What is natural? – the tussocklands of Otago

The dramatic tussock-lands of Lindis Pass are one the iconic landscapes of the South Island, and much admired by the traveler on their way from Canterbury to Queenstown or beyond. So iconic has this landscape become, it is hard to believe that while the tussock vegetation is “indigenous”, it is not “natural”. Rather, it is a human-induced landscape.

Lindis Pass is part of an extensive “dryland zone” which extends along much of the eastern part of the South Island [see map below right]. Located on the eastern side of the Southern Alps, this region is sheltered from the prevailing westerly wind, which drops the bulk of its rain on the western side of the mountain range. Therefore, this region is dry and drought-prone.

By the time Europeans began exploring the interior of the South Island, this region was “characterised by an almost total absence of forest” (according to botanist John Buchanan in 1869), but logs scattered across the hills indicated that forests had until recently been widespread. Palynological studies (the study of fossil pollen and other plant matter) and radiocarbon dating of partially fossilised logs and charcoal have since found that forest and shrubland used to be extensive in this dryland region, and that fire, since the arrival of humans, has dramatically reduced its distribution.

The composition of pre-human forests and shrublands is not known for certain, but it is likely to have consisted of beech (Nothofagus) forests and other mixed podocarp-hardwood forests on hill and mountain slopes, with small-leaved shrubs and vines dominating the driest intermontane basins and valleys. Pollen studies indicate that tussock vegetation was relatively limited in extent.

With the arrival of Maori in about the 13th century, large-scale fires became increasingly frequent, transforming the landscape from forest and shrubland to tussock of the genus Chionochloa, bracken fern, and shrubland dominated by kanuka and manuka (tea tree) – species which had previously occupied only marginal places in these landscapes. Unlike most of New Zealand’s woody species, tussock grasses can seed prolifically following fire. So they, as well as manuka, kanuka, and bracken, were better adapted than the original woody vegetation to spread rapidly and widely followed Maori arrival. The conditions that created these tussock grasslands were historically unique – a regime of occasional widespread fire disturbance, in the absence of fire-adapted weeds and of bird and mammal grazers.

From 1850, European settlement and pastural farming brought about a second wave of transformation on this dryland region. Pastoral (mainly sheep) farmers burned the vegetation at least annually to promote more vigorous regrowth, eliminating woody plants in all but the most protected valleys.  Sheep and rabbits soon destroyed any leafy plants that regenerated, and remnant vegetation soon became confined to inaccessible places such as cliffs. The effect of these two waves of transformation on the vegetation pattern can be seen in the diagram above left (C. rubra and  C. rigida are both large tussocks from the genus Chionochloa).

Most importantly perhaps, the current vegetation patterns have implications for the future of this landscape. Ecologists such as Susan Walker tell us that the native shrub species, such as matagouri, kanuka and bracken (the latter in particular being a very good host for woody regeneration) should not be regarded as weeds, but as a slow natural succession back to an inherently woodier landscape (one comprised of larger trees and shrubs).  This landscape will never resemble the pre-human one, by virtue of the fact that important grazing species such a moa are now extinct and that the environment as a whole is irrevocably altered by human intervention – however, our descendants may see the day when stands of beech trees again grace the slopes of the Lindis Pass and shrubs and vines fill its valleys.

Sources: “Long-term dynamics and rehabilitation of woody ecosystems in dryland South Island” (2009), New Zealand, by Susan Walker et al, in R.J. Hobbs and K.N. Suding (eds.) New models for ecosystems dynamics and restoration; Presettlement ecosystems of the Wakatipu Basin (presentation to Forest & Bird, September 2009) by Susan Walker.

Photo: View of Lindis Pass from SH8 (Photo by Rainer Kant). Map and diagram not to be reproduced without permission from Susan Walker.

Acknowledgements to Dr Susan Walker (Landcare Research) for kindly providing copies of her papers and presentation and providing comments on the article.

See also: Challenging assumptions – what is natural? ; What is natural? – The case of the Christchurch Port Hills.