Agriculture Reference
In-Depth Information
absence of disturbance, both in highly fire-prone shrublands (Mesle´ ard & Lepart
1989
; Keeley
1992a
) and mesic rainforests (Marrinan
et al.
2005
). Thus, many
shrubs that resprout after fire continue to produce new basal sprouts throughout
their life span, although depending on growing conditions many of these never
recruit into the canopy.
Generally, resprouting is a species-specific trait; however, there are notable
examples where different populations of the same species have diverged in
resprouting capacity, with both resprouting and non-resprouting populations
(Keeley
2000
; Ojeda
et al.
2005
). The relevant selective factors might be expected
to vary with different lineages since resprouting is associated with a different set of
traits in different floras (Pausas
et al.
2004b
).
Seedling Recruitment
Many fire-prone ecosystems in the world are resilient to fires but most species have
not capitalized on fire as an opportunity for seedling recruitment. In contrast,
many species in MTC ecosystems and related mediterranean-type vegetation
(MTV) have fire-dependent reproduction. This is most prominent in closed-
canopy shrublands where a predictable fire cycle produces postfire conditions that
represent resource-rich sites for seedling recruitment. This includes creation of
canopy gaps, reduced competition for soil moisture resources, accelerated miner-
alization, reduced herbivore populations and synchronized recruitment that sati-
ates predators.
However, there are potential costs to postfire recruitment. One is delayed
reproduction, often for many decades; rather than spreading recruitment risks
out over climatically different years, species gamble on a single pulse of recruit-
ment in the first postfire year. The potential costs of delayed reproduction are of
such a magnitude that they are not likely to evolve except under extraordinary
conditions (Gadgil & Bossert
1970
), and these are discussed in
Chapter 9
.In
MTC regions other costs of recruiting on recently burned sites are associated
with greater exposure to more intense summer drought stress, which requires
specialized physiological and anatomical traits (S.D. Davis
et al.
1998
;Pratt
et al.
2008
). Specialization to postfire recruitment is linked to traits that diminish
recruitment success in the shrub understory and thus many species have
specialized on postfire recruitment whereas other species have a very different
character syndrome specialized on fire-independent recruitment in the under-
story (Keeley
1998
).
Postfire seedling recruitment requires seedbanks that are cued to germinate by
fire. Some species exhibit fire-dependent reproduction because essentially all
seedling recruitment is restricted to the first postfire year. This pulse of recruit-
ment results in stands of single-aged cohorts as with many woody plants (Bond &
van Wilgen
1996
; Keeley
et al.
2006b
). Other species are more opportunistic and
couple postfire recruitment with seedling recruitment between fires as with many
