Agriculture Reference
In-Depth Information
low-intensity surface fires (Finney & Martin
1992
; Brown & Swetnam
1994
).
Undoubtedly this latter pattern is the result of human interference resulting from
Native American land management (Stephens & Fry
2005
). This is supported by
the fact that the region has a very low lightning-ignited fire frequency (Keeley
2005
), and historically it had one of the highest Native American densities in
North America (Heizer
1978
).
Prior to the entry of people into this region in the late Pleistocene, the fire
regime in these forests was undoubtedly characterized by long-interval crown fires
driven by unusually severe and widespread droughts at rare intervals. Such
conditions would be needed to adequately dry understory fuels in these closed-
canopy forests as well as enhance the probability of fire spread from surrounding
lightning-rich areas in the interior. Consistent with this crown fire regime is the
fact that redwood (
S. sempervirens
), the tallest tree in the world, is a vigorous
resprouter after high-intensity crown fires (Stuart
1987
), an unusual trait in
conifers and not to be expected in a surface fire regime (Keeley & Zedler
1998
).
Under these extended fire-free periods, other disturbance events such as tree
falls likely played a very important role in structuring these forests (Hunter &
Parker
1993
).
Mediterranean-type Vegetation outside the
Mediterranean-climate Region
Mediterranean-type vegetation in other climatic zones may be viewed at two
levels: (1) California Floristic Province taxa that range far beyond the MTC and
(2) communities outside this climate that are composed of different taxa that are
structurally similar to those that comprise our MTC communities.
Chaparral Outside the Mediterranean-type Climate
Chaparral is the most widespread vegetation within the MTC region of California.
Components of chaparral vegetation are also found east of the California MTC
(
Table 5.3
), in Arizona with a bimodal rainfall pattern and northeastern mainland
Mexico with a summer drought/winter rain climate (see
Fig. B1.1.1
). The primary
feature these MTVs have in common with chaparral is in the predominance of
sclerophyllous-leaved evergreen shrubs, and they do share a few shrub species
(Muller
1939
,
1947
; Shreve
1939
). In addition, because they form dense shrublands
in seasonal environments, they are all subject to periodic crown fires.
The vast majority of California chaparral shrub species are endemic to the MTC
region, although there are noteworthy exceptions. Shrubs such as
Malosma laurina
are distributed south along watercourses into arid deserts of Baja California
(Shreve
1936
) and
Heteromeles arbutifolia
is disjunct from southern California to
the tip of Baja California in a winter drought/summer rain climate (Phipps
1992
).
