Influence of climate and community composition on the population demography of pasture species in semi-arid Australia

Abstract

Substantial recruitment of Callitris glaucophylla in woodland, Sclerolaena birchii in cleared woodland, and Astrebla lappacea in grassland is related to catastrophic events of the past century in the form of interactions between climate, the impact of European land use (sheep, cattle, rabbits) and the rabbit myxoma epizootic. The direct effect of rainfall on the demography of these species and its indirect effect through competition via suites of accompanying plant species are examined. Major long-term changes in plant populations are generated by extreme sequential events rather than by random isolated events. One of the most potent climatic agents for change in eastern Australia is the E1 Nifio/Southern Oscillation phenomenon.

Demographic patterns of a widespread long‐lived tree are associated with rainfall and disturbances along rainfall gradients in SE Australia

“…Farms In contrast, C. glaucophylla has recruited frequently over the past 60 years in high rainfall areas (>405 mm mean annual rainfall), consistent with trends in a continentalscale survey by Prior et al (2011). For the first half of the 20th century, C. glaucophylla populations contained plants derived from the late 1800s and earlier, with no younger cohorts (Lacey 1972;Austin and Williams 1988). Repeated recruitment since1950s means that population density has increased (Lacey 1972;Austin and Williams 1988) and mean population age has declined.…”

“…Rainfall in these years was well above the mean annual rainfall or in the top decile rank (Bureau of Meteorology 2005). These events triggered widespread recruitment of woody plants including C. glaucophylla in SE Australia (Austin and Williams 1988;Harrington 1991;Denny 1992;Read 1995;Noble 1997;Parker and Lunt 2000), but only limited recruitment of woody species in arid and semi-arid regions (Hall et al 1964;Chesterfield and Parsons 1985), and sometimes only within fenced areas (Hall et al 1964). Consequently, the absence of C. glaucophylla recruitment during the 1900s from this low rainfall region (especially winter-rainfall region) is unlikely to be due to inadequate rainfall.…”

“…In a continental-scale survey of C. columellaris, recruitment was successful in tropical and temperate climates but absent in arid climates and linked to domestic stock grazing (Prior et al 2011). In smaller scale studies, recruitment of Callitris species was associated with above average rainfall and absent under the influence of grazing by sheep or cattle and high intensity fires (Austin and Williams 1988;Bowman and Latz 1993;Noble 1997;Prior et al 2010;Russell-Smith et al 2012). No studies have aged C. glaucophylla individuals to determine relationships between recruitment, rainfall events, and disturbance regimes across climatic gradients.…”

How do slow-growing, fire-sensitive conifers survive in flammable eucalypt woodlands?

“…Trees are killed by 100% crown scorch, although larger trees may survive low-intensity fires (Lacey 1973). Seeds are shed on maturity (Lacey 1973) and seedling establishment is dependent on above average rainfall (Austin & Williams 1988). By comparison, Eucalyptus species in the study area resprout and regenerate from seedlings after fires (Lindsay 1967;Gill & Bradstock 1992).…”

Synergy between small‐ and large‐scale feedbacks of vegetation on the water cycle

“…The idea that woodlands and herbaceous cover may represent alternative stable states is also in line with the observation that extreme events may be needed to trigger a shift. For example, in semiarid Australia successful seedling establishment of mulga (Nicholls, 1992) as well as Eucalyptus and conifer woodlands (Austin & Williams, 1988) has been confined to extremely wet periods associated with ENSO events. Similarly, in the Chihuahuan desert large increases in shrub cover over the past decades has been related to extreme episodes of high winter precipitation (Brown et al, 1997).…”