Fire in California - Fire in Mediterranean Ecosystems - page 113

5 Fire in California

On the west coast of North America lies the state of California, USA ( Fig. 5.1 ),
the bulk of which is dominated by a mediterranean-type climate (MTC). Eleva-
tions range from sea level to over 4000 m. Mountain ranges are largely oriented
north to south with a major valley between the coastal ranges and the interior
Sierra Nevada range. In the rain shadow east of the interior mountain ranges the
climate is more continental with much colder winters and increasing proportion of
summer precipitation eastward. This easternmost part of the state has steppe
climates in the northern portion and desert climates in the south. In Arizona
and a few other parts of southwestern USA and northeastern Mexico are disjunct
patches of sclerophyllous-leaved vegetation that closely resembles California
MTC vegetation. These include evergreen shrublands, broadleaf woodlands and
conifer forests and represent mediterranean-type vegetation (MTV) under non-
MTCs. Further east at similar latitudes but under different climates are sclero-
phyll forests with many similarities to MTC conifer forests.
The California Floristic Province (Raven & Axelrod 1978 ) essentially cir-
cumscribes the MTC vegetation of North America and extends across the latitu-
dinal range of the state. On the western slopes of the major mountain ranges is a
rich diversity of vegetation types that change along the elevational gradient.
Ascending the coastal mountains the main vegetation types sort out along
gradients of decreasing aridity in the following order: grasslands, semi-deciduous
woody sage scrub, evergreen chaparral shrublands, oak woodlands and conifer
forests. A similar pattern is evident on the west side of the interior Sierra Nevada
except for the absence of sage scrub. These vegetation types exhibit marked
differences in fire regime and tolerance to disturbance tied to the different patterns
of fuel structure resulting from changes in dominant growth forms along the
elevational gradient. Along this gradient there is an interaction between fires
and aridity such that lower fire frequency is required to displace shrubland
associations with grasslands and other herbaceous vegetation on xeric than on
mesic landscapes (Keeley 2002b ). Consequently there are complex local mosaics
due to differences in aspect and fire history (see Fig. 1.6c ) .
Associated with these vegetation types are gradients in primary productivity,
seasonality and ignitions that result in very different ecosystem roles for fire.
For example, the fuel structure in chaparral and sage scrub shrublands main-
tains biomass throughout the shrub profile resulting in all fires burning as