The first assessment, using a rangeland monitoring system, of change in shrub and tree populations across the arid shrublands of Western Australia
I. W. Watson A C , P. W. E. Thomas B and W. J. Fletcher AA Department of Agriculture and Food Western Australia and Centre for Management of Arid Environments, PO Box 483, Northam, WA 6401, Australia.
B Department of Agriculture and Food Western Australia and Centre for Management of Arid Environments, Locked Bag No. 4, Bentley Delivery Centre, WA 6983, Australia.
C Corresponding author. Email: iwatson@agric.wa.gov.au
The Rangeland Journal 29(1) 25-37 https://doi.org/10.1071/RJ07018
Submitted: 28 January 2007 Accepted: 10 April 2007 Published: 14 June 2007
Abstract
For the first time, a region-wide assessment of vegetation change across the southern shrublands of Western Australia is reported, using information from 965 shrubland sites of the Western Australian Rangeland Monitoring System (WARMS). The majority of sites were installed between December 1993 and November 1999, and were reassessed between July 1999 and November 2005, with an average interval of just over 5 years. Shrub and tree species density, canopy area and species richness remained the same or increased on the majority of sites. The results were similar when considered at a species level, with most species showing an increase in density, canopy area and the number of sites on which they were found. Recruitment of new individuals to the population was commonplace on virtually all sites and for virtually all species. High rates of recruitment, on many sites, were observed for long-lived species such as Acacia aneura Benth., A. papyrocarpa Benth., Eremophila forrestii F.Muell. and Maireana sedifolia (F.Muell.) Paul G.Wilson. Increases in density, i.e. where recruitment was higher than mortality, were observed for many shorter lived species which are known to decrease in response to excessive grazing (i.e. decreaser species) such as Ptilotus obovatus (Gaudich.) F.Muell., Atriplex vesicaria Benth., A. bunburyana F.Muell. and Maireana georgei (Diels) Paul G.Wilson. However, this result should be tempered by the understanding that acute degradation processes may still be occurring, especially within and surrounding drainage lines, which are away from where the WARMS sites are typically located. Grazing was implicated in decreased density on some sites, particularly those which had experienced below average seasonal conditions. On these sites, decreaser species were particularly affected.
Additional keywords: arid zone shrubs, pastoralism, range condition, range health, shrub demography, woody thickening.
Acknowledgements
The authors thank the editors of this special edition of papers from the 2006 Australian Rangeland Society Conference. Mark Alchin and Rob Richards provided helpful comments on a draft of the conference paper. The paper was considerably improved by the comments of two anonymous reviewers and Paul Erkelenz. Josh Smith produced the map. Funding from the Australian Collaborative Rangeland Information System (ACRIS) partially supported Ian Watson during the writing of this paper. Vanessa Chewings helped calculate the seasonal context data as part of ACRIS. Gary Bastin, as ACRIS co-ordinator, provided much thoughtful discussion on presenting WARMS data, as has Hugh Pringle in the past. The Department of Agriculture and Food Western Australia funds WARMS as a core activity.
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