Agriculture Reference
In-Depth Information
vulnerable to fires. Also, invasion by some understory alien species may act as
ladder fuels, increasing the potential for crown fires (see
Fig. 12.1
).
Following fire, mature oaks resprout epicormically if fires are not too intense,
and some resprout from the base when immature (Allen-Diaz
et al.
2007
). Oak
species differ in canopy architecture, bark thickness, self-pruning and various
other traits, but there has been little comparative study of how these differences
affect fire regimes and ecosystem responses. For example, the savanna blue oak
Q. douglasii
is something of an anomaly because it retains a substantial amount of
dead branches, and this lack of self-pruning would seem to be disadvantageous in
the current surface fire regime. However, Cooper (
1922
) suggested that the current
savanna habitat is a modern anomaly and this oak was originally part of a
shrubland assemblage with crown fires. In this context, lack of self-pruning may
have selective value in a crown fire ecosystem where localized higher fire intensity
could eliminate potential competitors (e.g. Bond & Midgley
1995
). Consistent
with Cooper's idea of a blue oak shrubland community is the observation that
seedling recruitment of
Q. douglasii
is facilitated by shrub nurse plants (Callaway
et al.
1991
). In a similar vein, Wells (
1962
) proposed that prior to increased
anthropogenic burning in the Holocene, the understory of
Q. agrifolia
savanna
was not grass, but rather sage scrub shrubland. Thus, many of the current oak
savannas may be human artifacts derived from oak shrubland associations.
Below the conifer belt on more mesic slopes, or sites free of fire for long periods of
time, broadleaf evergreen trees form closed-canopy woodlands. Common tree taxa
include
Quercus wizlizennii
,
Q
.
chrysolepis
,
Lithocarpus densiflora
,
Arbutus menziesii
and
Umbellularia californica
, and these oftenmerge with riparian woodlands (Sawyer
et al.
1977
). In southern California, foothill woodlands are dominated by winter-
deciduous
Juglans californica
in association with several obligate resprouting ever-
green chaparral species (
Heteromeles arbutifolia
,
Prunus ilicifolia
and
Rhamnus crocea
)
that persist as gap-phase arborescent shrubs (Keeley
1990b
). At higher elevations
throughout the state the dominant oak is the winter-deciduous
Q
.
kelloggii
,andit
merges with conifer forests.With one exception, all of these species are endemic to the
MTC California Floristic Province;
Q. chrysolepis
is disjunct to Arizona. In average
rainfall years fuel moisture remains high and fires dampen down and are largely
extinguished by evergreen woodlands, but in drier years these woodlands are subject
to crown fires. All tree species resprout epicormically if not burned too intensively,
and some resprout from the base, often from massive burls.
None of the dominants in these vegetation types have postfire seedling recruit-
ment; rather they produce transient seedbanks with little seed carry-over
from year to year. Successful seedling recruitment is largely restricted to fire-free
intervals and is often dependent on a sequence of high-rainfall years that promote
seed production in one year and seedling establishment in the following year.
Foothill Coniferous Trees
Included here are trees that occur in small patches juxtaposed with chaparral,
including:
Pinus attenuata
,
P. coulteri
,
P. muricata
,
P. radiata
,
P. sabiniana
,
