Agriculture Reference
In-Depth Information
1995
; Vesk & Westoby
2004
; Falster & Westoby
2005
; Knox & Clarke
2005
).
The extent to which these differences affect competitive interactions between
obligate seeders and facultative seeders is a matter of some controversy (Keith &
Myerscough
1993
; Clarke
et al.
2005
; Wardell-Johnson
et al.
2007
) and is discussed
further in
Chapter 9
.
Seed Supply Constraints
The bulk of species have persistent seedbanks in which there is some seed carry-
over from year to year and seeds are usually available for germination
in situ
following fire (Bell
1999
; Auld
et al.
2000
; Pausas
et al.
2004b
; Enright
et al
.
2007
;
Lamont
et al.
2007
; Pausas & Bradstock
2007
). The formation of persistent
in situ
seedbanks is tied to deep dormancy and relatively short range dispersal for
the bulk of the seed pool (long-distance dispersal of some of these is known,
Bradstock
et al.
1996
; Lamont
et al.
2007
). Such species are not reliant on annually
seeking gaps for establishment, but instead are dependent on disturbance
in situ
for the provision of gaps.
Seedbanks stored in the canopy through serotiny (see
Fig. 3.5
and
Table 9.2
), a
form of bradyspory, represent about 25% of the postfire seeders in MTC ecosys-
tems in Australia (Lamont
et al.
1991
,
2007
). Soil-stored seedbanks comprise
about 70% (Keith
et al.
2002
; Pausas
et al.
2004b
). Soil seedbanks of woody
species may accumulate more rapidly and have greater longevity than their
serotinous counterparts. This is due to several factors. Species with soil seed
storage tend to mature earlier (Auld
1987
; Bradstock & O'Connell
1988
; Keith
et al.
2002
; Abbott & Burrows
2003
; Burrows
et al.
2008
). Also, soil-stored
seedbanks possess dormancy mechanisms that may enable seeds to outlive parent
plants (Bell
1999
; Auld
et al.
2000
; Whelan
et al.
2002
; Wills & Read
2007
). In
addition, there is some evidence that soil seedbanks are not entirely exhausted by
postfire germination and may persist through repeated fires (Whelan
et al.
2002
;
Tozer & Auld
2006
); see also
Chapter 7
.
Canopy storage in obligate seeder species of shrubs generally take 4-10 yrs to be
initiated. Slower rates of maturation occur in extreme habitats such as rock
outcrops (Clarke
2002a
; Yates
et al.
2003a
,
2003b
,
2007
; Burrows
et al.
2008
)
and in gymnosperms such as
Callitris
spp. (Bradstock & Cohn
2002b
). Canopy
storage in facultative resprouters is replenished more rapidly because flowering
begins very early in resprouts and thus this secondary juvenile period is much
shorter than the primary juvenile period following germination (Bradstock
1990
;
Lamont
et al.
1998
,
2007
). Compared with congeneric obligate seeders, seed crops
of facultative resprouters with canopy storage also tend to be lower (but not
always, see Enright
et al.
2007
)(Zammit&Westoby
1988
;Bell
2001
; Pausas
et al.
2004b
;Lamont
et al.
2007
). Serotinous storage generally reaches maximum levels
>
10 yrs postfire, with rate of growth of storage declining at this time, due to losses
from predation (vertebrate and invertebrate) and spontaneous release (Gill &
McMahon
1986
; Bradstock & O'Connell
1988
;Bradstock
1990
;Lamont&
Groom
1998
; Lamont
et al.
2007
).
