Quick Dips

"Warm" Westland Grasslands

Imagine a Westland without trees ... a bleak landscape of grass and low, shrubby bush. That's how it was as recently as 14,000 years ago, at least around Okarito, and that's how it could be again in about 30,000 years after the present inter-Ice Age warm spell runs its course.

How do we know? The answer is in the soil, or more accurately within fossil soil preserved beneath modern soil. Last August, road construction within Okarito Forest revealed a scientifically tantalising discovery -- a layer of peat, five metres down, containing well-preserved plant remains and even whole beetles.

The area is managed by Timberlands West Coast Ltd and, recognising the rarity and importance of the discovery, they promptly notified Dr Peter Almond of Lincoln University's Department of Soil Science. Almond assembled a team of scientists from New Zealand and overseas.

"The geological deposits of Okarito Forest were all formed by glaciers and the presence of peat sandwiched between two packages of gravel tells us that there must have been a time when the glaciers had receded -- a warm period," says Almond.

The timing of past glacial advances and retreats is important in understanding the history of the south Westland landscape, and this is particularly important to Almond's work because it allows an understanding of the ages of land surfaces and soils. In Westland with its very high rainfall, soils change quickly with time and rapidly become very infertile, he says.

"Within peats like the one found at Okarito organic material is well preserved and this includes micro-fossils such as pollen which provides a detailed record of the changes in vegetation and climate throughout the time the peat built up.

"From the types of vegetation identified through the pollen we can tell whether the period during which the peat was formed was as warm as today, or cooler. The last time New Zealand experienced a climate as warm as the present period was about 80,000 years ago, before the last glaciation."

Pollen analysis by Dr Neville Moar of Landcare Research has shown that although the Okarito peat was formed in a warm period it was not as warm as today. The landscape was covered in an open shrubland which passed away with time into a bleak grassland.

"No pollens are present in the layers of gravel below or above the peat so we can assume that when they were laid down it was colder than when the peat accumulated," says Moar.

Knowing when this period of peat formation occurred is important to the scientists working on the "big picture" of Westland's climatic and landscape patterns over the past millennia. The Institute of Geological and Nuclear Sciences is contributing to the project by doing radio-carbon dates and Victoria University's Dr Olav Lian is applying a dating technique new to New Zealand called Optical Luminescence Dating.

Results so far have lived up to their early promise, says Almond.

"We can be quite certain now with this find and others that the tall forests we all associate with Westland today are a feature only of the warmest periods of our geological past. During the last Ice Age, even in the warm periods of it, the vegetation was nothing more than a low, stunted shrubland.

"Although the findings do not have immediate implications for current day to day forest management, they do highlight the dramatic alterations in vegetation that take place with natural climate changes."

If the "big picture" of geological history remains consistent, nature will one day again bring about a decline of Westland's tall forests, but that's still many thousands of years off, says Almond.