Fjölrit RALA - 05.12.1999, Blaðsíða 130

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BIOMASS AND SOIL NUTRIENT POOLS IN NEWZEALAND
shrubby vegetation formed scrub or heath; where it was open, shrubland. Only in
situations where topography offered no refuge ffom recurrent fire for woody plants
were grasslands free of shrubs. The varied density and stature of shrubs at the onset of
pastoralism may have reflected differences in lapse of time since the last fire. Early
European explorers of the interior frequently met conditions which required fíre to
allow easy passage.
For the high country pastoral settlers of the late 1850s, fire was often essential for
access with new flocks of sheep, especially because of scrub and the great mass of
vegetation. Following initial depasturing of animals, pastoralists soon leamed the use-
fulness of fíre to aid the management of grazing (Butler 1862). Buchanan (1868),
fresh from his earlier experience in Marlborough (Buchanan 1867), wamed against
buming in dry districts of Otago. Thousands of hectares at a time of tall tussock
grassland or scmb were burned in the first decade of pastoral occupation. Scmb was
often slow to recover, whereas over most of their natural range tall tussocks recovered
readily from fíre. ff, however, grazing pressure after buming was sufficiently sus-
tained, pastoral impact often achieved the transformation of such tall tussock grass-
lands to short tussock grasslands, usually of Festuca and Poa communities. Festuca
novae-zelandiae and Poa colensoi became the most abundant tussocks of induced
short tussock grasslands in montane and lower subalpine zones of interior subhumid
and humid districts from Southland to North Canterbury.
Although this primary conversion of dense tall tussock grasslands to more open,
shorter grasslands more suitable as pasturage, was the conscious work of managed
pastoral impact, and was recorded by early surveyors (O’Connor 1986), the dramatic
nature of this early transformation was not properly appreciated by scientists until
relatively recent times. The careful phytosociologic analyses of Connor (1964, 1965)
indicated derivation of many short grassland communities from tall tussock Chiono-
chloa grasslands, dominated by C. flavescens, C. macra, C. rigida, C. rubra, or C.
pallens. Until that revelation, opinions of resident and visiting scientists generally fa-
voured the interpretation that climate, especially summer droughtiness, controlled the
distribution of short grasslands.
For nearly a century, there is little suggestion in the writings of any naturalist or
scientist of any awareness of this early vegetation transformation. It is evident fforn
the descriptions of the grasslands in Otago by Petrie (1883, 1912) and by Leonard
Cockayne (1899) in Canterbury that any major transformation of tall tussock grass-
lands to short grasslands must have occurred in the early years of pastoral settlement.
Under the combined influence of grazing by rabbits since 1870s and sheep which
reached their maximum by late 1880s, unimproved rangeland began to decline in live-
stock carrying capacity. Analyses of local histories and of statistical records of live-
stock and land use (O’Connor and Kerr 1978, O’Connor 1980, 1982, 1986, O’Connor
and Harris 1992) have established that livestock numbers on unimproved rangeland
generally reached their peak within 25 years of its first pastoral occupation, oscillated
for a decade or more, and thereafter declined. Such declines were especially large and
sustained in drier regions. This feature has been demonstrated (Harris and O’Connor
1998) especially in drier sectors of the Mackenzie Country, even though Mackenzie
County as a whole had not shown serious decline.