Demographic characteristics of mallee pine (Callitris verrucosa) in fire-prone mallee communities of central New South Wales

Abstract

Aspects of the demography of Callitris preissii subsp. verrucosa (Cunn. ex. Endl.) J.Garden populations were studied in semi-arid mallee communities situated on sandy soils in central New South Wales. Rates of survival and fecundity were estimated in a range of populations of differing age (year of origin or last fire: 1920, 1957, 1972, 1974 and 1985) that were monitored between 1987 and 1997. Rates of survival of juveniles and adults were high (>0.99 p.a.), although juveniles less than 5 years of age survived at lower rates. Estimated seedbank at the time of first tagging (no. of closed cones per plant) followed an increasing trend with plant age. Density, overstorey, topography and within-age-class site factors had significant effects on stored seedbanks in particular age-classes. There was high interannual variation (1989 v. 1990) in the size of crops of new cones. Trends in seedbank with plant age and seed release rates from tagged cones indicated that the species exhibits strong serotiny. Measurements of litter weight and depth beneath plants from 1920 and 1957 age-classes, indicated relatively low fuel loadings (about 0.2�kg m–2). Bark was sufficiently thick in c. 60-year-old plants to render stems resistant to low-intensity fires. Thus, it is predicted that the probability of propagation of fire in populations >50 years old may decline and that survival of fire in this age range may increase. Given patterns of survivorship, fecundity and seedbank accumulation, populations may decline when fires are relatively frequent (<15-year interval). At longer intervals there is potential for population densities to remain stable or increase, although the nature of variability in sizes of individual seed crops may have a predominant influence on dynamics. The combination of strong serotiny and negative feedback effects of plant populations on their flammability is a paradox, given the likelihood that seedling establishment is tightly keyed to fire.

Effect of landscape fires on the demography of the endangered New Caledonian conifer Callitris sulcata

“…The high proportion of dead trees, especially in the small size classes, recorded in the recently burnt plots is strong evidence for fire being a major cause of mortality for C. sulcata. This is also the case for many Australian Callitris species (Bowman and Panton, 1993;Bradstock and Cohn, 2002;O'Donnell et al, 2010;Prior et al, 2010;Cohn et al, 2011).…”

The tortoise and the hare? Post-fire regeneration in mixed Eucalyptus - Callitris forest

“…Australian ecologists have commonly interpreted Callitris dynamics in fire-prone ecosystems in relation to immaturity risk, or the risk of repeated fires occurring before regenerating plants set adequate seed Russell-Smith et al 1998;Bradstock and Cohn 2002;Thompson and Eldridge 2005). This attribute is clearly important at Mt Pilot, as elsewhere.…”

“…Callitris populations at Mt Pilot will remain susceptible to fires for many years as only 2% of plants had formed seed cones within 7 years of the 2003 fire. A fire-free period of at least 15 years is considered necessary to maintain populations of Callitris species elsewhere (Price and Bowman 1994;RussellSmith et al 1998;Bradstock and Cohn 2002).…”

“…Clayton-Greene 1981; Clayton-Greene and Ashton 1990), even though Callitris occur in heathlands, woodlands and forests dominated by Eucalyptus and other woody plants (Bowman and Harris 1995). In savannah and arid woodlands, fire regimes and Callitris dynamics are regulated by dominant grasses rather than woody plants Bradstock and Cohn 2002). Indeed, we know of only one study (an unpublished seedling pot trial) that has documented competitive interactions between Callitris seedlings and other woody taxa (Clayton-Greene 1981).…”

“…Callitris dynamics have received considerable attention from fire ecologists as Callitris are often killed by fire and usually do not resprout after being subjected to 100% leaf scorch (Lacey 1973;Bradstock and Cohn 2002;Russell-Smith 2006). Consequently, population dynamics are highly sensitive to changes in fire regimes (Bowman and Panton 1993;Bradstock et al 2006;Prior et al 2010).…”