Plant Diversity and Fire - Fire in Mediterranean Ecosystems - page 316

generally comprise open-canopy vegetation due to stressful substrates, reduction
in understory species richness often occurs on sites that support taller stature
shrubs (Cowling & Gxaba 1990 ; Keith & Bradstock 1994 ; Specht & Specht 1999 ).
After fire the change in diversity is substantially higher on those sites with
greater prefire canopy coverage (Specht 1981 ; Bond & Ladd 2001 ). This effect of
oligotrophic soils is evident in other shrubland comparisons. For example,
comparing sclerophyllous shrublands on sandy substrates in non-MTC Florida
with MTC California chaparral reveals that the more open Florida scrub com-
munity has higher diversity prior to fire and much less increase in diversity
after fire (Carrington & Keeley 1999 ). A similar situation is evident in nutrient-
stressed serpentine chaparral with higher diversity prefire and a less striking
postfire diversity increase compared with chaparral on non-serpentine substrates
(Safford & Harrison 2004 ) and a similar fire response is evident in serpentine and
non-serpentine grasslands (Harrison et al. 2003 ).
As a result of these structural differences between MTC communities, and the
manner in which diversity interacts with fire, comparisons of diversity between
mature communities (Cowling et al. 1996 ) have not fully captured diversity
patterns between MTC regions. In general, MTC communities have remarkably
similar diversities ( Table 11.2 ) but peak diversity is closely tied to fire in some but
not others. When recently burned sites are compared, the diversity patterns are
remarkably similar. At the 1-m 2 scale, all regions typically have between 9 and 15
species but all regions show high variability with anywhere from 2 to 25 species. At
the 0.1-ha scale MTC communities typically have between 50 and 80 species,
although site to site variability is extraordinary, ranging from 10 to 180. Differ-
ences between MTC ecosystems emerge as they recover from fire. In the northern
hemisphere, MTC shrubland diversity declines markedly with canopy closure, but
in the Cape of South Africa and southwest Australia diversity remains high in
their more open fynbos and heathland communities, respectively.
Fire does not affect diversity solely by niche differentiation but rather there
are stochastic processes related to postfire recruitment that can play roles
in determining community diversity (Laurie & Cowling 1994 ; Lamont &
Witkowski 1995 ). For example, lottery type recruitment of postfire seeders has
the potential for altering community diversity patterns. Population fluctuations
in different fire cycles cause species to change their contributions to postfire
diversity and this is potentially controlled by stochastic effects between, during
and after fire.
Regional Variation in Factors Driving Fire-Diversity Interactions
Despite the apparent convergence in dominance of fire-prone sclerophyll shrub-
lands across all five MTC regions, and remarkable similarities in postfire regener-
ation ( Fig. 11.2 ), there are marked differences in the relationship between diversity
and fire (more details in Chapters 4 - 8).