Agriculture Reference
In-Depth Information
Promoting Postfire Recruitment with Enhanced Flammability
We hypothesize that the evolution of postfire seeding has been coincident with the
selection for traits that enhance flammability and thus increase sites for recruit-
ment (
Fig. 9.5
). In the paleorecord there is evidence that changes in plant structure
are associated with increased fire activity (Belcher
et al.
2010
), but this is inter-
preted as the result of climatic change altering plant structure and, as a conse-
quence, changing fire regimes. The idea that plant structure might evolve to
enhance flammability is something rather different. The idea was first proposed
by Mutch (
1970
), and although he saw a connection between fire-prone commu-
nities and species with enhanced flammability, he didn't tie it to postfire seeding.
Evolution of flammability has been criticized on a number of grounds (Snyder
1984
; Troumbis & Trabaud
1989
). As with many fire-adaptive traits, it is reason-
ably argued that there are other selective factors such as drought or herbivory that
could account for flammability of leaf and stem characteristics. Another argument
is that fire is a complex of biotic and abiotic factors and thus not affected by
individual plant traits. Perhaps the harshest criticism has been that selection for
flammability is group selectionist and unlikely to evolve in the context of inclusive
fitness theory that forms the basis of modern evolutionary theory. This criticism
may have been avoided had the evolution of flammability initially been tied to
enhancing postfire sites for fire-dependent seedling recruitment.
This last issue was directly addressed by Bond and Midgley (
1995
) who placed
flammability in the context of postfire reproduction and provided sufficient
reasons to consider it a possibility that flammability characteristics could arise
through selection on individual fitness. They reasoned that if plants with fire-
dependent reproduction evolved characteristics that enhanced flammability or
intensity, which in turn created greater gaps for postfire seedlings by negatively
impacting obligate resprouters, then such traits could evolve within inclusive
fitness theory. Modeling studies suggest several genetic mechanisms for evolving
flammability and contend that it is a form of niche construction that alters
environments to enhance fitness (Kerr
et al.
1999
; Schwilk & Kerr
2002
). Some
of the traits that contribute to greater flammability in postfire recruiter species
include volatile compounds that promote ignition and fire spread (Rundel
1981b
),
retention of dead wood in the canopy that enhances combustion of associated live
foliage (Zedler
1995a
; Schwilk
2003
), and structural differences in leaf and stem
placement that likewise promote combustion (van Wilgen
et al.
1990b
).
Thorough tests of this hypothesis are lacking; however, in chaparral, some postfire
seeder species such as
Adenostoma fasciculatum
and species of
Arctostaphylos
and
Ceanothus
fail to self-prune dead branches.
Ulex parviflorus
(see
Fig. 4.5
)isan
outstanding example of an obligate seeder from the Mediterranean Basin that
retains large amounts of dead biomass (Baeza
et al.
2006, 2011
). It is difficult to
imagine other selective factors accounting for such retention. The selective value for
retention is that it greatly increases fire ignition and intensity, over what would
occur if those branches were dropped and remained as surface fuels (Schwilk
2003
).
Fire severity measures also show a positive relationship with obligate-seeding
