North Island seral tussock grasslands 1. Origins and land-use history

Abstract

A review of the history of seral tussock grasslands in the North Island reveals no evidence for their presence below treeline in pre-human times. Tussock grasses were confined to natural non-forest sites, which, below treeline, were dominated not by tussocks, but by low trees, shrubs, and, on oligo-trophic bogs, by Empodisma minus and Gleichenia dicarpa. Non-forest sites below treeline result from cold-air inversion, high water tables, and frequent geomorphic disturbance. Forest clearance by early Maori fires and subsequent recurrent burning of secondary vegetation induced 660 000 ha of seral tussock grassland in three areas of the central North Island: short tussock (Poa cita and Festuca novae-zelandiae) grasslands on the Volcanic Plateau surrounding Lake Taupo; tall tussock (Chionochloa rubra) grasslands on northern and eastern flanks of the Tongariro volcanoes; and tall tussock (Chionochloa rubra) on the Moawhango plateaus. The pattern of deforestation and spread of tussock grasses in a region of low economic value to Maori is correlated with extensive undulating topography at montane-subalpine altitudes, summer-dry climates in rain-shadow areas, and free-draining volcanic ash soils. Of the 660 000 ha in grassland in c. 1840, almost half (310 000 ha) had disappeared by c. 1940, and only 10% (64 000 ha) remains today, mostly in Moawhango Ecological District. Conservation status, reserve design, and the renewable resource value of the grasslands are discussed.

Keywords:

• Polynesian
• tussock grassland
• secondary vegetation
• vegetation change
• fire
• deforestation