Agriculture Reference
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Ceanothus
seedling density in chaparral (Keeley
et al.
2005b
) and in two species of
Proteaceae in fynbos (Bond
et al.
1990
), and increased growth rates of seedlings in
Mediterranean serotinous
Pinus halapensis
(Ne'eman 2000; Pausas
et al.
2003
).
In addition, associated non-fire-recruiting obligate seeders generally drop dead
branches (W.J. Bond unpublished data). Similarly, tree species with flaky barks
that readily carry fires into the crown typically have good postfire regeneration
capacities, such as the stringy-bark group of
Eucalyptus
(Burrows
2001
). However,
in this complex genus the evolutionary nexus between postfire seeding and bark is
complex as many bark types are associated with postfire seeding.
Keeley and Bond (
1999
) have hypothesized enhanced flammability was a select-
ive force behind semelparous flowering cycles of many bamboos. Delayed repro-
duction and semelparity (monocarpy) generate massive amounts of fuel, and the
gregarious clonal distribution produces a contiguous fuel load, all of which
encourage the propagation of high-intensity fires. This effectively eliminates
canopy trees and canopy tree recruitment. Mast flowering synchronizes seedling
recruitment with the creation of safe sites in canopy gaps. Semelparity, in addition
to setting the stage for disturbance, also concentrates reproductive allocation to
the optimum time for recruitment.
Another selective basis for flammability is that by retaining dead branches and
foliage in the canopy it reduces soil heating around soil-stored seedbanks and
underground vegetative structures (Gagnon
et al.
2010
). This hypothesis has not
been tested and although it may apply to flammability characteristics in some
taxa, it seems unlikely that it applies very widely. For example, obligate seeding
taxa may be split between those with soil-stored seeds and others with canopy seed
storage. One would expect that this would select for very different flammability
characteristics, but a casual inspection of MTC communities fails to support that
expectation. One of the intriguing aspects of this theory is that it provides a
mechanism for obligate resprouters to compete against seeders by not retaining
dead fuels in the canopy and accumulating sufficient surface fuels to kill soil
seedbanks. Since these obligate resprouters don't recruit after fire they may reduce
seedling and sapling competition. This effect is most evident where surface fire
regime forests are juxtaposed with crown fire shrublands. The prolific postfire
regeneration in the latter contrasts with the relatively depauperate regeneration in
the former, and the sharp boundary is controlled in part by the higher soil-surface
fire intensity in the forest communities. Even within surface fire regimes pines that
recruit in early postfire years generate surface fuels capable of achieving substan-
tially higher surface temperatures than litter from species such as oaks that do not
recruit after fire (Williamson & Black
1981
).
Evolution of Non-resprouting Obligate Seeding Taxa
A second significant evolutionary innovation in postfire seeding was to eliminate
resprouting altogether and depend entirely on postfire seedlings (
Fig. 9.5
). Wells
(
1969
) first made the surprising observation that many woody shrubs in California
