Agriculture Reference
In-Depth Information
appears sufficient for postfire regeneration of annual grasses and forbs. Also, the
relatively low fire intensities resulting from burning these herbaceous fuels lead to
high seed survivorship. Native perennial bunchgrasses, such as
Nassella pulchra
,
Poa secunda
,
Koeleria cristata
and numerous geophytes, resprout after fire. Since
the aboveground parts of these species die back each year regardless of fire, this
response is indistinguishable from the normal seasonal resprouting. Enhanced
flowering of herbaceous perennial resprouts after fire generates substantial seed
crops that recruit in the second and subsequent postfire years (Tyler & Borchert
2002
; Keeley
et al.
2006b
). Numerous native annuals share their distribution
between grasslands and chaparral, but have markedly different germination and
establishment patterns, suggesting ecotypic differentiation (Keeley & Davis
2007
).
This includes species of
Gilia
,
Lotus
,
Lupinus
,
Phacelia
,
Silene
,
Trifolium
and others.
In closed-canopy chaparral soil, seedbanks of these annuals are deeply dormant
until germination is triggered by either heat shock or smoke. In grasslands
most native annuals establish every year, although populations exhibit extraordin-
ary annual fluctuations. Some of this is tied to years of high rainfall but
other factors may play a role. For example, extremely dry years are commonly
followed by years of high annual presence and this may be tied to the reduction of
thatch during dry years and its potential for opening up larger gaps in the
following year.
California Sage Scrub
Sage scrub shrubland is of smaller stature than chaparral and tends to include
mostly summer deciduous subshrubs (
Fig. 5.2
) and is distributed from Baja
California to central California (Rundel
2007
). Sage scrub is found on more arid
sites, usually at lower elevation, than chaparral on both coastal and interior sites,
but is absent from the foothills of interior ranges such as the Sierra Nevada.
The fire regime of sage scrub is remarkably similar to chaparral. Crown fires kill
nearly all aboveground biomass. It is resilient to a wider range of fire frequencies
than chaparral, as it will tolerate shorter fire intervals. Coastal sites dominated by
this vegetation were likely burned at frequent intervals by Native Americans
(Timbrook
et al.
1982
), but in the absence of human ignitions fires likely spread
from associated shrublands and grasslands in the interior and would have
occurred at much longer intervals.
Regeneration is also much like chaparral. Most shrubs are facultative seeders,
both resprouting and recruiting from seed. The soil-stored seedbank is often
polymorphic with some seed recruiting between fires and other seeds more
deeply dormant and stimulated to germinate by fire (Keeley
1991
). Resprouting
is commonly more successful in younger plants and it has been suggested that
this reflects the herbaceous ancestry of most of these subshrubs (Keeley
2006c
).
The ephemeral postfire flora is
largely indistinguishable from chaparral
(Keeley
et al.
2006b
).
