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apparent is in woody obligate seeding taxa, a functional type where postfire
recovery is entirely dependent on seedling recruitment. This risky strategy makes
such taxa vulnerable to fires during the juvenile phase, and localized extirpation
follows fires at short intervals. Cowling (
1987
) hypothesized that localized extinc-
tion of obligate seeding fynbos populations has increased the isolation of taxa and
contributed to their divergence. Landscape diversity is thus promoted because of
subtle differences in substrate to which these fragmented populations adapted.
Where less subtle substrate variation exists, such as with mosaics of ultramafic
soils in California, landscape-scale diversity between serpentine chaparral patches
is high and obligate seeding endemics contribute significantly to these patterns
(Harrison & Inouye
2002
; Safford
et al.
2005
). Perhaps a similar mechanism
accounts for obligate seeding chaparral genera following a predictable pattern of
species turnover along climatic gradients, both latitudinally and from the coast to
the interior (Cody
1986
).
These landscape effects also add to regional diversity and differences in fire
regimes may explain some regional differences in diversity between MTC commu-
nities. One example may be the extraordinarily high floristic diversity observed in
the Cape region of South Africa, with species richness markedly higher than other
MTC regions, including the very diverse southwest Australian flora (
Fig. 11.1c
).
Extreme landscape heterogeneity is often cited as a primary driver in the high
Cape floristic diversity (Cowling & Lombard
2002
), and is thought to result from
mountainous terrain contributing to habitat heterogeneity. However, such terrain
also has huge impacts on fire regimes, potentially increasing fire frequency due to
the effect of steep slopes in driving fires when dead fuels and/or winds are insuffi-
cient to carry active crown fires. More frequent predictable fires could potentially
impact regional species pools by speeding up the rate of evolution in fire-depend-
ent obligate seeding taxa (see
Chapter 9
). Metapopulations have been hypothe-
sized to be a key factor in the evolutionary differentiation of such populations
(Harrison
1998
), and fire would be one of the more likely mechanisms behind the
hypothesized metapopulation effects.
Conclusions
Fire affects diversity differently in each of the five MTC regions. On nutrient-
stressed soils in the Cape region of South Africa and in Western Australia the vast
majority of the diversity is from perennials and their presence is not strictly tied to
fire. In these Gondwana heathlands, shrub spacing is sufficient to allow a diversity
of growth forms to persist between fires and postfire diversity is only modestly
greater than prefire diversity. The more fertile MTC communities of the northern
hemisphere tend to have closed-canopy conditions with low understory diversity.
In these landscapes fire creates a marked change in resource availability and
associated with this is a huge increase in diversity after fires, from both annual
and perennial herbaceous species. In a comparison of postfire communities, no
