The first assessment, using a rangeland monitoring system, of change in shrub and tree populations across the arid shrublands of Western Australia

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.