Fire in Southern Australia - Fire in Mediterranean Ecosystems - page 217

(e.g. Acacia , Allocasuarina , Leptospermum , Melaleuca , Callitris ), with consequent
shading of lower-stature shrubs and herbs, has been invoked as the cause of a
marked decline in diversity. The rise of Allocasuarina as an alternative dominant
to eucalypts when charcoal abundances were low during the Quaternary has been
documented in various paleoecological studies (Lynch et al. 2007 ), though there
are anomalies in this trend.
Callitris species are generally capable of recruitment in the absence of fire in both
eastern and western MTV (Bowman & Harris 1994 ;Enright et al. 1994 ), with these
species tending to be prominent in fire refugia in shrubland vegetation (Beard 1984 ;
Enright et al. 1994 ) and off-shore islands (Yates et al. 2003a ). High abundance of
Callitris and Allocasuarina pollen in the Quaternary paleo record is correllated with
low abundance of charcoal (inferred low fire activity, e.g. Thomas et al. 2001 ;
Kershaw et al. 2002 ). The relationship between rainforest and sclerophyll vegetation
has been extensively debated on the basis of both paleo and contemporary eco-
logical evidence (Bowman 2000 ;Kershaw et al. 2002 ;Lynch et al. 2007 ). Whether
this transition applies in dry, infertile or skeletal edaphic conditions is open to
question. Allocasuarina and Callitris spp. may fulfill this role in these habitats.
Community Responses to Fire
Predicted community responses of Australian MTV to fire regimes reflect the
outcome of life history variations, demographic processes, competition and habi-
tat effects. Aboveground plant diversity (measured as species richness, see Chapter
11 ) tends to recover rapidly, reaching a peak within about a decade after fire, with
subsequent stasis or decline ( Fig. 8.5 ; Gill 1999 ; Bell 2001 ; Burrows & Wardell-
Johnson 2003 ), conforming to an initial floristic composition model. Gill ( 1999 )
noted variations around this basic pattern such as the greater occurrence of peak
diversity immediately after fire from fire ephemerals in drier MTC environments.
Diversity of MTV communities is often inferred to be a unimodal function of
the length of time between fires, known as the fire return interval or inter-fire
interval, with a peak at 10-20 yrs and diminution at both shorter and longer
intervals. Such inference is based on extrapolation of trait responses via dynamic
models of individual species and functional types, in particular the sensitivity of
obligate seeder shrubs to length of the fire interval. This model includes the
assumption of strong buffering against major variations in fire regimes due to
the relatively high but variable proportion of resprouters present. Fire regime
variations are therefore assumed to largely affect the woody obligate seeder
component of the community (Gill & Bradstock 1995 ; Gill & Catling 2002 ; Keith
et al. 2002 ; Bradstock & Kenny 2003 ; Burrows & Wardell-Johnson 2003 ; Pausas
et al. 2004b ; Lamont et al. 2007 ; Burrows et al. 2008 ; Groeneveld et al. 2008 ),
though sustained high frequencies of fire may also deplete some resprouter popu-
lations (Watson & Wardell-Johnson 2004 ; Pausas et al. 2004b ; Lamont et al.
2007 ). The predicted amplitude of the diversity and fire interval relationship
( Fig. 8.6 ) is therefore mainly a function of the obligate seeder component of
MTV communities.