Fire in Southern Australia - Fire in Mediterranean Ecosystems - page 219

communities in relatively dry habitats are most sensitive to changes in fire
interval because of higher proportions of woody obligate seeders (e.g. Clarke
et al. 2005 ).
The length of the juvenile period and patterns of replenishment of seedbanks
following fire (e.g. Gill & Nicholls 1989 ; Burrows et al. 2008 ) affects the position
of the peak in the relationship between diversity and length of inter-fire interval
( Fig. 8.6 ). The domain of highest diversity is a minimum interval of about 10 yrs
and this equates to levels of seed accumulation in canopy storage that are suffi-
cient for population replacement of the slowest maturing species (Burrows &
Wardell-Johnson 2003 ; Bradstock & Kenny 2003 ; Lamont et al. 2007 ; Burrows
et al. 2008 ). Conversely, declines in serotinous taxa may occur with putative
senescence. Similar declines in soil storage are predicted, though longevity may
exceed the life span of established plants.
A predicted decline in diversity at longer intervals between fires ( Fig. 8.6 ) also
reflects the inhibitory effect of a lack of fire for many species. Release from
inhibition is correlated with degree of removal of litter and plant canopies by fire,
as a result of direct and indirect stimuli. Accordingly, plant establishment is a
general positive function of fire intensity. Where the probability of fire is lower
(e.g. semi-arid communities), dependence on direct fire stimuli can be lower. The
long-term absence of fire results in a decline in diversity through senescence, lack
of recruitment and possible competitive displacement by long-lived dominants
that may recruit in the absence of fire (e.g. Callitris spp.). Some or all of these
mechanisms may apply in differing communities. In a successional sense, MTV is
disclimax vegetation, with inherent high richness/diversity sustained by recurrent
fire on a decadal to multidecadal cycle. Rainforests, which have potential to
replace MTV in the southeast, may contain a completely different assemblage of
species, often with lower diversity than neighboring MTV (Clarke et al. 2005 ).
The fire regime model ( Fig. 8.6 ) corresponds with observed responses of species,
functional types and communities to different fire regimes based on opportunistic
analyses (i.e. natural experiments) and on meta-analyses (e.g. Pausas et al. 2004b )
in shrub-dominated communities. Corroboration of the model via field observa-
tions is biased toward studies from southeastern Australia (e.g. Nieuwenhuis 1987 ;
Cary & Morrison 1995 ; Morrison et al. 1995 ; Bradstock et al. 1997 ; Benwell 1998 ;
Ross et al. 2002 , 2004 ; Watson & Wardell-Johnson 2004 ; Myerscough & Clarke
2007 ), though empirical observations have been made on individual species'
responses to fire regimes in the south and southwest (e.g. Specht 1981 ; Gill &
McMahon 1986 ; Wooller et al. 2002 ;Yates et al. 2003b , 2007 ; Lamont et al. 2007 ).
Such work confirms that obligate seeders (up to 50% of the species) are relatively
sensitive to variations in fire interval in particular (i.e. as indicated by the ampli-
tude and modal tendency), but also demonstrate that resprouters exhibit sensitiv-
ity in this regard as well (Watson & Wardell-Johnson 2004 ). In particular the
serotinous obligate seeder subcomponent (e.g. usually less than 5% of species,
Pausas et al. 2004b ) has a pivotal role in shaping this relationship, given high
sensitivity to both frequent and infrequent fire.