Agriculture Reference
In-Depth Information
species present in the first growing season after fire are the result of colonization.
This potentially changes after the first year, but dispersal into burned areas is
affected by the perimeter to burned area ratio, and sources of colonizers along the
perimeter. In a detailed study in California chaparral it was found that five years
after fire about half of the species were not present on the site in year 1 (Keeley
et al.
2005a
). These
apparent colonizers
, however, did not represent colonization of
the burned area because almost all of these species were present in year 1 on other
sites within the burned perimeter. Thus, these were postfire species that were
relatively rare across the burned area in year 1 and through mass action effects
ended up in new sites by year 5.
Species that recruit seedlings in the first growing season after fire from dormant
seedbanks are widespread in MTC regions. Some of these woody species regenerate
after fire both by seedlings and resprouting (facultative seeders); however, a signifi-
cant number only regenerate with seedlings and are termed obligate seeding species.
These latter species lack all capacity for resprouting and are a major exception to
Vesk &Westoby's (
2004
) supposition that all species resprout and only differ in level
of resprouting. Obligate seeding species were recognized by Wells (
1969
)asbeing
unique because resprouting capacity is nearly universal in woody dicots and he
attributed the obligate seeding habit in shrubs as a trait selected for by fire. Obligate
seeders comprise a small subset of woody plants in crown fire regimes (Table
3.4
).
One of the key attributes of obligate seeders is their sensitivity to short fire return
intervals as they are extirpated from sites when fire return intervals are shorter than
the time required for reproductive maturation, evident in both empirical (Zedler
et al.
1983
;Pate
et al.
1991
; Gill & Bradstock
1995
; Keeley
2006a
; Pausas
2006
)and
modeling studies (Keeley
1986
;Keeley&Swift
1995
; Enright
et al.
1998
; Regan
et al.
2010
).
Species differ in mode of storage, with most accumulating seeds in the soil and
others by the accumulation of seeds in serotinous fruiting structures (a type of
bradyspory) retained on the parent plant. The selective trade-offs are considered
in more detail in
Chapter 9
.
Soil Seedbanks
Many species that disperse seeds at maturity have innate barriers to germination
and accumulate dormant soil-stored seedbanks, which are later triggered to ger-
minate by fire-related cues. These include many dominant shrubs and a huge
number of ephemeral species that spend most of their lifetime as dormant seed-
banks (Pate
et al.
1985
; Keeley
1991
; Bell
et al.
1993
; Keeley & Fotheringham
2000
). Fire-stimulated germination of soil seedbanks may be triggered by heat or
chemicals from the combustion of biomass. These are often considered rather
specialized germination mechanisms; however, in the latter case there is evidence
that this germination mechanism may be rather ancient, being found in basal taxa
within several different lineages (C.J Fotheringham personal communication,
2004; Pausas & Keeley
2009
).
