Agriculture Reference
In-Depth Information
ensure that seed supply is not limiting if recruitment opportunities present
themselves. One advantage to delaying dispersal until fire is that litter beds form
a surface barrier that restricts radicles of germinants from reaching the soil
(Bradstock
1989
; Tozer & Bradstock
1998
). However, disturbances other than
fire such as digging and raking of litter by animals may expose mineral soil and
present conditions for establishment (Kirkpatrick
1997
). Plant cover may inhibit
the early growth of seedlings through shading and competition for water (Specht
& Morgan
1981
; Enright & Lamont
1989a
; Wellington
1989
; Kirkpatrick
1997
),
but canopy disturbances from treefalls and other factors often provide recruitment
opportunities in the absence of fire.
Predation of seeds and seedlings is a consistent inhibitor to establishment under
unburned conditions (Auld
1986
; Andersen
1989a
,
1989b
; Enright & Lamont
1989b
; Wellington
1989
; Bradstock
1991
; Cohn & Bradstock
2000
; Hanley &
Lamont
2001
). Seed supply and seedling establishment is restricted by both
invertebrate and vertebrate herbivory, though postfire herbivory can also be
severe (e.g. Cohn & Bradstock
2000
). Diminished predation of seeds and seedlings
in burned compared with unburned conditions may be due to predator satiation
(O'Dowd & Gill
1984
; Wellington & Noble
1985
).
Senescence Risk
Long periods without fire may pose a risk that is dependent on longevity of plants
and their seedbanks. Estimates of life spans of perennial woody plants are largely
based on extrapolation of survival rates (Auld
1987
; Bradstock & Kenny
2003
;
Pausas
et al.
2004b
). Much of what is known about senescence and mortality in the
absence of fire is largely anecdotal and speculative.
Beard (
1984
) discussed obervations on the fate of kwongan shrublands in the long-
term absence of fire. He noted that structure and composition may change through
growth, death and the opening of canopies, and that the emergent conifers
Callitris
and
Actinostrobus
sometimes became more prominent. However, wholesale replace-
ment of the community was unlikely and these conclusions are largely endorsed by
Lamont
et al.
(
2007
) for the northern kwongan. Holland (
1986
) documented little
change in the composition of long-unburned mallee in western New South Wales,
though there were some structural changes such as a taller overstory and more bare
ground. Hopkins & Robinson (
1981
) hypothesized that woodland in southwestern
Australia may be the result of transformation of mallee-heath in the absence of fire.
By contrast, Gent & Morgan (
2007
) hypothesized that in the absence of fire coastal
woodlands on dunes in southern Victoria would eventually lose the trees and be
replaced by grassland. Eucalypt-dominated woodlands and forests may undergo
quasi-successional change to dominance by
Allocasuarina
or
Callitris
in dry sites or
rainforest in wet sites if fire is absent for long periods of 50-100 yrs (Close
et al.
2009
).
Decline in species diversity has been documented due to shrub invasion
under long-unburned conditions in heaths, shrublands and woodlands in eastern
Australia (Bennett
1994
; Cheal
1996
; McMahon
et al.
1996
; Kirkpatrick
1997
; Lunt
1998
). In these cases, dominance by long-lived woody shrubs and trees
