Agriculture Reference
In-Depth Information
year, and in subsequent years, may be due to different factors. Although diversity
appears to be driven by disequilibrial processes, the fact that peak diversity occurs
postfire from residual species present prior to the fire suggests a long-term equilib-
rium whereby community composition has stabilized according to species-specific
niche specialization.
Arizona chaparral under a non-MTC presents an interesting contrast. The
dominant sclerophyll shrubs in this community represent MTV under a non-
MTC bimodal rainfall regime. The postfire flora arising in the first spring after
fire is somewhat lower than observed in California. However, the Arizona chap-
arral communities produce a second flora in the autumn and the total flora for the
year is markedly higher than in California chaparral (
Table 11.2
). The bimodal
rainfall regime allows for greater niche separation of postfire annuals, with the
spring flora taxa having affinities to the California chaparral flora, and the
autumn flora taxa to subtropical floras (Fotheringham
2009
).
Each of the five MTC regions has a forest component to greater or lesser degrees,
and these communities exhibit a variety of fire regimes. Many conifer forests in
California have a surface fire regime and the understory regeneration is typically
shrubs and herbaceous perennials from resprouts and seed germination (Keeley
et al
.
2003
;Knapp
et al.
2007
). Diversity at scales of 1-1000 m
2
are roughly half that
observed in shrubland communities (Keeley & Fotheringham
2003b
). There is a low
level of postfire annuals in mixed conifer forests (Knapp
et al.
2007
). Some of these
ephemerals apparently arise from fire-stimulated germination of dormant soil-stored
seedbanks; however, almost all are found on other disturbed sites and none appear to
be strict fire-following species. Unlike crown fire shrublands where diversity is resili-
ent to a wide range of fire severities (Keeley
et al.
2008
), in forests high fire severity
punches holes in the canopy and increases diversity when understory species colonize
these gaps (Keeley
et al.
2003
). In contrast, diversity in these forests is much more
resilient to frequent fires than diversity of crown fire shrublands.
Chile
Chilean matorral diversity typically declines after fire because the community lacks a
pool of dormant soil-stored seeds and vegetative structures that can colonize after
fire. This is apparently due to the lack of a predictable source of natural fire ignitions
in central Chile: the absence of summer lightning storms is because they are blocked
by the Andean Cordillera (see
Chapters 6
and
10
). A potential exception to this
pattern is matorral shrublands in the southeastern end of the MTC region where the
Andes are low enough to allow summer lightning storms, and thus a predictable fire
regime. Here there is some evidence of a postfire annual flora comprising species of
Hydrophyllaceae, Boraginaceae and Portulacaceae (see
Fig. 6.7
).
Open Chilean matorral characteristically has its highest diversity concentrated
in the understory of the mature shrubs (S. Keeley & Johnson
1977
) and this pattern
remains after fire (
Fig. 11.2d
). Diversity increases slowly as species colonize
from outside the burned area (Armesto & Gutierrez
1978
;G´ mez-Gonza´ lez &
Cavieres
2009
).
