Agriculture Reference
In-Depth Information
There are many lineages where one could make the argument that postfire
resprouting evolved as a response to top-kill from fire. However, an obvious case
where resprouting is not likely an adaptation selected for by fire is in herbaceous
perennials (hemicryptophytes; see
Box 1.2
). This growth form is most abundant in
highly seasonal environments such as MTC regions, where the common cycle is
summer dieback followed by winter or spring sprouting, and thus sprouting is a
natural phenological stage that may occur annually regardless of fire. However,
although most herbaceous perennials may resprout in the winter and spring
without fire, they typically flower much more than usual following fire. In all
MTC regions geophytes (cryptophytes) are the most common herbaceous peren-
nial and typically comprise a conspicuous part of the postfire flora from resprouts
(Le Maitre & Brown
1992
; Rundel
1996
; Parsons & Hopper
2003
; Proches &
Cowling
2004
). In the absence of fire in these closed-canopy shrublands, geophytes
typically become dormant for extended periods of time, and many of these
respond to fire by coupling resprouting with profuse flowering that is often
synchronized across the population in the first growing season after fire
(Le Maitre & Brown
1992
; Tyler & Borchert
2002
; Borchert & Tyler
2009
), suggest-
ing some fire-adapted modification of the annual sprouting cycle. This growth form
seldom maintains dormant seedbanks and thus it is normally an obligate resprouter
(Keeley & Bond
1997
;Keeley
et al.
2006b
). In some cases it appears that soil
microclimate changes produced by removal of the shrub cover are the cue for
triggering postfire flowering (Stone
1951
), which is possibly a general seasonal
trigger for flowering in grasslands and other open sites in the absence of fire.
In woody plants a seemingly specialized resprouting mode is evident in a diverse
array of woody species that resprout from swollen lignified structures at the base of
the stems, known as basal burls or lignotubers (
Fig. 3.1
). Depending on the species
and number of fire cycles these structures may vary from a few centimeters to more
than a meter in diameter (
Fig. 3.2c
) or even larger in some
Eucalyptus
species (Lacey
1983
). These burls possess storage carbohydrates, inorganic nutrients and adventi-
tious buds that are considered critical to postfire resprouting (Carr
et al.
1984
;James
1984
). Basal burls are common in MTC regions (Keeley
1981
) but occur in other
ecosystems, often induced as a wound response to repeated coppicing or other
disturbance and can be found in tropical forests (Johnston & Lacey
1983
), tropical
grasslands (Davy
1922
; Lawson
et al.
1968
;White
1976
), and temperate forests (Cant
1937
;Ekanayake
1962
;Mallik
1993
; Stone & Cornwall
1968
). Such structures also
need not be basal as evidenced by tropical “living fences” that resprout and form
burls from repeated coppicing at fence height nearly 2 m above ground (Sauer
1979
).
Somewhat unique among taxa in MTC regions is that many woody resprouters
(
Table 3.2
) produce these burls as a normal ontogenetic stage in early development
(
Fig. 3.2b
). They commonly are initiated from axillary buds of the cotyledons and
expand until they coalesce into a swelling that encircles the stem (Kerr
1925
), but
there is significant variation in this ontogeny (Lacey & Jahnke
1984
; Mibus &
Sedgley
2000
). Canadell & Zedler (
1995
) suggest that the term
lignotuber
be
reserved for these structures that are genetically determined and the term (basal)
