Agriculture Reference
In-Depth Information
There are a multitude of factors that could work against postfire survivor-
ship of resprouters, including weak resilience to high fire intensities and long
fire-free periods, since resprouting is only adaptive when the fire cycle is
shorter than the plant's life span, whereas soil-stored seedbanks can persist
under very long fire cycles. Whether or not such factors favored obligate
seeders over resprouters would be controlled strongly by the mode of seed
storage. Long fire return intervals might favor soil-stored seedbanks but
definitely not canopy storage as these taxa, like the resprouters, are at a
disadvantage if they die before fire. This is one of the key reasons why models
based solely on fire frequency are not likely to fully capture the conditions
selecting for the non-resprouting mode (see #5 below).
Other situations that would produce gaps would be along arid margins where
growth and survival of shrubs over many decades is precarious and postfire gaps
are large, and this is generally consistent with the distribution of postfire seeders
(Keeley
1977
;Pausas
2001
; Clarke & Knox
2002
; Meentemeyer & Moody
2002
).
Some factors driving the obligate seeding mode are illustrated by several
Californian
Arctostaphylos
species,whichhavebothresprouting and obligate
seeding subspecies:
A. parryana
,
A. peninsularis
and
A. manzanita
(J.E. Keeley
unpublished data). Both morphotypes are found in ecosystems subjected to crown
fires but the obligate seeding taxa are distributed in open woodland habitats
where postfire gaps are large and the resprouting taxa are in dense chaparral
where close competition after fire has likely selected for resprouting over seeding.
A more generalized version of this idea is the resource-based model pro-
posed by Clarke that hypothesizes trade-offs between resource availability and
resprouting (Clarke
et al.
2005
). Presumably as resource levels increase,
resprout success increases due to unfavorable conditions for seedling recruit-
ment in the face of resprout competition. In addition, such resource gradients
affect fuel production and as fuels increase they potentially increase fire
frequency (Pausas & Bradstock
2007
). Thus, increased resprouting dominance
at higher resource levels may be due to both reducing opportunities for
seedling recruitment as well as creating higher fire frequency that extirpates
non-resprouting populations prior to reproductive maturity.
Ecologically, it would appear that facultative seeders include the best traits
of both postfire resprouting and seeding. Thus, the evolution of obligate
seeding would appear to hinge on a selective advantage to losing the resprout-
ing mode (
Fig. 9.5
). This would require that there be costs to diversion of
resources to resprouting, and these costs would need to be sufficient to reduce
seed production and thus reduce seedling recruitment, which ultimately would
translate into reduced establishment of adults. In some facultative seeders
there does not appear to be trade-offs between the extent of prefire flowering
and postfire resprouting (Cruz & Moreno
2001b
). One trade-off may be in
earlier flowering of juvenile seeders, perhaps due to different patterns of
resource allocation from resprouters (Bell & Pate
1996
; Verboom
et al.
2004
;
Schwilk & Ackerly
2005
). In general, there is no consistent pattern of greater
