Agriculture Reference
In-Depth Information
resprouting success and this makes the comparison of more limited value. Compli-
cating some comparisons is recognition that there is no reason to expect contem-
porary patterns are indicative of selective environments that occurred millions of
years ago (see
Chapter 10
). The genetic consequences of greater selection for
seedling over resprout regeneration would further enhance the advantages for
recruitment in postfire environments. These fire recruiters as a group exhibit
physiological and anatomical traits that favor recruitment in open sites under
stressful soil-drought conditions (S.D. Davis
et al.
1998
; Paula & Pausas
2006
;
Pratt
et al.
2007
,
2008
). Under summer-rain conditions such sites might be
restricted to rocky outcrops as in eastern Australia (e.g. Clarke
et al.
2005
) but
under MTC these conditions are much more widespread. The greater number of
sexual generations leading to faster selection for novel recombinant types could
enhance inclusive fitness of individuals and increase the capacity for recruiting in
fire-induced gaps and feed back to increased selection for obligate seeding. This
mirrors advantages of herbaceous taxa over woody taxa in climatic niche evolu-
tion (Smith & Beaulieu
2009
).
Obligate seeders might also enhance their ability to track environmental
changes through their greater mobility between generations. As environments
change, such as through glacial and interglacial Pleistocene cycles, resprouting
taxa would be at a decided disadvantage in tracking these environments,
since dispersal is critical to finding new sites as the conditions deteriorate at
present sites.
The Biography of Postfire Resprouting and Seeding
With increasing focus on global patterns there is a need to understand the relative
importance of postfire responses in vastly different ecosystems representing a
diversity of growth forms and fire regimes. Although one can detect local patterns
of resprouting across different fire frequencies (e.g. Clarke & Dorji
2008
), global
predictions need to consider fire in combination with climate and geology (see
Fig. 1.4
). The level of resources based on climate and soils will have a deterministic
role in setting the range of potential growth forms and these in turn will form the
fuels for determining fire regime characteristics (Pausas & Bradstock
2007
). Func-
tional types will be dictated by resources, fire regime and growth form, with
substantial shifts in type driven in some systems by herbivory and modified by
phylogenetic constraints.
Scaling resprouting vs. seeding responses to site productivity is complicated by
differences in fire regimes. In addition, growth forms with different rates of
maturity will differ in fire sensitivity. The important parameter for scaling would
be the ratio of time to maturity over fire return interval. Seeders would be at a
decided disadvantage when that ratio drops below 1.
Potential growth forms vary along the resource gradient from trees where
resources are high to herbaceous plants at the lowest end of the gradient and
multiple strata of herbs and trees at intermediate levels. The tipping point between
