Agriculture Reference
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history characteristics change state from a crown fire selective regime to a surface fire
selective regime. Despite radical differences in fire intensity and frequency, both
systems may have equal levels of resprouting and non-resprouting seeding species,
but for very different reasons that are tied to the different selective environments of a
surface fire regime vs. a crown fire regime (see
Chapter 2
). Because of the effect of
such spatial thresholds on fire behavior (e.g. Slocum
et al.
2010
), the most successful
analyses of trait evolution have been ones that have focused on patterns within a
single fire regime (e.g. Ackerly
2004a
;Pausas
et al.
2004b
).
The ambition of developing a unifying model of life history that applies
across a wide range of plant communities (Sparrow & Bellingham
2001
) is not
feasible without consideration of fire regime characteristics, particularly in the
fuels consumed. Different growth forms exist in very different selective envir-
onments that are critical to understanding the selective value of resprouting.
Complicating the picture even more is the fact that some growth forms such as
trees change their resprouting capacity at different life history stages (see
Chapter 3
). We do agree with these authors that there is a strong interaction
between disturbance history and site productivity, but maintain that the
primary impact is on fire regime characteristics (see
Fig. 2.7
) and the subse-
quent impact this has on life history options (Keeley & Zedler
1998
). As in
Sewell Wright's (
1932
) adaptive landscape model, we might think of a similar
metaphor with different fire regimes as representing adaptive peaks in which
selection pressures vary from one fire regime to another.
A Model for the Evolution of Non-resprouting Shrubs
To summarize these arguments, it should be recognized that evolutionary and
ecological models discussed above are not mutually exclusive and there is reason
to believe they have worked in concert to promote the postfire obligate seeding life
history. It may be useful to frame the evolution of this life history by analogy with
arguments surrounding the evolution of semelparity and iteroparity (Keeley
1986
). If we change our reference from annual cycles to fire cycles then facultative
seeders represent an iteroparous and obligate seeders a semelparous life history.
Charnov & Schaffer (
1973
) concluded that the evolution of iteroparity would be
expected when the average clutch size of a semelparous organism was increased by
P/C individuals, where P and C are adult and juvenile survivorship, respectively.
Thus, obligate seeding would be expected when resprouting success is low and/or
when seedling success is high. In environments where resprouting success is high,
P/C would be high, and obligate seeding would evolve only when large increases in
successful seedling recruitment could be achieved from diversion of resources from
resprouting to seed production. In contrast, where resprouting success is low,
obligate seeding could be adaptive with far less increase in seedling recruitment
and survivorship.
Bond & van Wilgen (
1996
) investigated the trade-offs between resprouting and
seeding with a simple logistic model and found that different components had
different effects on the outcome. Although both resprouter mortality and level of
