Fjölrit RALA - 05.12.1999, Síða 132

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BIOMASS AND SOIL NUTRIENT POOLS IN NEWZEALAND
much influenced by the nutritional status of the vegetation as by its botanical compo-
sition per se. In most situations, clovers (Trifolium spp.), especially those cultivars
which have been bred as forages, are grazed preferentially, but the acceptability to
sheep of many of the more abundant grasses in short tussock grassland, such as Fes-
tuca novae-zelandiae, Agrostis capillaris, and Holcus lanatus may be greatly affected
by the nutritional regime in which they are grown. The great variations in value in
which such species are held by farmers may be related to this phenomenon of nutri-
tional effects on palatability. For example, fescue tussock has had a changing reputa-
tion with farmers, just as it has with scientists, over more than a century. It was ini-
tially highly valued as feed by Buchanan (1880), and by Petrie (1895), less valued by
Petrie (1912) as it increased in prominence in short grasslands, progressively devalued
as feed by Leonard Cockayne and eventually appreciated by him only for its shelter
for other plants. In recent studies, it has been found to be of very low dietary signifi-
cance in grasslands not receiving fertiliser, but is highly responsive in growth and
composition to improved nutrition, and has often been eliminated by preferential
grazing after legume oversowing and topdressing.
In one of his major efforts to make his understanding of ecology usefol to New
Zealand grassland management through the Southern Pastoral Lands Commission
(1920), Leonard Cockayne distinguished between depletion and deterioration. He rec-
ognised that grassland depletion was the loss of vegetative cover and increase in bare
ground, a process leading to man-made deserts, a stage we now define as desertifica-
tion. What Cockayne termed grassland deterioration, a decline in the pastoral qual-
ity of unimproved rangelands by loss of palatable components and increase in unpal-
atable plants, is discemible in the botanical composition of many unimproved grass-
lands, just as it is to be found in mountain grasslands under repetitive systems of live-
stock use in various parts of the world. We can now more easily discem how the proc-
esses of vegetation change induced by animal selection in repetitive grazing systems
can generate need for farther change in management.
Relating such causes and effects in chronological terms is difficult by current
vegetation analysis alone. It may be that many of the continuing floristic changes in
the grasslands, including some invasions, are part of the adjustment to stresses in-
duced by the major pastoral disturbance of more than a century ago. There are in-
stances where forther degradation of vegetation has continued even when current
pastoral impact has ceased. There are other instances where cessation of grazing has
been accompanied by an arrest of degradation, or even its reversal. In many situations,
ecological degradation has been sustained or accelerated under continuing pastoralism
(Treskonova 1991, Connor 1992).
Cultural revegetation of degraded rangeland
Deteriorating nutritional regimes for livestock and concem for resource conservation
if existing conditions were allowed to continue have together prompted gradual de-
parture ffom traditional pastoralism. Over the past four decades, substantial efforts
have been made by scientists and pastoral managers to revegetate degraded rangeland,
especially in the montane zone. For pastoral improvement of sub-humid and humid
range, primary emphasis has been given to legume oversowing with the application of
sulfar and phosphorus materials as fertiliser. Afforestation with conifers has also been