Fjölrit RALA - 05.12.1999, Page 127

RALA Reportno. 200
Changes in biomass and soil nutrient pools of
tall tussock grasslands in New Zealand
Kevin F. O’Connor1, Alan H. Nordmeyer2, and Kristín SvavarsdóttiK
' Centre for Moimtain Studies, Lincoln University, Canterbury, New Zealand
2 New Zealand Forest Research Institute, Rangiora,,Canterbury, New Zealand
Land Resources Division, Knight Frank Ltd, Christchurch, New Zealand;
formerly Hellabv Indigenous Grasslands Trust Research Fellow at Department of Plant Science,
Lincoln University, Canterbuiy, New Zealand
Abstract
This paper addresses the question whether nutrient loss from ecosystems is a cause of rangeland de-
sertification or an outcome.. The history of vegetation changes since humans arrived in New Zealand
is summarily reconstructed, emphasising the early effects of fire and grazing in European pastoralism
in reducing above-ground biomass of tall tussock grasslands that resulted from Polynesian buming of
forest. The paper cóllates available quantitative information on biomass and nutrient pools in soil and
vegetation at different stages of degradation in high country grasslands of South Island. Biomass
quantities in different compartments and N, P, K and Ca contents in green shoots of tall tussocks at
nine different localities are presented, showing great variation that is not well explained by species or
soil differences. From the one comparison possible, reduction in nutrient pools in biomass was largest
from mountain beech forest to tall tussock grassland. Comparable data sets of C, N, P, K and Ca in
biomass compartments and soil layers are presented ffom three sequences of ecological degradation in
different climatic zones. Reduction in nutrient pools in above-ground biomass with degradation from
tall tussock grassland to weedy short grassland is generally significant. Changes in roots were not
consistent. When soil nutrients were included, no difference in N and P total ecosystem pools were
found between stages of degradation. These results were related to historical phases of vegetation
change in the high country with early pastoral fire being credited with major impact on above-ground
nutrient pools. Nutrient conservation by soil microbial systems under degraded vegetation is sug-
gested as an explanation for failure to detect change in soils in moister environments, in contrast with
nutrient losses from less vegetated soils recorded or inferred in dry and or steep terrains. Nutrient
pools in biomass increased with revegetation with both pines and lupins.
Key words: desertification, ecological degradation, New Zealand, nutrient balances, pastoral impact,
tussock grasslands.
INTERPRETING CHANGES 1N RANGELAND NUTRIENT POOLS:
THE OBJECTIVE OF THIS PAPER
A summary account of nutrient pools and fluxes in different plant communities in
New Zealand was included with a review of biomass accumulation by Wardle (1991).
Principal biomass measurements in mountain grassland communities have been those
of Williams (1977), Meurk (1978), and Evans (1980). Williams et al. (1977a, 1977b,
1978a, 1978b,) recorded nutrient concentrations in the principal tall tussock grasses
over their geographic range, indicating some of the relationships of nutrient concen-
trations to selected topsoil properties. The only studies with comprehensive soil
chemical data and biomass nutrient pools from the same sites were the records of car-
bon and nutrients in soils, plant litter and living vegetation, compiled by Forest Re-
search Institute scientists (Kelland 1978, Nordmeyer 1980a, 1980b, Evans 1980).
These studies included nutrient comparisons of natural forest and grassland communi-
i
Corresponding author: Professor Kevin O’Connor, 21 Tuawera Tce., Sumner, Christchurch, New Zealand.
Tel:/Fax: 64 3 326 6594; E-mail: oconnork@Iincoln.ac.nz