Tradeoffs between pasture production and plant diversity and soil health attributes of pasture systems of central Queensland, Australia

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Abstract

The clearing land of trees and introduction of exotic pastures to enhance pasture production and associated monetary gains has been a common practise in Queensland. Previous studies on tree clearing emphasised the gains in pasture production, but over periods of less than 10–15 years after clearing, thus potentially misleading land managers who plan to continue grazing beyond that time. The present research follows an integrated approach to quantify the pasture yield and the effects of tree clearing on pasture species composition, soil properties (organic carbon, available N (NO3), pHw and microbial biomass (C and N)), and litter production over time-since-clearing on a grazing property in central Queensland, and to evaluate the implications of our findings for the region. The cleared pasture systems were taken at <5, 11–13 and >33 year age of clearing in comparison to their paired uncleared pastures for three major tree communities representative of the region: Eucalyptus populnea, Eucalyptus melanophloia and Acacia harpophylla. The paper evaluates the effects of clearing on individual attributes as well as an integrated effect of these attributes, i.e. overall ecological services. Pasture production generally increased with clearing but plant diversity, litter production and potential return of N and P through litter decreased. Among soil attributes, clearing of trees adversely impacted upon soil pH and microbial biomass, which play an important role in nutrient availability and mineralisation. This, the initial gains in pasture production are not sustainable over time. The multivariate analysis for such ecological attributes suggests that at the >33 year age of clearing, the ecological state of pasture systems changed compared to that at 5 year or 11–13 year or to the uncleared system. A disturbed pasture system will most likely take longer to revert to the original state compared to the time that would have taken to harvest the benefits. The results are important for landholders and policy makers to comprehend the real gains and losses following tree clearing for pasture development over the long term.

Introduction

Tree clearing has, until recently, commonly been practised to achieve greater pasture production and monetary gains in grazing systems of Queensland. Most of the cleared land is sown to exotic grass species such as Cenchrus ciliaris L. following raking and burning of wooden logs, leading to monoculture pastures. The high rates of clearing in the past especially after the 1950s, and even recently (e.g. in 1999–2001, 577,000 ha of land were cleared per year (Department of Natural Resources and Mines, 2003)), were mainly to develop pastures (94% of total cleared land in 1999–2001). Until 1985, government policies favoured clearing to develop land for pastoral and agricultural systems (Boulter et al., 2000), and thereafter permission was mandatory from the Queensland Department of Natural Resource to clear land, even for clearing of non-remnant vegetation. From September 2004, a change in government policy stopped land clearing. This was predicated mainly on the losses of biodiversity, and not based upon access to information on the long term effects of clearing on productivity of pasture systems or of the effects on soil properties which are equally important for a landholder. Thus, it is difficult for landholders to understand and adapt to the changes in governmental policies, from promoting clearing to that more recently of prohibiting clearing, without provision of unequivocal evidence on the long-term effects of clearing.

Most studies to date (Scanlan and Burrows, 1990, Burrows, 1993, Burrows et al., 1999) have highlighted the production gains from clearing, but these were limited to <10 years of age of cleared pastures (Scanlan, 2002). Indeed, the initial gain in pasture production with clearing is the only attractive phase for the landholders to clear land for pastures. These earlier studies highlighted the gains in pasture production, but did not consider the associated loss of other ecological services. There are increasing concerns that most of the development that occurred in the past involved little understanding of ecosystem functions and of the inherent potential of natural resources available on the Australian continent (Boulter et al., 2000).

To address the ecological issues of clearing trees for gains in pasture production, three major woodland communities, dominated by Eucalyptus populnea F. Muell., E. melanophloia F. Muell. or Acacia harpophylla F. Muell. ex. Benth. were selected on one property to quantify the impacts of clearing on pasture production and composition, soil properties (organic carbon, available N (NO3), pHw and microbial biomass (C and N)), and litter production. The impacts were measured over a time scale since clearing of recent (<5 years), medium (11–13years) and old (>30 years)) age of cleared pastures in comparison to their uncleared (intact) woodland pastures of each tree community.

The present research provides important information for landholders and policy decision makers to guide them while making objective decisions for the good of future pasture systems based upon the integrated effects of long term clearing on production and other ecological attributes of pasture systems. It is based upon a detailed study conducted on a grazing property in central Queensland region.

Section snippets

Materials and methods

Paired sites of cleared and uncleared woodlands for E. populnea (poplar box), E. melanophloia (silver-leaved ironbark) and A. harpophylla (brigalow) communities were selected across three age groups of clearing, i.e. recent (<5 year), medium (11–13 year) and old (>33 year) on a property “Avocet” (30 km south of Emerald) in central Queensland, Australia. The sites were selected with the guidance of research staff at Department of Natural Resources and Mines, and at Environmental Protection

Pasture above-ground biomass

Pasture biomass was on average greater at cleared compared to uncleared sites, with maximum production at medium age of clearing for E. populnea and A. harpophylla, and at recent age of clearing for E. melanophloia (Table 3). However, the gains in pasture biomass were not consistent over time-since-clearing and tended to decline at old compared to medium (in E. populnea and A. harpophylla) or recent age (in E. melanophloia) of clearing in all the tree communities. Interestingly, in E.

Discussion

In the woodlands of east-central Queensland, pasture yield has been shown to increase upon clearing by Burrows (1993); Burrows et al. (1999) and Scanlan and Burrows (1990), and was also evident in the present study. The present research furthermore demonstrated that much of the benefit was evident during the initial years of clearing and was not maintained at the same level over a longer term (>30 years). Cattle benefit more from the greater biomass of introduced exotic pasture species, but for

Conclusions

Increase in pasture production upon clearing was offset by loss of plant species diversity and their functional diversity, and loss of litter production and nutrient return. Together with the loss of soil microbial biomass and the increase in soil pH, these could notably affect other associated ecosystem functions such as nutrient mineralisation, and hence the soil processes that support plant growth. A compromise must be searched for when clearing trees to promote pasture growth while

Acknowledgements

We are thankful to the landholders Mrs. and Mr. Spooner for permission to carry out this research on their property and for their co-operation during the field work. Mr. Robert Lowry helped in soil sampling and processing. Funds were provided by Central Queensland University, Queensland. The statistical advice by Mr. David Reid, Department of Primary Industries, Rockhampton, is gratefully acknowledged. We acknowledge the improvements to the manuscript as suggested by the editor and two

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