Agriculture Reference
In-Depth Information
shrubs recruit seedlings in the first postfire year and most are in genera that include
species with both resprouting and postfire seedling recruitment. Most obligate
seeders lack any capacity for resprouting; however, facultative seeder populations
exhibit a range of resprouting levels and this is a function of species-specific
characteristics and tolerances to fire, as well as to characteristics of the fire event,
particularly fire intensity and degree of prefire and postfire drought.
Wells (
1969
) contended that resprouting was an ancestral trait and loss of the
ability was derived. In a broad sense this is certainly true as nearly all woody
angiosperms have the capacity to resprout and where obligate seeding has evolved,
the closest sister genera are nearly always resprouters. Within the very large genus
Erica
, there is evidence that obligate seeding arose as a result of suppression of bud
development responsible for lignotuber formation (Verdaguer & Ojeda
2002a
).
However, within some lineages there is evidence that resprouting has been derived
from obligate seeding ancestors (Bond & Midgley
2003
; Boatwright
et al.
2008
).
This even appears to be the case at the subspecific level as illustrated by Califor-
nian
Arctostaphylos
spp., which have both resprouting and obligate seeding sub-
species (J.E. Keeley unpublished data). Also, aberrant resprouting individuals of
non-resprouting species have been observed in Lamiaceae in the Mediterranean
Basin (J.G. Pausas personal observations), in Ericaceae and Fabaceae in the South
African Cape region (Ojeda
1998
; Schutte
et al.
1995
) and
Banksia
and
Hakea
in
Australia (R.A. Bradstock personal observations).
Since the initial observations of highly diverse obligate seeding genera in MTC
regions, at least five hypotheses have been put forth to explain the origin of this
life history.
(1) Wells (
1969
) hypothesized that the obligate seeder mode had been selected for
because it forced the population to turn over 100% of the individuals every
fire-induced generation and was able to evolve more quickly, and Raven
(
1973
) suggested such obligate seeders were thus better able to track changes
in the environment from generation to generation.
(2) An ecologically based explanation was proposed by Keeley & Zedler (
1978
;
see also Keeley
1986
) who contended that obligate seeding populations would
gain an advantage under conditions where it was more advantageous to
regenerate after fire by seedlings than by resprouts (
Fig. 9.5
). On one hand
the postfire strategy is one of
fire recruiter
and on the other hand one of a
fire
persister
(Keeley
1991
). Indeed, Bond & Midgley (
2001
) have proposed the
concept of the
persistence niche
; thus, the balance between factors favoring
recruiting over persisting will drive the evolution of reprouting and obligate
seeders. What are those conditions? In the immediate postfire environment
seedlings are at a distinct disadvantage in close competition with resprouts so
conditions that favored large postfire gaps would favor seedlings and not
resprouts. This model hypothesizes that obligate seeding is selected for when
postfire environments produce safe sites for seedlings and unfavorable condi-
tions for resprouts.
