Fjölrit RALA - 05.12.1999, Blaðsíða 128

126
BIOMASS AND SOIL NUTRIENT POOLS IN NEWZEALAND
ties with planted forests on the Craigiebum Mountains. They were used by Nordmeyer
et al. (1987) to examine some aspects of nutrients and reforestation.
Recent studies to model historical changes or measure current rates of change in
nutrient balances in South Island tussock grasslands have been summarised by Mcln-
tosh (1997). He listed measurements of biomass in different forms ofhigh country
vegetation, and from them presented a single, composite account of reduction in bio-
mass nutrient pools, compiled from a wide range of different environments. He also
summarised results of studies of changes in nutrient concentrations in topsoils at sev-
eral sites, including gains under pastoral development as well as losses under tradi-
tional use. He concluded that annual rates of nutrient loss calculated from reduction in
biomass under pastoral impact, or calculated from estimated losses in grazing systems,
were greatly exceeded by losses measured in soil chemical studies, generally in drier
zones, of recent changes in unamended topsoils.
To help establish a basis for current and fiiture studies, we have begun to collate
from different climates and soils, measurements of element pools in soil and biomass
for least-modified mountain vegetation. We also recorded data on levels of element
pools at different stages in localised sequences of vegetation change, both ecological
degradation under pastoral impact, and with cultural renewal by sown legumes or
planted forests. In this paper we report progress results, orienting these to the ques-
tion: is loss of nutrients from ecosystems a contributor to range deterioration and
thereby to desertification, or is it a consequence of desertification?
This question is of considerable interest in New Zealand and in rangeland environ-
ments in many parts of the world. The comparatively short history of grazing use of
New Zealand grasslands makes it possible to study this issue in a genuinely diachronic
perspective, thereby assisting scientifíc understanding of desertification elsewhere, as
well as assisting New Zealand in its own quest for sustainable land management as
part of the intemational movement for sustainable development.
Our approach is first, to summarise the vegetation changes from tall tussock grass-
land through short tussock grassland to weedy, short grassland that has occurred ex-
tensively under pastoral occupation over the last 140 years. The cultural development
of legume-rich pastures and plantation forests in place of deteriorated grassland over
recent decades is also outlined. Second, we present biomass and nutrient data from
studies selected to represent tall tussock grasslands at different altitudes at several
high country locations. Third, we demonstrate from particular localities, comparative
biomass and soil nutrient pools at different stages of vegetation degradation or cultural
development. In discussion, we try to locate in historical time the losses or gains rep-
resented by these comparisons, and place our results in a geographic perspective with
earlier reported studies.
Changes in tall tussock grasslands under pastoral occupation
Climatic zones and ecological regions of South Island high country used in this paper
are outlined in Wardle (1991), generally following the altitudinal zonation of Cock-
ayne (1928). Since the last major glacial recession, tussock grasslands, herbfíelds and
related formations have established and persisted above timberline in the penalpine
zone and in the alpine zone above. Below timberline, in the driest zone of sub-