Fire in California - Fire in Mediterranean Ecosystems - page 142

Due to these two seasonal floras, the total flora in the first postfire year in Arizona
chaparral sites is substantially higher than observed in California chaparral.
The similarities between California and northeastern Mexico chaparral include
patterns of postfire shrub resprouting and seedling recruitment. Lloret et al.
( 1999a ) concluded that because the mexical vegetation is burned infrequently,
yet possesses many of the fire traits evident in frequently burned California
chaparral, such traits are pre-adaptations to fire. However, the current fire regime
in California is anthropogenic. In the absence of a constant bombardment of
human ignitions in densely populated southern California (Keeley 1982 , 2006a ),
fire regimes would be much more similar (Rodrı´ guez-Trejo 2008 ). Thus, the
similar postfire responses are not unexpected and can't be used to infer selective
differences between regions.
In general, chaparral vegetation is widely distributed in both MTC California as
well as non-MTC Arizona, with bimodal rainfall, and northeastern Mexico, with
winter drought and summer rains. Other components of these communities are
radically different and thus these “chaparrals” do not represent the same plant
ecosystems. This is interesting in terms of paleoecology because shrub leaves are
the most likely plant parts to be preserved, and one might expect that fossil floras
of these three regions would tend to point toward the same ecological commu-
nities, when in fact they represent very different ecosystems.
Coniferous Forests outside the Mediterranean-type Climate
Sclerophyllous-leaved MTV conifer forests with surface fire regimes are not
restricted to MTC California. Indeed, the two most widespread forest types,
ponderosa pine forest and the mixed conifer forests, occur outside the MTC
region. The two dominants of these forests, Pinus ponderosa in the former and
Abies concolor in the latter forest type, are also dominants in similar forest types
under non-MTCs in other parts of the western USA.
Pinus ponderosa has roots back to the mid Tertiary (see Fig. 9.1 ) and through-
out this period climates have shifted and the taxon has migrated in step. The
present distribution dates to only the last glacial episode (Anderson 1989 ). Pinus
ponderosa is one of the dominants of the MTC mountains, throughout California
and into Oregon, Washington and Idaho ( Fig. 5.8 ), all of which have some
semblance of a MTC with winter rains and summer droughts. However, this
species is also widespread throughout the interior non-MTC Rocky Mountains.
The primary climatic difference between regions is not as much in temperature as
in the seasonal distribution of rainfall (Pearson 1951 ; Norris et al. 2006 ). Interior
populations of P. ponderosa occur under a continental climate with summer rains;
however, as with MTC populations, their distribution is characterized as occur-
ring on sites exposed to prolonged soil moisture deficit (Barrett et al. 1980 ), and
numerous factors affect soil moisture, including precipitation, degree of evapo-
transpiration, and soil structure (Stephenson 1998 ).