this connection is that New Zealand (and all southern latitudes) have a solar radiation budget effectively out of phase with global cooling and warming: during the coldest glacials, New Zealand experiences high summer insolation (as much as 10% or more above that of the present) and low winter radiation. The reverse occurs during warm interglacial peaks. It is also now well established that carbon dioxide levels have fluctuated roughly in synchrony with glacial-interglacial cycles, lowest levels coinciding with glacial cold and highest with interglacial warmth, although there can be prolonged periods where they are out of phase.

Basic information on the nature of climate change during the last 2.5 million years includes studies of glacial deposits, soils, vegetation history, plant isotopic investigations, and multi-proxy investigations of offshore and deep sea marine cores. However, we are still far from a comprehensive understanding of the nature of these changes. There is a need for comprehensive analysis of different climate proxies, and for climate modelling to investigate the relationship between local New Zealand and global climate change.

One promising avenue is the calculation of the direct effects of solar insolation for which recent results have demonstrated a striking correlation between differentiated global ice volume and mean summer solar radiation (5). Edvardsson in collaboration with our group has recently begun calculating radiation curves for New Zealand latitudes. An early finding from this work is that direct summer solar radiation in mid to high southern latitudes appears to explain subtle differences in temperature records from northern and southern polar sites. This is the first direct indication that mid- to high latitude summer insolation has markedly affected the degree of global cooling or warming experienced in the south. Edvardsson's calculations provide information independent from empirical measures to help understand the evolution of New Zealand habitats in the late Tertiary and Quaternary and as such they are helping provide us with a climatic framework to investigate the biological consequences of climate change.

 

 

References cited:

1. Fleming, C. A. (1979) The Geological history of New Zealand and its Life (Auckland University Press., Auckland).
2. McGlone, M. S. (1985) New Zealand Journal of Botany 23, 723 - 749.
3. Cooper, A. & Cooper, R. A. (1995) Proceedings of the Royal Society of London, Series B. 261, 293 - 302.
4. McGlone, M. S., Duncan, R. P. & Heenan, P. B. (2001) Journal of Biogeography 28, 199 - 216.
5. Edvardsson, S., Karlsson, K. G. & Engholm, M. (2002) Astronomy and Astrophysics 384, 689 - 701.