Is ‘vegetation thickening’ occurring in Queensland’s mulga lands – a 50-year aerial photographic analysis
G. Bradd Witt A B , Rebecca A. Harrington A and Manda J. Page AA School of Integrative Systems, The University of Queensland, Gatton Campus, Gatton, Qld 4343, Australia.
B Corresponding author. Email: bwitt@uq.edu.au
Australian Journal of Botany 57(7) 572-582 https://doi.org/10.1071/BT08217
Submitted: 10 December 2008 Accepted: 9 November 2009 Published: 21 December 2009
Abstract
Changes in the density of woody vegetation in arid and semiarid rangelands have the potential to dramatically reduce productivity as well as adversely affect ‘natural’ ecosystem processes. Many parts of Australia are believed to have experienced thickening of woody vegetation since European occupation and the associated changes to fire and grazing regimes that followed. Unfortunately there is little empirical evidence to support a widely held perception of thickening. This study analyses the available historical aerial photographic coverage for the mulga lands bioregion of south-western Queensland, Australia; a record spanning the second half of the twentieth century. Changes in woody vegetation canopy cover were assessed for 190 sites. More than half of the sites had no evidence of mechanical disturbance and thus reflected general, or background trends in woody vegetation cover. The region-wide average extent of change on these undisturbed sites was estimated to be approximately a 3.6% increase in canopy cover over the study period. Thus, a trend toward vegetation thickening was detected. However, large variation was observed depending on the land systems and rainfall zone where sites were located. The results are discussed in the context of century scale climate variability and perceptions of vegetation change and a tentative explanatory model is presented to account for the observed data.
Acknowledgements
We wish to thank Department of Natural Resources Mines and Energy, Queensland staff for assistance with access to archival aerial photography and for access to the SLATS site data. Without these contributions the study would not have been possible; Ms Tara Bell for diligent and comprehensive photographic analysis and field work assistance. Particular acknowledgement needs to be given to landholders in the mulga lands bioregion for support and assistance for use of their properties for ground-truthing and providing accommodation for field work. We thank Mr Russell Fairfax (EPA Queensland) for advice and assistance with research design and his extensive experience with aerial photographic interpretation. Thanks to Allan Lisle who provided advice on analytical and statistical aspects of the study. Jennifer Silcock and Michelle Noël provided welcome feedback on earlier drafts of this manuscript. The project was funded by an Early Career Researcher Grant through The University of Queensland.
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