Agriculture Reference
In-Depth Information
germination after a long duration at lower temperature (e.g. 1 hour at 70
C).
The same applies to lethal temperature regimes: for example, large-seeded
species survive a short duration at high temperature but are killed by a long
duration at lower temperature, whereas very small seeds exhibit the opposite
pattern (Keeley
1991
). Such differences in stimulation/tolerance regimes may
explain some of the variance in microhabitat segregation of postfire floras
(e.g. Davis
et al.
1989
).
Since many species are stimulated by a long duration at 70-80
C, they are not
strictly tied to postfire environments as such conditions may be encountered by
seeds exposed to direct sun rays on open sites. Thus, heat-shock-stimulated
germination does not limit recruitment to burned sites, rather such species can
establish in gaps created by other types of disturbance as well. Also, since
unburned landscapes often comprise a heterogeneous collection of suitable and
unsuitable recruitment sites it is not surprising that most heat-stimulated species
have polymorphic seed pools (Keeley
1991
,
1995a
). Thus, while the bulk of the
seedbank may be deeply dormant, a portion may germinate readily and establish
in the absence of fire.
It has only recently become evident that in some fire-prone environments the
majority of species that recruit after fires lack heat-stimulated germination, but
rather chemical products of biomass combustion contained in charred wood or
smoke provide the cue that triggers germination and heat plays no role in their
germination (
Box 3.1
). This response is present in many of species from MTC
ecosystems in southern Australia, South Africa, California and the Mediterranean
Basin (Wicklow
1977
; Keeley
1987
,
1991
; de Lange & Boucher
1990
; Brown
1993
;
Dixon
et al.
1995
; Moreira
et al.
2010
). The chemical trigger is transferred to seeds
either in the smoke aerosol during fire, in aqueous leachate or vapors from smoke
or charred wood after fire or even secondarily transferred after adhesion to soil
particles (Keeley & Fotheringham
1997
), and there is evidence for both inorganic
(Keeley & Fotheringham
1997
) and organic (Flematti
et al.
2004
) compounds as
the trigger. This fire-generated chemical stimulus for germination is found in a
wide diversity of plant families (
Box 3.1
).
Longevity of soil-stored seed is not well documented but circumstantial evi-
dence suggests seeds of many MTV species are very long lived. The longest record
for shrublands is that inferred from the study of California chaparral burned after
approximately 150 yrs (Keeley
et al.
2005b
). Many of the ephemeral species that
are restricted to the immediate postfire years were present in the first growing
season after fire in these ancient stands. Colonization from outside was impossible
considering the 25 000-ha size of the fire, lack of dispersal capacity and short time
interval between fire and establishment, indicating these seeds had persisted for
roughly a century and a half as dormant seeds. Some shrubs common in MTC
shrublands also play a role as a seral stage following crown fires in forests and in
one case there is evidence the seedbanks survived over 200 yrs between fires
(Gratkowski
1962
).
