Fire-adaptive Trait Evolution - Fire in Mediterranean Ecosystems - page 238

expense of woodlands, particularly in light of the evidence that C 4 grasslands were
expanding into more mesic habitats (Keeley & Rundel 2005 ). Further evidence for
such a fire origin for these grasslands is that maintenance of these ecosystems is
today dependent on frequent fires. Whether or not these represent qualitative
changes in the earth system (cf. Bowman et al. 2009 ) or just an expansion of fire-
prone environments is yet to be sorted out.
Origin of Fire-adaptive Traits
Many plant traits have been interpreted as adaptations to fire but that implies the
trait was selected for that function, and this is often not known. It is considered
important by some to distinguish adaptations from exaptations (Gould and Vrba
1982 ); the latter are traits that serve a particular function, but originated through
selection for some other function. This is a theoretically valuable concept but
linking traits to selective environments at the time of their origin is a huge
challenge. Even if one could demonstrate a factor is responsible for the origin of
a trait, it doesn't rule out other factors playing a role since evolutionary pressures
do not necessarily act independently. Distinguishing between adaptations and
exaptations may be shaded by one's perspective on what are believed to be the
most important selective factors in the environment. For example, climate has
long been considered the major driver in plant trait evolution; however, it is now
clear fire has great antiquity and fire-prone environments have persisted through
multiple climatic changes. The limitations to recognizing exaptations from adap-
tations are by no means unique to traits with adaptive value in fire-prone habitats,
but apply to most traits and thus this historical definition of adaptations is fraught
with numerous difficulties (Lauder et al. 1993 ; Keeley et al. 2011). In many cases it
is doubtful one can clearly distinguish between adaptations and exaptations, and
thus the term “apparent adaptation” may be appropriate.
Origin of Resprouting
Resprouting from vegetative structures after top-kill is a widespread regeneration
mechanism following many types of disturbance, and although of apparent adaptive
value in fire-prone landscapes, it is common inmany vegetation types where fires are
rare or uncommon such as tropical forests (Putz & Brokaw 1989 ; Kauffman 1991 ).
In both tropical and temperate forest trees resprouting has adaptive value in recover-
ing following major wind events such as hurricanes and tornados (Glitzenstein &
Harcombe 1988 ; Bellingham et al. 1994 ; Paciorek et al. 2000 ), although fire cannot
be ruled out as a factor since these wind events create fuel loads that induce
widespread burning (Liu et al. 2008 ). Making a case for fire as the selective
force in the evolution of resprouting is made difficult by the observation that
resprouting has been around for a very long time, as suggested by its near universal