David Cairns, of Levin, asks :-
I was fascinated by your explanation of the change in vegetation of the Sahara. I have often wondered about our south Island eastern side. Is it naturally tussock, what was it like in pre-human times, what would happen if it was left to nature? Would forest come or is it too dry?
Matt McGlone, an ecologist with Landcare Research, responded.
From Gisborne south, the tall mountain ranges create rain-shadows in the lowlands to the east. Hawkes Bay, parts of the Wairarapa, Marlborough, Canterbury and Otago all have annual rainfall less than 1000 mm per year. In summer, the air becomes very dry, scorching winds blow across the mountains and soils dry out.
But, as can be seen today, planted trees will survive quite well in most areas. Before humans arrived, despite the rain-shadow climate being dry by New Zealand standards, it was well within the tolerance of a number of native trees and shrubs. Tall podocarp-hardwood forests and, in somewhat less droughty areas, beech forest, covered most of the landscape. Tall scrub was common on poor soils and in the most droughty areas. However, only in the very driest inland basins of central Otago can we be sure that tussock grassland was part of the natural vegetation cover. We are not clear exactly how much, but the dry terraces and steep rubbly slopes along the rivers in Central Otago probably had a thin grassland and thereby turf. Elsewhere, only large peat bogs had a tussock cover.
Maori settled New Zealand in the late 13th century, maybe as many as 750 years ago, and immediately began to burn the dryland forest and scrub. Within a couple of hundred years they had cleared almost the entire rain-shadow zone of forest. It is not clear why so much land was burnt but one of the major reasons may have been simply to open up the landscape and encourage tussock, scrub and fern land where there was a rich supply of food. Nearly all the lowland and montane tussock grasslands we see today have been created and maintained by humans.
New Zealand vegetation evolved in a land where fire-creating thunderstorms are really rather rare. The vast majority of all forest, scrub and grassland fires in New Zealand are lit by humans. If we stop firing or farming the landscapes, eventually forest will return – even to the very driest areas. Thin-bark totora, once a major forest tree, is making a comeback in many areas. However, because we have introduced so many plants, many of these trees and shrubs will be exotics. Lodgepole pine (Pinus contorta) will be a big winner, but there are many others such as douglas fir, hawthorn, broom, and wild roses. The Te Ara Encyclopedia of New Zealand website has more information, in particular under Ecoregions.